Introduction

Develo** alternative energy sources has become necessary due to the world's economy and population growth, as well as the resulting demand for oil and gas resources (Razeghifard 2013; Shuba and Kifle, 2018). This rising energy demand is related to the transportation sector, where the most demand is met by using fossil-based fuels (Asomaning et al. 2016); the transportation sector consumes approximately 28% of total global energy and is heavily reliant on fossil fuels. Nevertheless, 71% of crude oil contributes significantly to total greenhouse gas emissions (Leite et al. 2013). The use of fossil fuels has significantly contributed to air, water, and soil pollution, which has a negative impact on public health, an energy crisis caused by the irreversible depletion of global fossil-fuel reserves, and increased climate change (Fawzy et al. 2020). Biomass is the world's third-largest energy resource after coal and oil (Tumuluru et al. 2011; Osman et al. 2021a, b). Biomass for power generation is gaining popularity globally due to its long-term sustainability, potential, and environmental benefits. Furthermore, the produced biomass is almost carbon neutral and has the potential to significantly reduce net carbon emissions as well as hazardous emissions (Tumuluru et al. 2012); therefore, the need for sustainable and renewable energy sources has become an urgent demand (Silva et al. 2019).

Biofuels are typically derived from biomass and are composed of organic or biological components that can exist as solid, liquid, or vapors (Osman et al. 2021a). Over the last five decades, researchers have developed biomass as a feedstock for the first, second, third, and fourth generations of bio-based energy (Chowdhury et al. 2019). Third-generation biofuels, which are primarily based on microalgae, are thought to be a viable solution to the problems associated with first and second-generation biofuels. Algae (Fig. 1) are ubiquitous organisms in various habitats, but their commercialization as a substitute for fossil fuels is limited (Gajraj et al. 2018). Because of their rapid growth rate and high lipids, protein, minerals, and carbohydrate contents, microalgae are regarded as a viable feedstock for various bioproducts and bioenergy carriers (Hamed et al. 2020). On an industrial scale, the literature clearly demonstrates algae-based biofuels' positive economic and environmental impacts (Hamed et al. 2020a; Efroymson et al. 2021; El Shimi et al. 2018). After lipid extraction, the algal biomass residue can be converted into a variety of biofuels, including biomethane, bioethanol, and biohydrogen (Lam et al. 2019; Hamed et al. 2017).

Fig. 1
figure 1

Different types of algae cultivation, either in open or closed ponds. Algae growth is influenced by water supply and cultivation mode. Microalgae can be grown in two different ways. Ponds and lakes are examples of open systems, whereas photobioreactors are examples of closed systems. Every system has its benefits and drawbacks. Open farming (open pond) is regarded as the most basic and oldest method of producing and cultivating microalgae on a large scale

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Microalgae are photosynthetic microorganisms that form the foundation of most aquatic food chains and contribute significantly to oxygen production (Chen et al. 2018). Numerous studies have examined how their mixotrophic nutrition on organic compounds during growth aided carbon transformation and storage (Hamed et al. 2021; Abomohra et al. 2014; Heredia-Arroyo et al. 2011). They have been shown to use nutrients from wastewater (Abou-Shanab et al. 2014; Dong et al. 2014; Pittman et al. 2011), carbon dioxide and exhaust gases emitted through industrial processes (Lizzul et al. 2014). As a result, producing microalgal biomass concurrently with existing industrial or municipal treatment activities could significantly reduce economic and environmental costs while also providing a valuable remediation service (Mishra et al. 2017; Karpagam et al. 2021; Rawat et al. 2011; Park et al. 2011).

Microalgae produce a variety of biotic substances with diverse applications in the chemical, food, pharmaceutical, carbon sequestration, and biofuels industries (Siddiki et al. 2022). Due to technical challenges, large-scale microalgae cultivation is limited, which is one of the major influences on its commercialization (Lam et al. 2019; Hamed et al. 2020). Therefore, this review aims to evaluate and critically describe the key factors that can be used to improve microalgae-based biofuels production by utilising potential microalgal species while highlighting related technologies and problematic issues in their production. Furthermore, evaluate the application of microalgae in atmospheric carbon removal as a carbon sequestration tool.

Applications

Biodiesel has recently received much attention as a renewable, biodegradable, and non-toxic fuel that emits fewer pollutants than regular diesel (Antolın et al. 2002; Lang et al. 2001; Vicente et al. 2004). Biodiesel has better chemical and physical properties than petro-diesel fuel, such as a higher cetane number, lower sulfur concentration, higher flash point, and better lubricating efficiency due to the contained oxygen (Goodrum et al. 2005; Anastopoulos et al. 2001; Nabi et al. 2006). Compared to diesel, direct use of biodiesel or biodiesel blends positively affects exhaust gases (Nabi et al. 2006, Knothe 2006, Schumacher et al. 1996). Biodiesel's relatively high oxygen content significantly reduced combustion gases and decreased carbon monoxide emissions.

Moreover, biodiesel is free of aromatic compounds and other chemical substances; thus, it has no negative environmental impact. In 2003, global biodiesel production was around 1.8 billion liters (Fulton 2004). In recent years, there has been a significant increase in biodiesel production. Biodiesel production is expected to increase in response to increased global demand for fuels and cleaner energy. Microalgae-derived biodiesel can completely replace petroleum; however, the cost of microalgal oil production must first fall from approximately $ 2.80/L to $ 0.48/L (Chisti 2007).

Compared to other feedstocks, microalgae-based biofuels are the most cost-effective; photoautotrophic microalgae, for example, convert sunlight into biomass more efficiently than higher plants (Demirbas et al. 2011). Whereas terrestrial plants have a photosynthetic efficiency of less than 4%, algae have a photosynthetic efficiency of 3–9% (Dismukes et al. 2008). The high growth rate of microalgae and consecutive biomass production reflect this light utilisation efficiency. Furthermore, algae are more tolerant to a wide range of light intensities than higher plants, allowing them to live autotrophically through photosynthesis. Meanwhile, some microalgal species can produce a relatively high content of energy-rich compounds by utilizing organic carbon sources such as glucose (Lee, 2001; Hamed and Klöck, 2014; Hassan et al. 2012).

Heterotrophs are algae that can grow in the absence of light energy but feed on organic carbon (Heredia-Arroyo et al. 2011; Joun et al. 2021; Feng et al. 2014; Shen et al. 2019). Heterotrophic microalgal growth is less efficient than phototrophic growth because the organic source required in heterotrophic growth is produced by another photosynthetic crop (Heredia-Arroyo et al. 2011; Patil et al. 2008). As a result, energy must first be used to grow the crop in the heterotrophic mode, whereas in the photoautotrophic mode, energy is used directly for algae growth.

Algae have adapted to live in various ecosystems ranging from hot springs to snow (Harwood et al. 2009). Some algal species live in terrestrial habitats, but the majority live in water bodies, including freshwater, brackish, marine, and hyper-saline waters (Hu et al. 2008). Algae are classified into nine main groups: green algae, cyanobacteria (blue-green microalgae), diatoms, yellow-green algae, golden algae, red algae, brown algae, dinoflagellates, and pico-plankton (Hu et al. 2008). Because algae are diverse and largely unexplored organisms, there is potential for further developments and applications. Their diverse genetic and biochemical composition could explain their ability to survive in many environments (Faramarzi et al. 2008).

To summarize, there is real potential for using algae in biodiesel production; however, the cost of microalgal oil production must first fall from around $ 2.80/L to $ 0.48/L. Algae's ability to live in various harsh conditions makes them ideal for various applications, with the potential to further explore undiscovered organisms within algae.

Algal biomass

Sources

Freshwater

Freshwater accounts for less than 3% of all water on the planet. Approximately 69% of the Earth's freshwater is inaccessible to humans, such as ice in glaciers and polar ice caps and groundwater. The world's available surface fresh water is not distributed evenly. The majority of the world's surface freshwater is located in Brazil, Russia, Canada, Indonesia, China, Columbia, and the United States. Lakes, rivers, wetlands, streams, and ponds are examples of freshwater habitats. Various types of algae and cyanobacteria can be found in low salt concentration ponds and freshwater lakes that can support diverse flora and fauna. For example, favorable conditions such as nutrient availability, adequate light, and temperature promote algal bloom biomass production (Gatamaneni et al. 2018). To reduce the negative environmental impact of algal blooms, researchers converted naturally occurring algal blooms into biofuel production (Kuo 2011; Ghosh et al. 2019). Streams and rivers have different environments, such as higher oxygen levels and faster flow. Because of agricultural runoff, streams have high nutrient content, such as nitrogen and phosphorus (Díez-Montero et al. 2020). Algae are usually abundant in the middle of the river due to the lower water flow in this section than in others. The river mouth is unsuitable for algal growth as the water becomes murky. Algal growth is also diminished due to limited light penetration and stagnant water conditions (Chew et al. 2018). Overall, using freshwater resources for algal biomass production is not economically feasible.

Saltwater

Saline or salt water constitutes the vast bulk of Earth's water. Saline water includes oceans, marginal seas, saline groundwater, and closed lakes. The availability of saltwater from the oceans introduces a low-cost option for microalgae growth and biomass production. The marine ecosystem is home to a diverse range of algae, including microalgae cyanobacteria as well as macroalgal species. Marine macroalgae are commonly known as seaweeds, which include three different classes red algae (Rhodophyta), brown (Phaeophyta) and green macroalgae (Chlorophyta). Marine macroalgae commonly occupy intertidal and sublittoral-to-littoral zones on rocks and other hard substrata. Marine algae are an important component of the marine ecosystem because they absorb sunlight energy, water and carbon dioxide to produce organic compounds and release oxygen to the ambient environment. This cycle contributes to the balance of the ocean's life cycle.

The unicellular microalgae, known as phytoplankton, constitute the base of the marine food chain. It is typically found near the water's surface, capturing sunlight. Marine phytoplankton plays a critical role in the biogeochemical cycling of the oceans. Their varied genome structure explains their adaptation to thrive in various conditions. Microalgae have become a dominant force within marine ecosystems. For example, coastal regions' microalgae are contended with high nutrients, low light and turbulence conditions, whereas open ocean microalgae are adapted with low nutrients and high irradiance. Polar microalgae are acclimatized to high nutrients with freezing temperatures and long periods of light and darkness.

Because salinity is an important factor in these environments, saltwater is a typically ideal condition for develo** microalgae resistant species to high salinity. The average salinity of marine water is 35%, though this varies slightly according to the amount of runoff received from surrounding lands and rainfalls. Overall, marine water is an attractive resource for microalgae scales up, however  cultivation in the marine environment requires careful monitoring to ensure optimal biomass productivity.

Wastewater

Wastewater is a rich nutrients source for microalgae growth. Phyco-remediation is a wastewater treatment method that employs microalgae for nutrient removal from wastewater, which can then be reused for multiple purposes (Lage et al. 2018). There are several types of wastewaters, including industrial, domestic, refinery, agricultural, and leachate. The organic composition of all wastewaters is comparable; however, the inorganic content varies among wastewater sources. Carbohydrates, proteins, lipids, amino acids, and volatile acids account for three-quarters of the organic carbon in sewage.

These wastewaters typically contain high nitrogen, phosphorus, and other inorganic compounds, which cause eutrophication to local water sources and impose ecological risk to aquatic life. Thus, employing wastewater for algal mass production could be a  promising solution to replace synthetic culture media, which is currently prohibitively expensive to use on an algal scale. For instance, wastewaters of palm fertiliser, fruit bunches, and palm oil mill effluent are rich with high nitrogen and other nutrients content; they have been proposed as good culture media.

Although high nutrient levels in wastewater promote algal growth, the high levels of toxic compounds, emerging contaminants and pathogens can inhibit algal growth. Therefore, resistant microalgal species that adapt to the wastewater environment must be carefully selected for optimal growth and high biomass yields. For example, struvite is a wastewater nutritive medium that has been investigated as a microalgal supporting medium to reduce cultivation costs (Chew et al. 2018).

Types

Freshwater species

Freshwater microalgae could provide a promising feedstock for biofuels production due to their fast growth rates, high biomass yields, high carbon dioxide sequestration ability, and their strong potential to grow on marginal lands that do not compete with agricultural crops (Yun et al. 2014; Ramaraj et al. 2010; Saraf and Dutt, 2021; Al-Lwayzy et al. 2014). Every species has a distinct growth rate and innate metabolic profile. Consequently, selecting microalgal species with high biomass production and promising properties for large-scale production are important factors for sustainable biofuel technology.

Freshwater microalgae have been previously used for human and animal nutrition. These species can rapidly absorb nutrients from the liquid phase and thrive in the environment. Numerous studies showed the high ability of freshwater microalgae in biomass for bio-based energy production, such as Chlorella vulgaris (Al-Lwayzy et al. 2014), Chlorella pyrenoidosa (Yang et al. 2015), Muriellopsis sp. and Scenedesmus subpicatus (Gómez-Serrano et al. 2015), Ankistrodesmus falcatus (George et al. 2014), Coelastrella sp. (Narayanan et al. 2018), Asterarcys quadricellulare (Sangapillai and Marimuthu, 2019), Scenedesmus obliquus (Liu et al. 2013) and Tribonema sp. (Wang et al. 2014a).

Significant efforts have been made to use marine macroalgae and cyanobacteria for the biofuels industry; however, much less concern has been placed on using freshwater macroalgae (e.g. eukaryotic Chlorophyta) (Yun et al. 2014; Grayburn et al. 2013; Lawton et al. 2013; Demirbas 2010; Khola and Ghazala 2012). Nevertheless, freshwater macroalgae may have significant potential for liquid and solid biofuels that can be combusted directly or co-combusted with more traditional energy sources (Tumuluru et al. 2012; Grayburn et al. 2013). Moreover, biomass harvesting, represented as dense floating mats, is much easier and cheaper than dewatering equivalent biomass of suspended microalgae (Grayburn et al. 2013; Hillebrand 1983). For example, using an algal turf scrubber for large-scale freshwater macroalgae cultivation has been elucidated as a cost-effective and eco-friendly approach (Yun 2014). This technology combines nutrient removal in wastewater with bioenergy production. Several common freshwater macroalgal taxa, e.g., Oedogonium, Rhizoclonium, Ulothrix, and Microspora, have been reported (Adey et al. 2011, Kebede‐Westhead et al. 2003, Mulbry et al. 2008a, Pizarro et al. 2006).

Although the elemental biomass composition of common freshwater macroalgae has been analyzed, relatively few studies have evaluated the efficiency of freshwater macroalgae as a feedstock for biofuel (Tumuluru et al. 2012; Lawton et al. 2013). At the industrial scale, it is critical to investigate how variable environmental conditions such as temperature, salinity, light, and nutrient availability affect the growth of freshwater macroalgae, biomass productivity, and energy efficiency (Smith et al. 2010; Sturm et al. 2012; Shurin et al. 2013). In this context, nutrient limitation remarkably induced lipids accumulation in several eukaryotic algae; however, the yields and productivity of algal biomass were noticeably reduced (Shurin et al. 2013; Subramanian et al. 2013).

As a result, determining the net energy yield of freshwater macroalgal cultivation systems necessitates a thorough examination of biomass productivity and energy content. In this study, the exhaust emissions and fuel characteristics of the biodiesel produced from a mixture of freshwater macroalgae Cladophora and Rhizoclonium were nearly identical to petro-diesel (Grayburn et al. 2013). Furthermore, three common freshwater macroalgal species were found to have high bioenergy potential in terms of higher heating value and productivity (Lawton et al. 2013). Hence, more investigations on the effectiveness of freshwater macroalgae as a bio-based energy carrier are needed.

Marine or saltwater species

Marine microalgae are less diverse than freshwater microalgae due to their toxic and hazardous nature (Chew et al. 2018). Because of their large surface area, algae in seawater and oceans have evolved to outperform freshwater algae. The photoconversion rate of marine microalgae has increased to better use the abundant sunlight and synthesize biomass more quickly (Wei et al. 2013). Certain algal species can accelerate the production of valuable byproducts if exposed to harsh environmental conditions. The laboratory study of Rodolfi et al. (2009) on 21 different marine microalgal strains, e.g. Tetraselmis suecica, Phaeodactylum tricornutum, Chaetoceros calcitrans, Isochrysis galbana Nannochloropsis oculata, Pavlova lutheri, Skeletonema sp. revealed variable biomass productivities which ranged between 0.04 and 0.37 g/L/day and lipid productivity (17.4–61.0 mg/L/day) (Table 1). The marine diatom Chaetoceros muelleri exhibited moderate growth production (0.272 g/L/day) and high lipid productivity (51 mg/L/day) on synthetic saline water medium under phototrophic cultivation with high carbon dioxide aeration levels (10–20%) (Wang et al. 2014b) (Table 1). The marine microalgal strains Chlorella salina, Neochloris sp. and Nannochloropsis sp. were demonstrated for lipid-rich biomass production with lipid contents 28, 46 and 52% using synthetic medium and enriched seawater medium under phototrophic growth conditions (Surendhiran et al. 2014; Moazami et al. 2011). This implies the possibility of lowering the cost of biomass-derived biofuel production, which is currently not economically viable.

Table 1 Influence of different cultivation conditions on algal biomass and lipids productivity potential as a bio-based energy source. The freshwater green microalgae Scenedesmus obliquus, Neochloris oleoabundans, Rhizoclonium hieroglyphicum, Chlorella protothecoides and Chlorella vulgaris have significant biomass yields.
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Wastewater species

The commercialization of algae-based biotechnology for the manufacturing of bio-based energy is restricted by the high production costs (Hamed et al. 2020). Thus, several ways have been proposed to make this technology economically feasible, such as using industrial effluent (Gómez-Serrano et al. 2015; Kim et al. 2015), municipal wastewater (Abou-Shanab et al. 2014; Dong et al. 2014; Sturm et al. 2012; Zhang et al. 2014; Han et al. 2016; Mahapatra et al. 2014), and dairy manure effluent (Kebede‐Westhead et al. 2003; Mulbry et al. 2008a; Pizarro et al. 2006; Wahal and Viamajala, 2016) as a culture media to reduce cultivation costs and freshwater requirements.

No specific species can grow in wastewater; instead, robust strains should be carefully chosen to survive under these harsh conditions (Lage et al. 2018). Oscillatoria, Euglena, Chlamydomonas, Chlorella, Scenedesmus, Navicula Nitzschia, and Stigeoclonium are the eighth-most tolerant genera (Randrianarison et al. 2017). Scenedesmus sp. (He et al. 2019) and Chlorella sp. (Makareviciene et al. 2011) are widespread in freshwater bodies of various types as primary producers cleansing the eutrophic waters (Abdel-Raouf et al. 2012).

Several laboratory studies on microalgae biomass production using wastewater have been investigated either in bioreactors, small semi-continuous culture systems, or batch cultures. Data in Table 1 showed that microalgae yielded differential lipid contents ranging from moderate such as Botryococcus braunii (17.85% dry wt) (Órpez et al. 2009), to high lipid content 23, 30 and 40% in Micractinium reisseri (Abou-Shanab et al. 2014), Chlorococcum sp. RAP13 (42%) (Ummalyma and Sukumaran, 2014), Chlorella sorokiniana (61.52%) (Ramanna et al. 2014). The study of Chinnasamy et al. (2010) revealed that Butyraceous Braunii, Chlorella saccharophila and Dunaliella tertiolecta grown in industrial wastewater (treated carpet mill) showed biomass productivity ranging from 0.016 to 0.038 g/L/day, and the estimated lipid productivity was ranged from 2.72 to 4.4 mg/L/day depending on species type. Implying that this kind of wastewater could be a sensible option for biomass-derived energy yield.

In this context, the green microalga Chlamydomonas reinhardtii grown in batch culture using 100% municipal wastewater showed reasonable lipid productivity (16.6% dry wt) coupled with high biomass productivity (Kong et al. 2010). Moreover, this green microalga had the potential to grow vigorously in the wastewater for 1 month when transferred to a biocoil with a relatively high lipid content (25.25% dry wt) and considerable biomass and lipid productivities of 2.0 g/L/day and 505 mg/ L/day in addition to high removal efficiency of nitrogen and phosphorous from the ambient medium, as shown in Table 1 (Kong et al. 2010). Botryococcus braunii grown in secondary treated municipal wastewater showed considerable levels of total lipid content (17.9% dry weight) and biomass productivity of 0.35 g/L/day, indicating that wastewater nutrient status influences biomass and lipid synthesis (Órpez et al. 2009).

The use of dairy manure as a microalgae nutritive supplement could increase the algae-based biofuel production potential. Where the lipid content of microalgal consortia of Chlorella sp. Micractinium sp. and Actinastrum sp. grown in anaerobically digested dairy manure wastewater in outdoor batch culture showed considerable lipid content of 14 to 29% dry wt depending on the used concentration of wastewater, giving estimated biomass and lipid productivity of 0.06 g/L/day and 17 mg/L/day (Woertz et al. 2009). A similar finding has been reported in Chlorella sp. using 20% diluted anaerobic digester effluent samples treated with dairy waste giving biomass and lipid productivity 0.34 g/L/day and 37.0 mg/L/day (Wahal and Viamajala, 2016). Remarkable increases in biomass and lipids productivity were observed in Rhizoclonium hieroglyphicum grown in dairy effluent enriched with carbon dioxide and manure (Mulbry et al. 2008b) and in Chlorococcum sp. RAP13 when grown in dairy effluent supplemented with 6 % waste glycerol (Ummalyma and Sukumaran, 2014).

To summarize, the high production costs limit the commercialization of algae-based biotechnology for the production of bio-based energy. Microalgal biomass production in agricultural, municipal, or industrial wastewater is a low-cost option that could significantly reduce economic and environmental costs while also providing a valuable remediation service.

Characterization

Microalgae have fast-growing cycles, more acceptable forms of stored carbon, and their ability to survive in sewage or saline water, making them a more appealing biofuel feedstock. The number of microalgae species ranges from 70,000 to one million. Only about 44,000 species are known, with new species and genera being discovered all the time. The continuous discovery of new species adds to the search for strains capable of high growth rates and lipid accumulation. A combination of microscopic and phylogenetic analysis is used to classify the performance of strains (Chew et al. 2018; Neofotis et al. 2016).

Potential as a fuel

Biofuel has been proposed as a potential future energy source. Liquid biofuels have experienced a rapid global expansion in recent years. The first-generation biofuels mainly depend on using sucrose producing plants (e.g. sugar cane, sugar beet and sweet sorghum) or starch-based crops such as wheat, corn and barely. This kind of biofuel is already commercially viable in the United States, Brazil, and the European Union. The impact of the first generation biofuels on the transportation industry is currently limited due to increased competition on agricultural land for food production. The second-generation biofuels are derived from agricultural lignocellulosic waste and non-edible crops, including agricultural residues, wood chips, straw and grass. Nevertheless, cereal and sugar crops have also been used as feedstocks for second generation processing technologies. Also, plant-based biodiesel production necessitates the utilization of cultivable lands for food production to grow oil producing crops such as palm soybean, oilseed rape and sunflower, which negatively affects food security. Therefore, governments limited the amount of these feedstocks for biofuel production (Shuba and Kifle 2018). At the economic scale, second generation biofuels have been reported to be not commercially viable due to extensive processing technologies, poor conversion rates and low net energy production (Milano et al. 2016, 2018).

Furthermore, vegetable and animal fats have been proposed to produce biodiesel using nonpolar solvents and catalyst, but there damaging effect on diesel engines, low oxidation stability and volatility, increased viscosity and density, fuel atomization, and a higher ratio of greenhouse gases emissions make them an unacceptable option. The third-generation biofuels are derived from microalgae and are considered to be a viable alternative energy resource. Microalgae have an ability to grow quickly and they can survive under harsh environments. Their metabolic profile can be easily engineered to produce high value added byproducts by  selecting appropriate species and adjusting cultivation conditions (Milano et al. 2016).

Microalgae biomass is regarded as one of the most efficient renewable energy sources for bio-oil production with increased energy demands in addition to their role in mitigation of greenhouse gas emissions Lee et al. (2011). Algal oil (oilage) is a biodegradable compound that has gained popularity as a primary substrate for biodiesel manufacturing, due to its low emissions and adequate physio-chemical properties that positively affect efficiency of diesel engines. Algal biodiesel is produced through transesterification of algal lipids in a two steps process which, yields more fatty acid methyl esters than the direct transesterification  process. Previous studies revealed that oilage feedstock surpasses the best seed crop oil in terms of productivity (Arvindnarayan et al. 2017). Algae-based fuels are environmentally friendly, non-toxic, and potentially reduce global carbon dioxide emissions (Pienkos et al. 2009). It has been stated that 1 kg of algal biomass can fix 1.83 kg of carbon dioxide. Interestingly some microalgal species can also fix sulfur and nitrogen oxides as nutrients source (Tu et al., 2019).

In conclusion, microalgae as a feedstock can be viewed as a potential alternative for balancing and compensating for the rising demands for biofuels. In terms of nutrient requirements and carbon dioxide sequestration capacity, wastewater combined with an inorganic carbon source (industrial flue gases) may be the most economically viable option for scale-up over freshwater resources. However, algal fuel technology is still in its early stages, and more work is required for commercialization.

Cultivation

One of the most critical stages in the development of algal biomass is the design of affordable and efficient microalgae culture. The medium is considered a necessary component in cultivation because it regulates algae growth and reproduction. As a result, the medium must contain all necessary components for growth, including minerals such as phosphorus, nitrogen, magnesium, sulfur, calcium, manganese, silicon, and iron in sufficient quantities. Successful microalgae cultivation for sustainable biofuel production requires a high rate of productivity, a low cost of production, and a low cost of maintenance (Voloshin et al. 2016; Ullah et al. 2015). Microalgae can be grown in two different ways; indoor (closed system) such as tubular, flat-panel and vertical column photobioreactors and outdoor (open system) conditions such as ponds and lakes. Every system has its benefits and drawbacks. Open farming is regarded as the most basic and oldest method of producing and cultivating microalgae on a large scale (Fig. 3).

Influence of cultivation conditions on biomass and lipid productivity

Develo** novel strategies for increasing microalgal biomass rich with lipid content would result in a low-cost, long-term biofuel production process. Several publications in the last decade investigated various strategies for inducing biomass production and lipid accumulation in microalgal biomass. This section will look at strategies that include various species types under various cultivation conditions. Cultivation conditions significantly impact microalgal growth characteristics and composition. There are four types of microalgal cultivation conditions: photoautotrophic, mixotrophic, heterotrophic and photoheterotrophic. The biomass and lipid productivity of different microalgal species under different cultivation conditions are summarized in Table 1, where the following sections discuss each type of cultivation in more detail.

Phototrophic cultivation

The most frequently used conditions for microalgae cultivation were reported in many publications (Illman et al. 2000; Mandal and Mallick, 2009; Liu et al. 2013; George et al. 2014; Mandotra et al. 2016; Narayanan et al. 2018; Sangapillai and Marimuthu, 2019; Saraf and Dutt, 2021). Microalgae use inorganic carbon sources (e.g., carbon dioxide) and light as the energy source to produce high energy organic compounds via the photosynthesis process. Overview of phototrophic cultivation conditions (Table 1) revealed a large variation in the biomass productivity of microalgae, ranging from 0.01 (Takagi and Yoshida 2006; Liang et al. 2009) to 0.7 g/L/day (Gómez-Serrano et al. 2015; An et al. 2003) and lipid productivity ranged between to 0.2 mg/L/day to (Illman et al. 2000) to 505 mg/L/day (Kong et al. 2010) depending on the type of microalgal species, cultivation system and nutrient status. Previous studies revealed significant increases in the lipid content of microalgae when nitrogen or nutrient deficiency was applied (Fernandes et al. 2013; Li et al. 2013; Hamed et al. 2020). However, achieving significant lipid productivity resulted in lower biomass production. Thus, lipid content is not the only factor influencing the energy value of microalgae. As a result, both biomass and lipid productivities should be considered simultaneously. Lipid productivity is a more accurate indicator of a microalga's ability to produce lipid because it combines the effects of lipid content and biomass production. Chlamydomonas reinhardtii recorded the highest lipid productivity by 505 mg/L/day under phototrophic cultivation. The microalga was grown in a biocoil photobioreactor using municipal wastewater (the centrate) (Kong et al. 2010) (Table 1). In this context, remarkably high lipid productivity was found in Chlorella sp. (121.3–178.8 mg/L/day) and Nannochloropsis oculata NCTU-3 (84.0‒142.0 mg/L/day) under phototrophic condition supplemented with different carbon dioxide concentration (2%, 5%, 10%, and 15%) (Chiu et al. 2008). The use of autotrophic cultivation for microalgal growth and lipid production has a significant benefit in terms of carbon dioxide sequestration as an essential carbon source. Accordingly, the microalgae's cultivation sites should be close to power plants or factories in order to provide a sustainable source of carbon dioxide for microalgal growth. Furthermore, autotrophic cultivation has a lower contamination rate than other types of cultivation. As a result, phototrophic cultivation conditions, such as open ponds and raceway ponds, are commonly used in microalgal outdoor scale-up.

Heterotrophic cultivation

Similar to bacteria, some microalgal species can grow in the absence of light by utilising organic carbon as a source of energy and carbon. This mode of nutrition is known as heterotrophic cultivation (Xu et al. 2006; Cheng et al. 2009; Ummalyma and Sukumaran, 2014). This cultivation method is suitable for large-scale photobioreactors as it avoids the hurdles of light limitation in the phototrophic conditions that could impede high cell density. Compared to other cultivation methods, heterotrophic cultivation recorded the highest biomass and lipid productivities (Xu et al. 2006; Li et al. 2007; ** countries have expressed a growing interest in identifying renewable feedstock for bioenergy production in order to meet global energy demand. Microalgae are being researched as viable sources that have traditionally contributed to producing various compounds and extracts, including carotenoids and proteins. The global market for keratin oils and proteins is expected to reach 2.0 and 35.54 billion USD in 2024, respectively (Kannah et al. 2021b).

Consumers are increasingly interested in reducing environmental pollution while extending their lives and avoiding the emergence of chronic diseases. This rising demand has resulted in a growing shift toward the production of microalgal biomass as a fossil fuel substitute in order to improve environmental conditions (García et al. 2017). Due to a lack of demand for petroleum-based fuels, the commercialization of bioenergy from microalgae has increased. It has resulted in economic growth benefits while being facilitated by a new technology directly involved in lowering production costs. However, there are some difficulties in producing microalgae and marketing them for commercial use. Microalgae biorefinery is ideal for overcoming these challenges and generating lucrative income (Camacho et al. 2019).

One of the most popular biorefinery techniques is the use of lipid extracted microalgae for bioenergy production. The extracted lipid is converted into biodiesel, and the residual is used in the anaerobic digestion process to produce biomethane. The global average market price for biodiesel and biomethane is 0.83 USD/L and 0.76 USD/L, respectively. In develo** countries such as India, the market price of biomethane and biodiesel is around 0.59 and 0.89 USD/L, respectively (Kannah et al. 2021b). Increased production quantities and bulk synthesis of lipids, carbohydrates, and proteins using microalgae as cell factories are required in the near future. With significant fixed capital expenditures and labor expenses, economies of scale play a critical role in the process's capital and operational expenditures (Camacho et al. 2019). Even though commercial production and microalgal biofuels are still in their infancy due to cost inefficiency, algal cultivation for value-added product extraction and biofuels can strengthen the process due to the high likelihood of scale-up and profitability.

Outstanding issues

Over the last five decades, extensive research has been conducted on microalgae-based biofuel production. However, due to constraints such as strain selection for higher biomass production, microalgae culture system selection, quantity and quality of bio-based product recovery from microalgae, and operational and environmental variables, commercial microalgae production has yet to be recognised and implemented in the real world. For the successful implementation of large-scale microalgae production for bioenergy, it has been suggested that a few key elements, such as biomass composition and productivity, bioconversion platform selection, and other technical and administrative costs, be considered. A few phrases in microalgae processing for bioenergy, such as growth and harvesting, continue to be a major concern for cost-effective methodologies.

Microalgal harvesting loans account for 20–30% of total biogas production costs. Microalgae cultivation pond costs 10 to 20%. The commercialization of microalgal biofuels faces several challenges, including the inability to produce cost-effective fuels due to substrate composition, conversion platform, and technology. An expansion of large-scale examination and optimization of commercial microalgae production and cultivation was occurred. The research revealed that producing commercial-scale microalgae takes a long time. One of the challenges is the scaling-up process, which includes seed culture preparation for a large-scale manufacturing facility of approximately 300,000 L. Simultaneously, there is a scarcity of handling equipment and skilled labor for large-scale production. Several governments in developed countries are assisting this industry in develo** a sustainable and environmentally friendly manufacturing system. By producing biomass, microalgae can help to mitigate global warming. Microalgae require 1.8 kg of carbon dioxide to produce 1 kg of biomass. This methodology has two advantages: treating wastewater while reducing environmental concerns and producing biomass at a low cost, resulting in the recovery of value-added products. The combination of bioenergy and value-added product recovery processes has the potential to increase biofuel market demand while decreasing production costs (Kannah et al. 2021b). For example, the cultivation and harvesting of microalgae for bioenergy remain a major concern for cost-effective methodologies.

Carbon sequestration

Carbon is identified as a vital component required to maintain ecological stability by providing a distinct cycle of capture and accumulation. Human intervention has significantly disrupted this equilibrium. The ecosystem has suffered negative consequences because of the disruption caused by increased industrialization (Osman et al. 2021b). Furthermore, unrestricted use of natural resources contributes to the implementation of this negative impact (Jaiswal et al. 2021).

The post-industrial era has resulted in increased atmospheric carbon dioxide and long-term viability, posing a threat to global ecosystems. Carbon dioxide contributes to global warming, accounting for 68% of total greenhouse gas emissions. Thus, the carbon capture storage and utilisation approach is extremely important, where carbon capture employs three major technologies: pre-combustion, post-combustion, and oxyfuel combustion routes. The first two routes accounted for 96.6% of the literature work until 2018, while oxy-reforming technology accounted for only 3.4% of total publications (Osman et al. 2021b). Carbon dioxide can be sequestered using three efficient strategies: chemical, physical and biological. Every methodology has advantages and disadvantages, whereas the earlier based on washing with alkaline solutions or carbon dioxide immobilization with the use of multi-walled carbon nanotubes, adsorption material, and amine coated activated carbon are examples of chemical approaches for carbon dioxide sequestration. Direct injection of pollutants into the ground, oceans, depleted oil/gas wells and aquifers are examples of physical techniques. The biological fixation of carbon dioxide involves photosynthetic microbes, algae, and plants (Shukla et al. 2017).

Physical methods involving direct injection are feasible for large-scale carbon dioxide sequestrations. This, however, necessitates the availability of geological and geomorphological structures, separation equipment, and carbon dioxide collection and compression technologies. This brings uncertainty and an increased risk of long-term leakage. Chemical neutralization methods are safer and provide long-term carbon dioxide fixation; however, the high cost of the reagents required for neutralization limits their use. Both physical and chemical methodologies face challenges in capturing carbon dioxide from diffused or nonpoint sources at low concentrations. Microalgae are photosynthetic organisms that use their photosynthetic machinery to sequester carbon dioxide from the environment with increased photosynthetic efficiency of 10 to 15 times that of traditional plants (Zhou et al. 2017).

Microalgae species absorb and sequester carbon dioxide, and their photosynthetic systems can harvest light photons and inorganic carbon. These microbes effectively capture and use carbon dioxide for biomass production, making them a viable source for the bioenergy and food industries (Jaiswal et al. 2021).

Microalgae can sequester 1.3 kg of carbon dioxide to produce 1 kg of biomass. Microalgae absorb light energy and convert light into adenosine diphosphate, nicotinamide adenine dinucleotide phosphate (NADP), adenosine triphosphate, and nicotinamide adenine dinucleotide phosphate reduced form (NADPH). This energy is then channeled into the dark cycle, which converts carbon dioxide into viable organic compounds via the Calvin Benson cycle. Carbon dioxide sequestration is possible in this case because this sequestration is global in scope, in contrast to other site-specific methods. This possibility has resulted in identifying potentially useful flora and fauna, such as microalgae. To achieve maximum carbon dioxide sequestration, optimal conditions such as temperature, pH, salinity, aeration, nutrition, and illumination should be maintained. To maximise production, the demand for a closed system, i.e., bioreactor, increases where the conditions can be adjusted for increased productivity. The point of focus is the selection of potential microalgae and design parameters for bioreactors in combination with carbon dioxide sequestration. Efficient design can be considered a breakthrough in sustainable sequestration by microalgae (Verma et al. 2018).

Mechanism and tolerance of carbon dioxide sequestration of microalgae

Microalgae are autotrophic photosynthetic microbes whose total metabolism exceeds higher plants of the same weight. Carbon, nitrogen, phosphorus, potassium, magnesium, calcium, and sulfur are the major nutrients for microalgae growth, with carbon being the most important. To adapt to changes in the concentration of inorganic carbon in water, various types of microalgae initiate a mechanism in which inorganic carbon is actively converted in their cells. This is referred to as carbon dioxide concentration mechanisms, which is a critical mechanism for microalgae because carbon dioxide concentration mechanisms are the only way to use carbon dioxide during their photosynthetic process. Organisms with concentration mechanisms have a high affinity for carbon dioxide, which is a major physiological characteristic that allows these organisms to efficiently utilize low carbon dioxide concentrations to fulfil their photosynthetic needs. Ribulose‒1,5‒bisphosphate carboxylase/oxygenase (RuBisCo) limiting enzyme is catalytically immobilized in vivo because RuBisCo has a low affinity for carbon dioxide and for normal reactions, requiring a high concentration of carbon dioxide (Xu et al. 2019).

The affinity and tolerance level of carbon dioxide varies between microalgae strains. Microalgae can survive in varying carbon dioxide environments. At carboxylated sites, microalgae have developed mechanisms like concentration mechanisms to survive in environments with low carbon dioxide concentrations. An increase in carbon dioxide concentration has an anesthetic effect on microalgal cells, inhibiting photosynthesis and algal growth. The initial concentration of carbon dioxide influences growth, which also influences lipid yield and composition. The synthesis of fatty acids is inhibited by low carbon dioxide concentration, whereas increased carbon dioxide concentration increases fatty acid accumulation regardless of the inhabited carbon change the saturation and elongation (Zhou et al. 2017). This cycle consists of 13 steps divided into three categories: fixation, reduction, and regeneration.

Carboxylation stage: 3-phosphoglyceric acid is produced by catalyzing carbon dioxide and ribose-1,5- diphosphate under the action of ribose-1,5- diphosphate carboxylase.

Reduction stage: Adenosine triphosphate is used for the acidification of 3-phosphoglyceric acids, which is then converted into 1,3-diphosphate glyceric acid under the action of the 3-phosphoglyceric acid kinase. This is then reduced into glyceraldehyde-3-phosphoric acid by NADPH under the action of phosphoglyceraldehyde dehydrogenase.

Regeneration stage: consisting of the regeneration of ribose 1,5-diphosphate. The glyceraldehyde-3-phosphate molecule is then acidified in ribonuclease 1,5-diphosphate under the action of Adenosine triphosphate and enzyme. Carbon dioxide fixation occurs due to the generation of ribose 1,5-diphosphate. This recycling of biochemical processes utilised carbon dioxide in the photosynthetic action (Xu et al. 2019).

Bicarbonate and gaseous carbon dioxide are used by microalgae as carbon sources. However, bicarbonate is considered a dominant carbon species in the most frequent pH range (6.5–10) in the medium of microalgae production. When an industrial flue gas stream is fed into microalgae cultures, the carbon dioxide content is generally greater than in ambient air, which results in better biomass production. The dissolved carbon dioxide in media is used as a buffer, increasing biomass productivity by enhancing carbon content. In microalgae, chloroplast produces lipids. Chloroplast fixes atmospheric carbon dioxide as an indigenous source of Acetyl-CoA and subsequently into carbon in the fatty acid chain. Chlamydomonas reinhardtii, Chlorella sp., Nannochloropsis sp., Ostreococcus tauri, Phaeodactylum tricornutum are considered the most promising microalgae for the synthesis of lipids and triacylglycerol.

Microalgae accumulate monosaccharide glucose through photosynthesis. This glucose serves as an energy source, as well as a source of proteins, lipids, and other carbohydrates. With increased irradiance or nutrient depletion, the cell's glucose production can exceed its consumption rate. Excess glucose can disrupt the cell's osmotic balance; thus, excess glucose is converted into stored products such as polysaccharides and lipids. In the future, these products will serve as an energy source and a carbon source (Choi et al. 2019).

Advantages of microalgae-based carbon sequestration

Cultivation of photosynthetic microalgae can provide a sustainable substitute for carbon sequestration compared to terrestrial plant systems, as illustrated in Fig. 7. Simple harvesting, quick production, low requirements, increased tolerance to environmental stress, increased carbon dioxide tolerance, high photosynthetic ability, and increased biomass production rates are important characteristics that make microalgae a promising solution. Several algal species grow at an exponential rate, doubling their biomass production. Solar energy is efficiently converted into biomass by the microalgae's ability to tolerate higher carbon dioxide concentrations, increasing their optimum growth through more efficient carbon dioxide fixation than higher plants. Carbon dioxide in the atmosphere and soluble carbonates can be used as a carbon source to cultivate microalgae. Microalgae can also fix the increased concentration of carbon dioxide in industrial flue gases. Because of the ability of microalgae to thrive in wastewater and the use of various trace elements, including heavy metals, microalgae are the most prominent substitute and alternative to dealing with rising environmental concerns. As a result, microalgae are recommended for bioremediation, particularly for wastewater treatment and heavy metal removal from water bodies (Banerjee et al. 2020).

Fig. 7
figure 7

Cultivation of photosynthetic microalgae can provide a sustainable substitute for carbon sequestration compared to terrestrial plant systems, where microalgae can grow at a rate ten times that of terrestrial plants. Simple harvesting, rapid production, low requirements, increased tolerance to environmental stress, increased carbon dioxide tolerance, high photosynthetic ability, and increased biomass production rates are all important features that make microalgae a promising solution. Microalgae can also fix increased carbon dioxide concentrations in industrial flue gases. Because of microalgae's ability to thrive in wastewater and use various trace elements, including heavy metals, microalgae are the most prominent substitute and alternative to dealing with rising environmental concerns. Microalgae can grow in agricultural and industrial wastewaters, especially those containing high levels of nitrogen and phosphorus. Microalgae biomass can also be converted into a carbon–neutral biofuel such as bioethanol, biodiesel, or biogas. CO2 is referred to carbon dioxide and O2 for oxygen

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Yadav et al. (2019) used organic and inorganic nutrients derived from industrial wastewater and coal-fired flue gas to cultivate microalgae in closed photobioreactors for waste bioremediation and biomass production. In the industrial wastewater, Chlorella sp. and Chlorococcum sp. were grown with varying concentrations of coal-fired flue gas ranging from 1 to 10% carbon dioxide. The results showed a 1.7 fold increase in biomass production, while the microalgae cultivated with industrial wastewater with flue gas containing 5% carbon dioxide showed maximum growth and carbon dioxide fixation (Yadav et al. 2019).

Tu et al. (2019) studied the impacts of power plant tail gas to reduce carbon dioxide by using the tail gas as a carbon source and cultivated a freshwater microalga Chlorella pyrenoidosa. An increase in dry weight and lipid production by 84.9 and 74.4% was observed in the presence of power plant tail gas. Optimum carbon fixation sequestration of microalgae was 1.12 g/L having an average carbon fixation rate of 0.21 g/(Ld), which was 134.2% and 107.1% higher compared to the growth of microalgae in the open air. C. pyrenoidosa is tolerant to sulfur dioxide and nitric oxide, which is in accordance with the study mentioned above. The tolerance is 0.04%; however, pretreatment processes like desulfurization and denitrification are required (Tu et al. 2019) (Table 5).

Table 5 Characteristics of photobioreactor systems for microalgal cultivations for carbon bioconversion (Paul et al. 2021; Klinthong et al. 2015; Ibrahim et al. 2020; Severo et al. 2019; Ruiz-Ruiz et al. 2020)
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Aghaalipour et al. (2020), in their research study, analyzed the assessment of carbon dioxide by fixation of two microalgal species, Scenedesmus obliquus and Chlorella vulgaris. Along with that, two new species, Monoraphidium contortum, and Psammothidium sp., were also studied for their capability of carbon dioxide inputs in two types of photobioreactors, including glass bottles and vertical columns. This study aimed to assess the carbon dioxide bioremediation rate, growth kinetics, and protein content of microalgal species of different types of photobioreactors with varying amounts of carbon dioxide ranging from 0.04% to 10%. According to the results, Chlorella vulgaris (3.35 g/L/day) was most significant as Chlorella vulgaris showed maximum carbon dioxide sequestration at 10% carbon dioxide in the vertical column photobioreactors, followed by Psammothidium sp. (3.24 g/L/day), Scenedesmus obliquus (2.40 g/L/day), and Monoraphidium contortum (1.40 g/L/day). Psammothidium sp. showed maximum carbon dioxide recovery (CR%), which was 41.70%. Chlorella vulgaris has also depicted maximum protein content during Chlorella vulgaris cultivation in a glass flask photobioreactor with 10% carbon dioxide (Aghaalipour et al. 2020).

Toxic pollutants in combustion flue gas

Sulphur dioxide

sulfur dioxide is considered a limiting factor for microalgal growth. An increase in 100 ppm sulfur dioxide concentration completely restricts microalgal growth. A few microalgal species have shown growth in high concentrations of sulfur dioxide; however, these species have depicted a longer lag phase. Increasing sulfur dioxide levels decreases carbon fixation and biomass production, ultimately inhibiting growth (Klinthong et al. 2015).

Sulphur dioxide is considered an important pollutant in flue gas and increases the acidification of microalgal culture, giving oxidative damage to the cells. Wang et al. (2020) utilize exogenous spermidine to reduce the negative impacts on Chlorella sp. and reduce the impacts of sulfur dioxide and 15% carbon dioxide impurity. Spermidine efficiently in habitat HSO3−/SO32- by penetrating through cell walls and protecting photosynthetic PSII structures and thylakoid membranes. It resulted in the recovery of chlorophyll from 0.5 mg/L to 8.5 mg/L while increasing the biomass yield recovery from 0.12 g/L to 1.52 g/L. The recovered lipid content of the biomass was improved from 5.28% to 19.12% (Wang et al. 2020).

Duarte et al. (2016) stated that sulfur dioxide and nitrogen dioxide injection until 400 part per million (ppm) do not affect the carbon dioxide bio-fixation in microalgae, although the optimum yield and results were obtained at low concentrations of industrial waste. This study examined that Chlorella fusca LEB 111 can grow under all conditions while showing resistance to sulfur dioxide and nitric oxide concentrations up to 400 ppm. Chlorella fusca LEB 111 can grow in a culture medium with 40 ppm ash and be unaffected by sulfur dioxide and nitric oxide concentrations of up to 400 ppm. The optimum carbon dioxide fixation efficiency was observed with 10% carbon dioxide, 200 ppm sulfur dioxide along with nitric oxide and 40 ppm ash (Duarte et al. 2016).

According to Song et al. (2021), the cultivation of microalgae with real flue gas has the probability of containing an impurity, reducing cost in the treatment of flue gas, and increasing the benefits of microalgal carbon sequestration. On the bubble dissolution characteristics of 15% carbon dioxide, the impact of sulfur dioxide impurity was observed in different sulfur dioxide concentrations, solution pH, different culture media, initial bubble sizes, and biomass concentrations in the microalgal genus Arthrospira. Arthrospira's photosynthetic growth and biomass yield were increased by 24% to 5.04 g/L with 200 mg/m3 sulfur dioxide in simulated flue gas containing 15% carbon dioxide, compared to without sulfur dioxide impurity. Arthrospira solution has an alkaline medium that nullifies sulfur dioxide's toxic effects (Song et al. 2021).

Nitrogen oxide emissions

The nitrogen oxide emission level in flue gas ranges from several hundred to several thousand parts per million with more than 90 to 95% nitric oxide and 5 to 10% nitrogen dioxide. After releasing flue gas, removing such emissions is still at the 50 to 200 ppm-level. Nitric oxide cannot directly impact microalgal growth through pH in the cultivation media. The concentration of nitric oxide poses a two-sided influence on microalgal growth. Low nitric oxide concentration can be absorbed by the cultivation media and converted into nitrogen dioxide, which acts as a source of nutrition for microalgae. Increased nitric oxide concentrations are observed to decrease microalgae growth for various species. More than 300 ppm of nitric oxide decreases microalgal growth. A selective catalytic reduction process and flue gas desulfurization can separately be used to treat oxides of sulfur and oxides of nitrogen. Sometimes, the combined treatment systems can be used simultaneously before entering the gas stream into a microalgae reactor ( Klinthong et al. 2015).

To summarize, microalgae can be used to efficiently capture carbon dioxide and convert microalgae to bio-fuel, simultaneously solving two major issues of the world. The point of focus is the selection of potential microalgae and design parameters for bioreactors in combination with carbon dioxide sequestration. Efficient design can be considered a breakthrough in sustainable sequestration by microalgae. To use inorganic sources for carbon dioxide, e.g., flue gas, a gas treatment system should be used to reduce or eliminate inhabitation factors. Furthermore, microalgae can sequester 1.3 kg of carbon dioxide for every kg of biomass produced. Using potent microalgal strains in efficient bioreactor designs for carbon dioxide sequestration is thus a challenge. In open and closed cultures, microalgae can theoretically use up to 9% of light energy to capture and convert 513 tons of carbon dioxide into 280 tons of dry biomass per hectare per year. Algal biomass cultivation should be coupled with thermochemical technologies, such as pyrolysis, to design an efficient atmospheric carbon removal system.

Conclusion

The value of biofuels extends beyond their use as a transportation fuel; the economic and environmental benefits of biofuel co-products should be considered. Material item development can play an important role in preventing future environmental damage. Various generations of biofuels reduce greenhouse gas emissions while increasing reliance on crude oil, encouraging energy diversification and the creation of a large number of rural jobs (Ahorsu et al. 2018). To accelerate implementation, the primary goal of integrated algae waste operations should be to maximise productivity and product accumulation while minimising energy, water, nutrients consumption, and land footprint, particularly for large-scale production and future research and development. Biorefinery technology capable of producing a wide range of high-value products will be required to fully implement algal biomass and enable commercially viable bioenergy co-production (Dayton and Foust 2019).

Microalgae are regarded as a viable biodiesel production option. The combination of microalgae and wastewater purification can reduce carbon dioxide emissions while also lowering biodiesel production costs, providing a way for practical application. Temperature, salinity, pH, light intensity, photobioreactor configuration, nutrient ratio, and carbon dioxide flow rate influence microalgae productivity and efficiency. One of the major constraints is the successful extraction of oil from microalgae biomass. Transesterification is a common procedure used in the production of biodiesel. (Li et al. 2021). While the use of microalgae in carbon sequestration as an effective carbon removal strategy should be considered in the near future.

Several methods are used to extract energy from algae, each with its own set of advantages and disadvantages. A few of these strategies are still in the early stages of development, and algae-based biofuel generation is deemed economically but commercially unfeasible. Given the early stage of research and the high cost, it is reasonable to conclude that there is still a long way to go in terms of influencing the process of algae biofuel production (Pragya et al. 2013). After liquid extraction, the residue algal biomass has improved economic viability for value-added products generation and biorefinery technology (Subhash et al. 2014). It is possible to use a single-step integration technology that combines cell destruction and liquid extraction procedures (Vasistha et al. 2021). While the use of algae in atmospheric carbon removal should have lower constraints than biofuel production (biodiesel), such as high oil content or oil extraction, what is required in terms of carbon sequestration is a species with a high growth rate and low oil content, as algae, in this case, will be converted into solid biochar.