1 Introduction

Emerging pollutants (EPs), also known as contaminants of emerging concerns (CECs), are commonly defined as pollutants that have recently appeared or have existed for some time, with concerns arising not long ago [134]. However, ambiguity often arises when categorizing a compound as EPs. For instance, a pollutant with concrete evidence of a known adverse effect and requiring a regular monitoring system by the authorities may still be considered as EPs. Depending on each region or country, their EPs could be different depending on their economics activities, pollutant treatment technologies, policies and regulations, education levels and lifestyle of their citizens [1]. For example, uranium-238 and thorium-232 has been considered as EPs in Malaysia due to the operation of Lynas Advanced Materials Plant (rare earth refinery) in Kuantan start from year 2012, and it is suspected to cause pollution in Balok River [2]. In the case of Indonesia, the discharge limits of thorium-232 was not set by the Indonesia government and no concrete evidence was found on its related pollution, and thus not considered as EPs in Indonesia [3]. Besides that, the EP status for Dichlorodiphenyltrichloroethane (DDT), which is a widely used organochlorine pesticide in the mid-1940s, could be different between countries as well. The US Environmental Protection Agency (EPA) totally banned usage of DDT in year 1972, and after decades of time, it can no longer be considered as EPs in the US although some of the DTTs and its metabolites still presence in the US environment [4]. However, in the European Union (EU) listed priority ‘emerging contaminants’, DDT is still considered as EPs, and it is still widely utilised and produced in India [5]. Thus, with the incorporation of the above statements, EPs could be newly found or existed substances, that appear in the natural environment naturally or anthropogenically, that originally did not contain the said substances, which concerns arise recently with or without concrete evidences to prove their impacts, and lack of proper regulations or enforcement in monitoring or controlling their presence in the environment of the specific region or country [6, 7].

Industry and home activities are frequently the most significant contributors to water pollution, and similarly to other pollutants, EPs make their way to the environment through point source and non-point source discharge. Some examples of ECs point source pollution could be personnel care products (PCPs) and pharmaceutical compounds leak or discharge from production facilities; biocides and estrogenic chemicals such as steroid hormones from livestock farms; mixed matrices of ECs from leachate of landfills; and heavy metals or radioactive substances runoff from active mines. For non-point source, the examples include pesticide, herbicides and fertilizers runoff from agriculture farmland; heavy metals and radioactive substances flow from abandoned mines; as well as ECs suspended in air flow into natural water bodies due to precipitations [8, 9]. Figure 1 demonstrated the EPs pathway to the environment. Polycyclic aromatic hydrocarbons, various pesticides and fungicide, flame retardants, per- and polyfluoroalkyl substances (PFAS), endocrine disrupting chemicals (EDCs), and pharmaceutical and PCPs are some examples of commonly spotted and observed emerging pollutants in water environment [10]. The continuous production and utilization of novel items, including industrial chemicals, agrochemicals, electronics, fragrances, pharmaceutical products, and others, have led to the ongoing discharge of numerous newly introduced chemical pollutants into aquatic ecosystems.

Fig. 1
figure 1

Movement of ECs from their source to the nature waters [Icon from Flaticon Basic License CC3.0 (Creative Commons)]. *WWTP = Wastewater Treatment Plant

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The necessity to access the risk and monitor their discharge into the environment lead to the establishment of the NORMAN project, which target to create a permanent network between references bodies, researches centres, industries, non-government organisations and other parties, in order to promote more rapid, broad and transparent information and data exchange, standardisation of measuring and monitoring methods for EPs, and the establishment of independent and competent forum for scientific debate on EPs related matters [11]. The original NORMAN list for EPs only consists only around 900 compounds, while in year 2018, the NORMAN Suspect List Exchange (SUSDAT) database have included more than 40,000 compounds. Currently, the SUSDAT list consist of almost 106,000 compounds with different validation level, and the increment of EPs proved the increasing in types of compounds synthesised or the reemphasize on compounds that previously ignored or being paid less attention on [12]. Dembrexine is used as seccretolytic drug to treat respiratory disease for horses [13]; Salinazid which used as antituberculous, antiasthmatic and anti-inflammatory drugs [14]; Atropine methyl often used to restrict production of saliva and bronchial fluids, and to treat organophosphorus agent poisoning [15]. Among the compounds above, atropine methyl and dembrexine can be considered as existing compounds that added into the suspect list recently as they have been registered in the PubChem library for a long time. Some of the EPs, including but not limited to pharmaceutically active compounds, personal care products, plasticizers, and brominated flame retard are restricted as “legacy persistent organic pollutants (POPs) “ by The Stockholm Convention of 2001 [16,17,18,19,20]. Due to their inherent physical and chemical characteristics, POPs exhibit persistence in the environment, widespread distribution, bioaccumulation in the food chain (including humans), and pose risks to both wildlife and human health [6, 10]. From this point onwards, the utilisation of extremophiles for the degradation of these compounds could come in handy. Table 1 below summarised some of the EPs and their adverse effect towards human and the environment.

Table 1 Type of EPs, their usage and potential impacts
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Extremophiles are categorized into various groups, including thermophiles, acidophiles, psychrophiles, halophiles, alkaliphiles, xerophiles, barophiles, and endoliths. Recently, researchers have shown considerable interest in extremophiles due to their ability to produce a multitude of enzymes called extremozymes, which can function under extreme conditions, affecting their substrates [21, 22]. Extremozymes showed some of the properties such as highly charged exterior surfaces, rigid folds sustained by several ion-pair networks and tightly packed hydrophobic core. Compare to normal enzymes, they are more dense and compact protein structures which partly caused by the high amount of acidic and basic amino acids [23]. In the biodegradation of organic pollutants, it is necessary to produce enzymes or microorganisms that can withstand extreme salinity, acidity, alkalinity, temperature, high pressure, and the toxicity of chemical pollutants. Extremophiles and extremozymes play a crucial role in lignin bioprocessing. These specialized microorganisms and enzymes have the unique ability to thrive and function under extreme conditions, making them ideal candidates for lignin degradation and utilization. Their involvement in lignin bioprocessing holds great promise for the development of sustainable and efficient methods to break down lignin and extract valuable products. By harnessing the power of extremophiles and extremozymes, researchers aim to enhance lignin valorization, biofuel production, and other applications in lignin-based industries [24, 25]. The production of active extremozymes enables extremophiles to carry out essential biochemical processes necessary for their survival in their extreme habitats. For example, thermophilic extremophiles living in high-temperature environments produce heat-stable enzymes that can continue to function even at temperatures that would render most enzymes inactive. Similarly, acidophilic extremophiles in highly acidic conditions produce acid-stable enzymes, while halophilic extremophiles in highly saline environments produce salt-tolerant enzymes [26].

Cultivating extremophiles necessitates specific nutrient, salt concentration, and temperature parameters. Microorganisms with moderate thermophilic, extremely thermophilic, and hyperthermophilic traits can thrive and propagate at temperatures above 45 ℃, 70–80 ℃, and beyond 80 ℃, respectively [27]. Some hyperthermophilic archaea able to thrive and flourish at temperatures exceeding 100 ℃. Extremophiles have distinct adaptations that enable their survival in environments with extreme heat, such as hydrothermal vents and hot springs. One of the most significant contributions of hyperthermophilic archaea lies in their production of thermostable enzymes. These enzymes can maintain their functionality and structural integrity even at extremely high temperatures, making them invaluable for various industrial applications [28]. Halophiles are organisms that thrive at high salinity conditions. The halotolerant and halophilic bacteria growing salinity condition can be 0.5–2.5 M and 2.5–5.2 M NaCl, respectively [29]. Alkaliphiles require a pH range of 8.5 to 11, with an optimum pH of 10. Haloalkaliphiles can survive and growth under alkali and high saline condition. A growing medium with a pH of 5 or less is suitable for isolating acidophilic and extreme acidophilic microorganisms [30, 31]. However, depending on the environment condition of the sample source, the setting for growth medium must be changed accordingly for the culture of extremophiles. For optimal growth, polyextremophilic bacteria require specific combinations of these extreme conditions, which vary depending on the species [32]. Scientists were intrigued by the possibility in commercial applications of extremophiles to accelerate processes under extreme environments. Extremozymes that exhibit excellent activity throughout a broad pH, temperature, and gasification range are utilized extensively in various industries, including dairy and beverages, bioenergy, and sustainable manufacturing of valuable chemicals [33, 34].

2 Extremophile and Industry

One of the most significant industries that use extremophiles is the pharmaceutical industry. Many of the compounds produced by extremophiles have potential medical benefits. For example, Thermus aquaticus, a thermophilic bacterium, produces Taq polymerase, which is used in the polymerase chain reaction (PCR), a widely used laboratory technique in molecular biology. The discovery of Taq polymerase has revolutionized the field of genetics and helped to diagnose and treat genetic disorders [35]. Similarly, the discovery of archaeosomes, which are new generation of liposomes made by natural ether lipids produced from archaea, has potential medical applications due to their stability in various condition and unique properties, such as their ability to transport drugs through the blood-brain barrier [36]. Extremophiles also have applications in the biotechnology industry. Extremophilic bacteria are known to produce enzymes known as extremozymes that exhibit remarkable stability in extreme environments and possess unique properties. Notably, some of these enzymes are capable of breaking down cellulose, which happens to be the most abundant organic polymer found on Earth. Extremozymes have found wide applications in various industries, including the production of detergents, textiles, paper, and biofuel [37]. The use of extremophile-derived enzymes in these industries not only enhances the efficiency of various processes but also contributes to more sustainable and eco-friendly practices, as they often require lower energy consumption and generate fewer harmful byproducts. The environmental industry also benefits from extremophiles. Some extremophiles have unique abilities to clean up polluted environments. For example, certain bacteria can degrade oil spills and heavy metals [38]. The discovery of these bacteria has led to the development of bioremediation techniques, which use microorganisms to break down pollutants in soil and water. Table 2 summarized the extremophiles applications in various industries. Despite the benefits of extremophiles, there are also some potential risks associated with their use in industry. One risk is the spread of these organisms to non-native environments. If an extremophile is introduced into a new environment, it could outcompete native organisms, leading to ecological imbalances. Additionally, some extremophiles are pathogenic and could pose a risk to human health if not handled properly [37, 39].

Table 2 Industrial utilization of enzymes derived from extremophiles
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One potential application of thermophile amylase in environmental remediation is in the degradation of starch-based pollutants. Starch is a common pollutant found in many industrial wastewaters, and its degradation can be difficult due to its complex structure. However, thermophile amylase has been shown to be effective in breaking down starch into simpler, more easily degraded molecules [40]. This can lead to more efficient and cost-effective removal of starch-based pollutants from wastewater. Another potential application of thermophile amylase in environmental remediation is the bioremediation of contaminated soil. Bioremediation is a bio-based remediation process that harnesses the power of microorganisms to degrade and eliminate contaminants from soil. However, the effectiveness of bioremediation can be limited by the temperature of the soil, as many microorganisms are unable to function at high temperatures [41]. Thermophile amylase, on the other hand, can function at high temperatures and can be used to improve the effectiveness of bioremediation in high-temperature environments. By breaking down starch-based pollutants in contaminated soil, thermophile amylase can help to facilitate the removal of other contaminants as well [35, 39]. Psychrophiles are organisms that thrive in cold environments, including polar regions, high-altitude locations, and deep-sea waters. One example of a psychrophile protease that has been used for environmental remediation is a protease isolated from a psychrophilic bacterium, Pseudoalteromonas sp. strain A28. This protease has been shown to effectively degrade several emerging pollutants, including naproxen, ibuprofen, and diclofenac, at low temperatures. Previous study proved that the Pseudoalteromonas sp. strain A28 protease was able to degrade over 80% of the naproxen in an aquatic environment within 24 hours at 10°C [42, 43].

3 Adaptation Mechanisms of Extremophiles and Their Enzymatic Processes Towards Pollutants

3.1 Halophiles

Halophilic microbes can survive in a high-saline environment, which impedes the survival of organisms due to osmolar imbalance and metabolic issues [31, 42]. Prior research on halophilic microbes identified two main adaption methods for survival in situations of extremely high salt concentration. The first is to employ a “salt-in” strategy, which refers to the accumulation of inorganic osmoprotectants such as KCl within the cell to preserve osmotic equilibrium both within and outside the cell [44]. By utilizing the adenosine triphosphate (ATP)-dependent potassium ion (K+) transport system (the KdpFABC complex), cationic amino acid transporter-3 (Cat3) and Na+ efflux antiporters (NhaC) for balancing the osmotic gradient between cell and environment under high-salt conditions, Halobacterium salinarum has been shown to store up to 3.97 M and 4.57 M of K+ and Cl ions respectively [45, 46]. Furthermore, halophilic microorganisms have developed various negatively charged aspartate and glutamate residues on the surfaces of their proteins. This feature allows them to retain water molecules around the cells, preventing protein precipitation and dehydration. By doing so, halophilic microbes effectively avoid drying out in their high-salt environments [46, 47]. The ‘compatible solutes adaption’ technique was adopted by certain halophilic and halotolerant bacteria. They make use of compatible organic solutes, such as polyols, glucosylglycerol, sucrose, trehalose, ectoine, and betaine, to maintain osmotic equilibrium in their high-salt environments. This allows osmotic equilibrium to be preserved by the bacteria in extreme conditions, and it is facilitated by the utilization of compatible organic solutes. [48, 49]. When exposed to a high salt concentration, the halophilic bacteria Spiribacter salinus M19-40 secretes increased quantity of suitable solutes which play a vital function in lowering the thermodynamic activity of water in order to compensate for the external osmotic pressure [44, 48]. Table 3 listed the halophiles and their enzymes in the degradation of various pollutants.

Table 3 Halophiles in the degradation of various pollutant. (discuss the tables, probably on how their enzyme could be use for degradation of certain EP, their metabolic mechanisms and enzymatic system)
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3.2 Acidophiles

Acidophilic microbes can survive in environments with an extremely low pH (less than pH 3) via managing proton permeation. Microorganisms from the genera of Thermoplasma, Ferroplasma, and Sulfolobus, for instance, have highly impermeable cell membrane primarily made up of tetraether lipids with a diverse formation of polar head groups and a bulky isoprenoid core, allow them to control proton permeation under extremely low pH environment [46]. Survival at low pH requires the control of proton influx through the proton pump system, putative proton pump proteins such as plasma membrane H + -ATPase, symporters, and antiporters such as NahP-/NhaA-type Na + /H + exchangers from Ferroplasma type II and Leptospirillum group II are implicated in pH homeostasis. Furthermore, it is known that F0F1-type ATP synthase is crucial for the regulation of proton permeation, while they can be found in acidophiles such as Bacillus acidocaldarus, Thermoplasma acidophilum, and Leptospirillum ferriphilum. Additionally, a few auxiliary mechanisms, such as chaperone which participates in protein refolding; and cytoplasmic buffering capacity, preserve intracellular molecules such as DNA, RNA, and proteins under extremely acidic situations [46, 50, 51]. With those surviving abilities, acidophiles has been utilized and studied for the treatment of various pollutants (Table 4).

Table 4 Acidophiles enzyme in degradation various pollutants
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3.3 Alkaliphiles

Alkaliphilic microbes are tolerant to high pH, as opposed to acidophilic microorganisms. Up to present, three important biological pathways have been found in these microbes as survival strategies. Under extreme alkaline conditions, certain alkaliphilic Bacillus species can boost proton motive force generation by synthesizing an acidic cell membrane, composed mostly of peptidoglycan, teichuronic acid (TUA), and teichuronopeptide (TUP), which benefits both energy production and pH balance. In Bacillus lentus C-125, the contents of TUA and TUP in cell wall greatly increase as the surrounding pH increase [52]. Secondly, sodium motive force can increase pH equilibrium in settings of excessively high pH. The intracellular pH is altered through the Na + /H + antiporter, Na + channels or stator force generator that triggers Na + -dependent motility, reduce the dependency on H + -potential base transport systems. It is useful when the alkaliphile is exposed to neutral conditions when the concentration of H + is very low [53]. Lastly, the generation of organic acids such as succinate, lactate and acetate by alkaliphiles for pH calibration is a crucial biological activity for pH maintenance [25]. Table 5 summarized the alkaliphiles utilized in the degradation of several pollutants.

Table 5 Alkaliphiles in degradation of various pollutants
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3.4 Thermophiles

Similar survival mechanisms are activated by thermophilies with a favoured temperature above 60 °C. In order to tolerate high temperatures, B. acidocalidus, stabilized the membrane lipid fluidity by increase the production of hopanoids (a subclass of triterpenoids). Thermophiles might also alter the iso or anteiso fatty acid composition in cell depending on the surrounding temperature. It is reported that the iso fatty acids in Bacillus spp. increase according to the increment of growth temperature while anteisofatty acid content increase with the decrement of temperature [54]. By the regulation of membrane lipid content, the thermophilic archaeon Metahnocaldococcus jannaschii can withstand extreme temperature. Diether lipids (archaeol-based) reduced from 80 to 20% when these microorganisms were subjected to high temperatures, but caldarchaeol- and cyclic archaeol-based lipids will increase by 30% to a total of 40% content in the cell [55]. Moreover, thermophilic microbes have evolved several biomolecules to induce heat stability, such as by increasing the guanine or cytosine concentration of DNA, or generating a positive supercoiled DNA shape. In addition to possessing an abundance of ribosomal proteins, these organisms have a proficient heat-shock response that permits the generation of normal protein even at elevated temperatures [46]. Thus, thermophiles have been utilized in several processes that run on elevated temperatures (Table 6).

Table 6 Thermophiles enzymes and their applications
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3.5 Psychrophiles

A superabundance of genetic redundancy is typically observed in psychrophilic microorganisms. This means that they encode multiple copies of tRNA species responsible for the biosynthesis of amino acids and have a high quantity and variety of chaperones. This genetic redundancy suggests that a high capacity for translation and post-translational processing is crucial for these organisms to thrive in low-temperature environments. Additionally, psychrophilic enzymes tend to exhibit higher structural flexibility, lower thermostability, and greater specific activity at low temperatures compared to their mesophilic counterparts [56]. Cold-adapted enzymes have higher catalytic activity and more productive than mesophilic and thermophilic enzymes under low temperature, thus provides an energy saving and economical pathways for low temperature processes. Some potential applications including biodegradation, biodiesel production, and food and beverage production through psychrophilic xylanase, lipase, protease and peroxidase [57]. Psychrophilic enzymes including proteases, lipases and chitinases have been applied in food processing activity including descaling of fish, skin and grease removal, and oil extraction, while some proteases are involved in meat tenderization and flavor enhancement [58]. Study found that psychrophilic protease synthesized from Pseudoalteromonas sp. SM9913 have a higher catalytic efficiency and released more taste amino acids and essential amino acids on the exterior of sea fishes, shrimps, and pork than their mesophilic counterparts during refrigerated storage [59].

3.6 Metalophiles

As minute amounts can be hazardous to public health and the environment, concerns over the toxicity of heavy metals have increased dramatically. In addition, chemical remediation of dangerous heavy metals under harsh environments are sometimes hindered by their limited accessibility. Thus, throughout the past several decades, the advancement of sustainable bioremediation technologies that employ extremophilic bacteria for the treatment of heavy metals has been researched. Acidophilic bacteria that can flourish under low pH circumstances have been employed in biomining processes including bioleaching and bio-oxidation as well as detoxification of heavy metals in the event of severely acidic environments [60, 61]. Metalophilic or some acidophilic microorganisms had evolved sharpened metal detoxification pathways and mechanisms to activate and/or inhibit anti-oxidativestress, metal-binding and transport, and membrane-permeability responses [38]. The most prevalent acidophilic and chemolithotrophic microorganisms are Acidothiobacillus strains, which have been used in the development of bioremediation techniques. For instance, Acidothiobacillus ferrooxidans has been utilized for bioleaching on an industrial scale for extracting metals out of sulfide ores. There are also growing interest to utilise the species for develo** of in situ resource utilization (ISRU) techniques such as mining from other celestial bodies under microgravity condition [60, 62, 63]. Previous studies reported the ex-situ bioremediation of Uranium (IV) from polluted mine water utilizing Acidithiobacillus ferrooxidans through bioadsoprtion, as well as applying both Acidithiobacillus ferrooxidans and Acidothiobacillus ferrivorans strains for sulfidogenesis process for the selective precipitation of copper from a zinc-copper solution under pH ranged from 2.6 to 2.6. Besides that, Acidocella aromatica PFBC were found able to efficiently reduce vanadium ions [vanadate; V(V)] to V(IV) under pH 2 or pH 4.5, aerobically or anaerobic condition [64, 65]. Table 7 concludes a few species that have been used in detoxification and removal of heavy metals.

Table 7 Extremophiles utilized for the detoxification and removal of heavy metals
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4 Potential of Utilizing Extremophiles for Biodegradation of EPs in Water

From previous part, it is mentioned that utilization of extremophiles has been widely commercialized in various industries including food and beverages, pulp and paper and biodiesel. However, very little is studied and utilized in water and wastewater treatment industries, especially on the degradation of EPs. Current conventional water treatment plants majorly relied on physical and chemical treatment methods in the removal of various pollutants, while wastewater treatment plants utilized both the methods together with biological processes [66]. However, conventional treatment methods are not specifically design for the removal of EPs, especially with most of them appears on trace level [67]. Their efficiency varies depending on the treatment methods and types of EPs, could be as low as around 20% to more than 98% removal [68]. Besides, conventional treatment methods bring their own limitations such as energy-intensive and the production of harmful by-product. Among the processes, biological therapy for pollutants removal appears to be more favourable and widely accepted as they are regard as environmentally friendly and sustainable [69]. Although processes such as settling, aerating volatilization, chemical precipitation and sludge absorption contribute to the removal of EPs, but biodegradation process plays the major role [67]. Harmful organic contaminants, such as petroleum hydrocarbons, aromatic petrochemicals, and various halogenated compounds, can be converted into non-toxic or less toxic molecules by a variety of microbes. This extensive transformation requires not only strong resistance to exposure to toxic organic pollutants but also the ability to utilize these harmful contaminants for cellular metabolism [46].

Industrial wastewater discharge is one of the main sources for EPs contamination. Industrial production plants including pharmaceutical, food and beverages, personal care products and pesticides production factories generally produce wastewater contained with EPs due to production waste, leakage, and by-products formation, contaminates the surrounding environment due to incomplete removal [70]. The wastewater produced from these industries often has broad range of compositions and characteristics, with some comes with extreme conditions such as low pH, high temperature and high in toxicity. Pharmaceutical wastewater normally characterized with high in organic matter, high salt concentration, microbial toxicity, high fluctuations in quantity and complicated compositions due to verities of raw material used during batch production process. The temperature of wastewater could range from 25 to 80℃, and pH from 1 to 8 [71]. Pesticides production industry wastewater contains high amount of toxic components, with high chemical and biochemical oxygen demand, high total dissolved solids, and generally low in pH [23]. The pH could range from 1.5 to 14, and chemical oxygen demand from 17,000 to 35,800 mg/L [72]. For personal care products (PCPs) industries, a case study on Brazilian cosmetic industries proved that the pH could range from 1 to 12, with maximum salinity of 2.4 parts per thousands (ppt) [10]. Moreover, the condition of incoming raw water or wastewater for treatment always fluctuates depending on different causes such as weather or season, incoming water flowrate, saline intrusion, time of a day, adjustment in production processes and much more [16, 44]. Conventional treatment methods often incorporate multiple processes to adjust the influent conditions such as pH and temperature as commonly used chemicals (coagulant, flocculant) and microbes in water treatment industry usually require a “sweet spot”, which commonly close to normal condition for optimal performance. These indicates extra capital and operation cost are needed to change the condition of influent for maximum treatment efficiency.

Hence, extremophilic microorganisms that have adapted over a long period of time to harsh habitats such as extreme temperatures, pH and salinity have the potential to be exploited extensively for the treatment of hazardous EPs under the corresponding conditions, without additional processes to change the characteristics of influent [42]. Only very limited studies have been done on the EPs degradation by extremophiles, while many focus on petroleum products, hydrocarbon compounds, dyes, and heavy metals [53, 72]. In terms of hydrocarbon compounds, X. Lin et. al. found that close to 90% removal in 28 days was achieved for degradation of mixed crude oil and diesel by Pseudoalteromonas sp. P29 at temperature of 5 ℃ [73]. Another study isolated thermophilic Bacillus licheniformis WY2 which able to degrade hydrocarbon by 52% at temperature range from 52 to 80 ℃ [74]. Srivastava et. al. discovered that Bacillus albus DD1 able to degrade azo dye Reactive Black 5 up to 98% in 38 hours under condition of 40 ℃ and pH 7, the bacteria species also showed degradation efficiency of 73% at pH 8 and pH 9 [75]. These could prove that extremophiles could undergoes metabolism activities and their enzymes able to work efficiently even under extreme conditions. More researches could be done to discover their potentials in the bioremediation of EPs including but not limited to pesticide and fungicide, PCPs and pharmaceutical drugs due to their increment in usage as well as the pollution causes by them.

5 Extremozymes Activities Against EPs

From Sect. 3, it is found that enzymes synthesis by the extremophiles plays an important role in degrading various EPs. In bioremediation, enzymes such as hydrolases, lipases, oxygenases, laccases and oxidoreductase able to degrade or detoxify most of the hazardous pollutants [76]. Oxygenases catalyse the oxidation of reduced substrates through transmitting oxygen atom from oxygen molecule (O2), by using coenzyme such as Flavin Adenine Dinucleotide (FAD), Nicotinamide adenine dinucleotide hydrogen (NADH), or nicotinamide adenine dinucleotide phosphate (NADPH) [77]. Oxygenases actively participate in the metabolism of organic compound, through increasing their reactivity or water solubility or breaking of the aromatic ring, and thus able to degrade wide range of toxic pollutants such as aromatic hydrocarbons, halogenated compounds and chlorinated aliphatics [78]. Oxygenases can be categorised into 2 main groups, which are monooxygenase, and dioxygenase. Monooxygenases catalyse the infusion of one oxygen atom into substrate, while another form into water; while dioxygenase catalyse both O2 atoms into the substrate [79]. Multiple tables in Sect. 3 stated different types of extremophiles synthesised various oxygenases enzyme in the degradation of pollutants, including phenanthrene, phenol, aniline, trichloroethylene, 4-HBA and others. Monooxygenases often participates in processes including dehalogenation, dealkylation, denitrification and hydroxylation processes [79, 80]. Dehalogenation process reduce the toxicity and degradation-resistance of a compound by removal of halogenated groups; Dealkylation process involved the removal of alkyl groups in a substrate, breaking it into 2 smaller substances; Denitrification process involve the changing of nitro group attached to aromatic ring into hydroxyl group; while hydroxylation by oxygenase often involve bond attachment of hydroxyl group onto aromatic rings, usually for the purpose of aromatic ring cleavage [81]. Dioxygenase catalyse several reactions including breaking of aromatic rings, dihydroxylation, and S-oxygenation of thiols to sulfunic acid [79, 80]. (choose a EP and propose a metabolic path way)

6 Factors Affecting Stability of Extremozymes

Enzymes are highly efficient catalyst that account for catalysing various biological reactions in every living cell [82]. Enzymes able to carry out molecular recognition and selective catalysis which are the key chemical processes in life, while also capable of speeding up bioreactions up to 17 orders of magnitude [60, 82]. With those characteristics, enzymes able to synthesis various complex biomolecules with distinct structural feature [60]. However, enzymes efficiency is highly dependable on the surrounding conditions such as temperature, pH and toxicity. Extremophiles have to ability to produce extremozymes that could withstand and works efficiently even under extreme and high toxicity conditions, which play a major role in metabolism activities and survival of extremophiles. For instance, thermophiles such as T. thermophilus, Geobacillus, and Thermosediminibacter species able to produce extreme pH and temperature stable laccases and azoreductases, which are competent for the removal of various toxic dyes, while some capable to produce detoxifying thermozymes for the degradation of toxic organic pollutants [83].

Although having a better adaptation towards extreme conditions when comparing to normal enzymes, the stability of extremozyme could still be affected by the surrounding conditions. Thermodynamic stability of enzyme is the capability of protein to unfold or refold after being subjected to stresses including increase in temperature, extreme pH or high toxicity condition. Enzyme denaturation happens when the tertiary structure unfolds to a disordered polypeptide where the residues are not functional or carry out structural stabilizing interactions, and it can be reversible and irreversible [84]. One of the primary factors affecting the stability of extremozymes is temperature. Extremophilic enzymes are typically adapted to function in environments that are far outside the temperature range that is conducive to the activity of mesophilic enzymes. Thermophilic enzymes usually work best in the temperature range between 55 and 121℃, while psychrophilic enzymes work efficiently from −2 to 20 ℃ [63]. When comparing to their mesophilic counter parts, thermophilic enzymes showed similar three-dimensional structures, but have increased charged residues on the surface and have different amino acid contents. They also have shorter loops to prevent nonspecific interactions due to their high flexibility at an elevated temperature. The numbers of bisulphide bonds formed between two cysteine residues are also higher to increase their structural rigidity for preventing unfolding at high temperature [63]. Besides, the available of salt bridges is also a distinct feature of thermozyme comparing to their mesophilic variant. Salt bridges help in stabilise the enzyme by increasing rigidity, with the penalty of increment in activation energy, and thus explained that thermophilic enzyme is usually less active at lower temperature [85]. Figure 2 showed the presence of salt bridge in thermophilic acylphosphatase, connecting the active side and C-terminus of the enzyme [86]. Thermophilic enzyme tends to have better adaptation towards enzyme unfolding cause by extreme heat, as their highly charged surfaces and increment in bonds could hold the structure closer together, giving it greater possibility to regain stability when condition returns to normal [87] (Fig. 3).

Fig. 2
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Comparison of thermophilic AcP to its mesophilic counterparts [86]

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Fig. 3
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Illustration on thermal decomposition on thermophilic and mesophilic enzyme [87]

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Psychrophilic enzymes usually have a flexible structure and low stabilities, and it is due to higher proportion of α-helix than β-sheets in their protein. They maintain high catalytic rate at a low temperature through the enhancement of the solvent connection and structural flexibility [63]. By reducing the salt bridges, disulphide bond, aromatic face-to-edge interaction, and bidentate short hydrogen bond, as well as increment in hydrophilicity in core clusters, the psychrophilic enzymes tend to increase their structural flexibility, require lower activation energy and thus able to maintain active at lower temperature [88]. Figure 4 stated the difference between psychrophilic α-amylase synthesised from Pseudoalteromonas haloplanktis to their mesophilic counterparts, with cyan colour represent psychrophilic enzyme and red as mesophilic enzyme. N150D showed the presence of salt-bridge in mesophilic enzyme; V196F present the increment of aromatic ring in mesophilic protein which strengthen the aromatic face-to-edge interaction; Q58C/A99C showed the availability of disulphide bond in mesophilic protein; T232V and Q1641 demonstrated the substitution of hydrophobic to a hydrophilic protein in the core cluster of mesophilic enzyme; and K300R illustrated the availability of bidentate binding though short hydrogen bond in mesophilic enzyme [89].

Fig. 4
figure 4

Comparisons between psychrophilic and mesophilic α-amylase [89].. *Lys = Lysine; Asn = Asparagine; Asp = Aspartic Acid; Val = Valine; Phe = Phenylalanine; Tyr = Tyrosine; Cys = Cysteine; Ala = Alanine; Gln = Glutamine; Trp = Tryptophan; Leu = Leucine; Thr = Threonine; Pro = Proline; Ile = Isoleucine; Arg = Arginine

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Although extremophiles can withstand greater range of temperature, they still subject to denature or become unstable when the temperature range exceeded their adaptation capability, either too hot or cold. The thermal unfolding of enzymes also often leads to aggregation [90]. In order to overcome this issue, extremozymes may be stabilized by adding certain chemicals such as trehalose, glycerol and dextran that can protect the enzyme from denaturation, or by modifying the enzyme’s amino acid composition to improve its stability at higher temperatures [69. 87]. Trehalose is usually used to prevent thermal denature of enzyme, with some theories explained its bioprotection action including vitrification, preferential exclusion, and water replacement theory. Vitrification theory stated trehalose forms a glassy matrix surround the enzyme or cell, physically protects it from abiotic stress; preferential exclusion theory explains that trehalose isolates enzyme away from water molecules, increase the enzyme stability by reducing its wetted radius and increasing its compactness; water replacement theory assumes the structure of enzyme is maintained through the replacement of water molecules by trehalose-forming hydrogen bonds [51]. Figure 5 demonstrated the stated theories.

Fig. 5
figure 5

Trehalose in stabilizing enzyme [11]

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Another factor that can affect extremozyme stability is pH. At extreme pH condition, enzyme unfolding happens as the formation of same charges occur due to ionisation of functional groups, causing an increment in electrostatic repulsion. Most enzyme will form molten globule state, which consist of considerable amounts of secondary structure and totally disrupted tertiary structure as the protein denatures gradually [11]. Totally unfold of protein may happened under prolonged extreme condition. Figure 6 illustrate the pH induced denature of enzyme, in native state, the surface charges is at a balance state with zero or minimum net charge. When the condition turns acidic, more hydrogen ions (H+) will presence in the environment and interact with the negative ions of the enzyme, turning the net charges into positive state, along with the drop in ionic strength. When condition prolonged or worsen, more negatively charges will be dissipated and lead to unfolding of protein.

Fig. 6
figure 6

Illustration on the pH induced enzyme denature process

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Generally, enzyme function optimally between range of 6 to 8, while extremophiles live in environments that often have extreme pH conditions, and their enzymes have evolved to function optimally under these conditions. For example, acidophilic enzymes function optimally at pH values below 5, while alkaliphilic enzymes function optimally at pH values above 9 [91]. Acidophilic enzyme usually demonstrated large area of negative charge at the active site, and overall have lesser positive charge compare to neutrophilic counter parts. As protonation of the negatively charged group happen more frequently at lower pH, this will balance out the charges of the enzyme and make it stable at lower pH [11, 90]. They also showed an increase in the number of hydrogen bond. Similar to acidophilic enzyme, alkaliphilic enzyme also showed more negatively charges on the protein surface. They also have an elevated number of hydrophobic residues and drop in polar residues to maintain better stability in alkali environment [89]. However, extreme pH conditions can also cause denaturation of the enzyme. To address this issue, extremozymes may be stabilized by altering their amino acid composition to improve their stability at extreme pH values, or by using buffering agents to maintain a stable pH in the reaction environment. Enzyme immobilization techniques such as nano-zeolite immobilization can be applied for increasing the pH stability of enzyme [92].

Salt concentration is another factor that can affect extremozyme stability. Enzyme are usually more soluble and active in a dilute salt concentration environment, as ionic-formed salt able to associate with opposite charges within the protein moiety, increase hydration on the protein surface. However, extreme salt concentration could remove the essential layer of water molecules from the enzyme, disrupting the salt bridge as well as certain level of ionic bond, which could lead to denature on quaternary and tertiary enzyme structure [31]. Halophilic enzymes are adapted to function in environments with high salt concentrations, and their stability is often dependent on the presence of salt ions. Halophilic enzymes thrive in high saline environment as the protein have higher ion-pair networks, reduction in hydrophobic surface patches, and abnormally high in number of ordered side chains [56]. The high negative charges could provide hydrated carboxylate groups that maintain the solubility of protein under high salinity condition, as well as offsets the stabilization from the salt’s enhancement of hydrophobic effect [43]. Figure 7 showed a comparison between halotolerant aminotransferase from Halomonas elongata and its counterparts from E. coli. Red parts indicate negatively charged aspartic and glutamic acid, while white parts indicate hydrophobic leucine, isoleucine, and phenylalanine protein. Halophilic aminotransferase showed a higher density of negatively charge residues, while its homologue showed a more evenly distribution of acidic and basic proteins, with net charge near to zero [93].

Fig. 7
figure 7

Comparison on the surface charge residues between halotolerant aminotransferase to its counterparts [93]

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However, at extremely high salt concentrations or very low salts concentration, denaturation can occur due to the disruption of the enzyme’s structure. To improve the stability of halophilic enzymes, the salt concentration of the reaction environment may need to be optimized, or the enzyme compositions may need to be modified to enhance its stability under extremely saline or low in salt concentration [31]. The presence of other chemicals or contaminants in the reaction environment can also impact extremozyme stability. Enzymes can be sensitive to certain chemicals, such as detergents, solvents, or heavy metals, which can inhibit their activity or cause denaturation. Some but not all extremozyme showed resistance or able to degrade the toxic compounds lie within the chemicals. To improve the stability of extremozymes in the presence of these chemicals, they may need to be modified to reduce their sensitivity, or stabilizing agents may need to be added to the reaction environment [94].

7 Key Challenges in Utilizing in Industries, Conclusions, Future Prospective

Overall, extremophiles exhibited promising characteristics such as able to thrive in harsh environment of extreme temperature, pH, salinity, and some of the properties help them to survive and utilized and degrade toxic compounds as their energy sources. These make them an excellent candidate in the treatment of EPs which commonly showed high toxicity towards other organisms. Utilizing extremophiles in the treatment of EPs will also bring other advantages such as reduce energy consumption, capital cost and operation cost especially in industrial wastewater treatment plant as the ability of extremophiles to survive in harsh condition able to reduce some steps to alter water conditions in treatment process. Studies can be started by finding the source and isolate the potential strains of extremophiles that could survive and degrade EPs under extreme conditions. Further research including but not limited to determine the extremozymes that helps in degradation and the optimization of efficiency can be done for commercial application of extremophiles in the water and wastewater treatment industries.