1 Introduction

Water, being the most essential natural component of the earth, is distributed mostly as sea water (97%) and the remaining (3%) occur as freshwater [1]. More than 80% population in the present world are suffering from freshwater scarcity [2]. Moreover, surface freshwater present in the water bodies like rivers, estuaries, and channels are being continuously polluted by anthropogenic activities such as urbanization, agriculture, and industries. [3]. Heavy metals (arsenic, cadmium, chromium, copper, lead, mercury, nickel, selenium, silver, zinc, etc.) representing the persistent toxic chemical elements, in nature after being released from several human activities (mining activities, the metallurgical industry, sewage, irrigation and agricultural wastes, pesticides and fertilizers) enter into the soil and water of the riverine system causing several human health problems worldwide [4] by not only acting as cytotoxic but also playing their roles as mutagenic and even carcinogenic pollutants [5]. On the other hand, waterborne pathogen-mediated human diseases are a major water quality concern for human beings especially in the develo** countries which have posed serious threats to the human survivability mainly because of lack of sanitation system, improper disposal of waste materials, and scarcity of potable water supply [6]. Heavy metals after being accumulated and magnified in many aquatic organisms [7,8,9] severely affect soil subsystem from where they are transported to upper trophic levels by food chain–food web processes [10]. Alongside heavy metals, biological oxygen demand (BOD) and chemical oxygen demand (COD) have been increased by an intricate interaction among bacterial populations with several other pollutants, both organic and inorganic in nature after entering the river ecosystem such as wastes from the paper mills, sewage from the food and pharmaceutical industries, from the leakages and drains of the septic tanks, pesticides from agricultural runoff, and heavy metals from mining and metal industries [11].

Microorganisms in the polluted riverine flows have shown to manifest different strategies to cope up with deteriorating ecological condition and also to survive in the ecologically stressed environment by adopting different detoxifying mechanisms such as bio-sorption, bio-accumulation, bio-transformation, and bio-mineralization [12]. The scientific principles behind such adaptive strategies and mechanisms of microorganisms can be manipulated and exploited for bioremediation either by ex situ or in situ mechanisms [13, 14]. Microbial biomass of surface soil in the benthic zone of river ecosystem is constituted by about 90% of bacteria and fungi, which tend to play decisive and regulatory roles in nutrient cycling [15, 16]. The previous studies have revealed the higher abundance of fungi in coarse sand fractions, whereas bacteria were recorded in higher densities in silt- and clay-dominated fractions of riverine sediment [17,18,19].

Waterborne pathogens have been recorded in increasing abundance in coastal-estuarine environment, which tend to pose serious threats to public health [20]. However, fecal coliforms, such as E. coli, were found both in bottom sediment and flowing water of river [21]. Higher abundances of pathogens in estuarine ecosystem are supposed to be due to the increased human activities, such as water transportation, and substantial recreation during tourisms [22, 23]. Rhodes and Katorin 1990 identified several pathogens in estuaries such as Vibrio cholerae, Giardia sp., Cryptosporidium sp., Salmonella sp., and Campylobacter sp. [24].

Many conventional methods (adsorption, photocatalytic degradation, dialysis, coagulation, and filtration) were used for bioremediation of heavy metals [25]. Microorganisms (fungi, bacteria, and algae) provide a good option for remediation of heavy metals, dyes, and other contaminants from wastewater [26,27,28]. Various fungal genera such as Penicillium sp, Aspergillus sp., and Rhizopus sp. have been used as potential microbial agents to remove of heavy metals from aqueous solutions [29]. ** of wastes and development of mining and mineral processing industries mostly at the upstream (mainly Ghatsila and Muri.) of this river have been contributing for causing the environmental degradation of the river basin during the last few decades [43].During the monsoon seasons, the suspended solids and heavy metal loads in the river water are increased due to the erosion through land runoff and transportation of wastes from different industries, exposed solid waste dum** sites and mining activities. Several environmental problems in the vast stretches of river basin have been resulted for the mining of granites, basalts, quartzite, dolerite, sandstone, limestone, dolomite, gravels, and river sands [44]. Besides that, the domestic and industrial wastewater generated from the urban areas, along the stretch of the river after being discharged into the river pose serious pollution threats to riverine flows in the river [45]. Pathogenic bacteria are being introduced into waters in various ways, including leaking of septic tanks, sewer malfunction contaminated storm drains, runoff from animal feedlots, human fecal discharge, etc. [46].

3.2 Coliform count by MPN and MFT method

The results after the bacteriological examination of water samples collected from the three study sites (S-I, S-II, and S-III) of Subarnarekha river are given in Table 1. According to guidelines of World Health organization (WHO), the permissible limit of drinking water is zero coliform/100 ml [37]. The total coliforms in all samples collected from the all study sites in different seasons were found to range from 540 MPN at S-I during premonsoon to > 2400 MPN at S-II during monsoon (Fig. 1). The considerable amount of coliform has been grown in all three study sites during all seasons of 2017–2018.

Table 1 Coliform count in different seasons at different sides of Subarnarekha river
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Fig. 1
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MPN test at premonsoon season in S-I site of Subarnarekha river water

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The highest coliform count in MPN test (> 2400 MPN/100 ml) was recorded at S-II followed by S-I and S-III during monsoon season. At premonsoon, the S- III showed lesser coliform count in comparison with the S-II and S-I (Table 1). The MPN study of water sample is shown in Fig. 1. The presence of > 10 coliforms/dl in water is designated as polluted or unhealthy for drinking purpose [38]. High MPN values in all the samples clearly indicate that the water is highly contaminated with coliform bacteria. In this study, total coliforms are found to be excessively high compared to the WHO and BIS guidelines. Total coliform test resulted in growth of coliform bacteria at a temperature of 37 °C. Coliforms that produce acid and gas from lactose at 42.5 ± 0.2 °C within 24 ± 2 h are also known as fecal coliforms due to their roles as fecal indicators. On a global scale, water contamination by coliform is a major cause of morbidity and mortality, especially in children. As indicated earlier, coliform are acquired directly or indirectly from a human or animal carriers. Risks from drinking water, therefore, only enunciate following the fecal contamination of the supplied water [47].

3.3 Fungus with potential heavy metal removing activity

A total of 112 fungal isolates have been successfully isolated and cultured from the water/sediments collected from three sampling sites of Subarnarekha river. Fungal isolates were then successfully screened against lead, cadmium, and mercury. From the preliminary screening, there were 16 fungi which showed different resistance patterns against at least five of the three chosen heavy metals. Major fungal isolates were closely related to Aspergillus sp. F12 (MN210327) (Fig. 2). It has been observed from previous studies that Aspergillus penicillioides an aquatic fungus had been isolated from soil sediment of Talsari [34]. All five isolates showed resistance against Pb(II) and Cd(II) up to 1000 ppm and Hg(II) up to 200 ppm. Basically, living organism can absorb metal ions by two ways, one is metabolism-independent way, where cell wall bound to metals ions; and another is intracellular metabolism-dependent pathway, where cell membrane can transport metal ions slowly [48].

Fig. 2
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Morphological structure of F12 strain under light microscope

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Cadmium is considered to be more lethal in later phase of life because of the increasing risk after exposure [49]. Higher concentration of Cd(II) was mainly due to the discharge of effluents from steel industries as well as household discharges, Mercury, being one of the global pollutants, has the ability to move a long distance away from the source [50]. The maximum concentration of Hg was found in the coal-based power plant [51], which can affect the human immune system [52] and cellular disruption [53]. Lead is a neurotoxic and nephrotoxic pollutant and comes into the Subarnarekha river water by the waste of industrial effluents [54].

Among all isolates, Aspergillus sp., Fusarium sp., Penicillium sp., Rhizopus sp., and Pythium sp. have shown heavy metal scavenging potential. Out of them, Aspergillus penicillioides F12 (MN210327) has the highest heavy metal (Pb, Cd) scavenging ability in an optimum PH, temperature, and time [34]. The scavenging activity of heavy metals (Pb, Cd and Hg) at optimum conditions by Aspergillus penicillioides F12 (MN210327) is shown in Fig. 3a, b.

Fig. 3
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Percentage of heavy metals removal by Aspergillus Penicillioides (F-12) biomass (a) and EPS (b)

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3.4 SEM and EDEX analysis

For SEM study, fungi sample was grown in equal concentration of heavy metal (Pb, Cd, and Hg) containing media. The SEM study revealed that the characteristic of the genus Aspergillus sp. F12 (MN210327) is the spore-like bearing structure (Fig. 4a) which produce extracellular polymeric substances (Fig. 4b). The conidia are produced on conidiophores arising from the foot cells of the hyaline and septate somatic hyphae. The hypha is branched and multinucleate. The EDEX study of the dry mass revealed the accumulation of target metals in the surface of fungal cell (Fig. 5). The maximum metal accumulation was observed in biomass of fungi, whereas almost double percentage of Pb and Cd has been removed than Hg when supplemented combined with equal concentration.

Fig. 4
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SEM images of fungal strain, Aspergillus Penicillioides biomass (a) and EPS (b)

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Fig. 5
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Obtained EDEX spectra of F12 strain of biomass

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3.5 Antibacterial activity of fungal extract isolated from Subarnarekha River

Microorganisms (bacteria and fungi) produce several bioactive compounds those have biomedical as well as eco-monitoring activities [55].The crude extract of fungal isolates with hexane, ethyl acetate, and methanol was screened for their antimicrobial potential. The present study has evaluated the antimicrobial activity of metabolites produced by fungal endophytes against four reference human pathogenic microorganisms (E. coli, Vibrio cholerae, Bacillus subtilis, and Staphylococcus aureus).The results on the in vitro antimicrobial activities of several fungi against four different bacterial strains are given in Table 2. The results reported in this present study demonstrate the potential of fungi, Aspergillus sp. F12 (MN210327), showed antibacterial activity followed by Pythium sp. Fusarium sp. Rhizopus sp., and Penicillium sp. Test with the crude extract produced by the aquatic isolate showed promising results for growth inhibition of human pathogenic bacteria (Fig. 6). Therefore, it indicates that these fungi can be an important source of bioactive substances of biotechnological interest.

Table 2 Antimicrobial activity of fungus (F12) against both gram-negative and gram-positive bacteria
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Fig. 6
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Antimicrobial sensitivity test of Aspergillus Penicillioides (F12) extract

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4 Conclusion

In the present study with the fungal isolate, Aspergillus penicilloides F12 (MN210327) was observed to display dual roles of antibacterial as well as heavy metal scavenging activity but most of the metal resistance fungi cannot show such antibacterial activity. Considering different research information generated out of the present study, it can be inferred that the water of Subarnarekha river in India at different locations have been polluted by the considerable amount of organic and inorganic wastes which have necessitated to undertake remedial measure for the cause of human society. The elemental concentrations of Pb, Cd, and Hg were found to exceed the permissible limits of WHO, whereas the prevailing higher abundance of bacteriological indicators in the river water was because of the discharge of wastes out of anthropogenic activities. Based on the results of water quality parameters as well as of bacterial counts, it is recommended that the water of this river in the existing state should not be used for the purpose of human use, especially as drinking water without proper treatment.