Bacterial biosorbents & their role in heavy metals clean-up!

Eeasha Mondal, University of Calcutta (LADY BRABOURNE COLLEGE)

Toxic heavy metals(HM) like mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), lead (Pb), manganese (Mn), and nickel (Ni) that are released from industrial activities pose a serious hazard to environmental and human health even when present in low concentrations. Certain conventional remediation methods are known are- Chemical precipitation, electrodialysis, electrodialysis reversal, Membrane filtration (ultrafiltration, nanofiltration, reverse osmosis), microfiltration, and photocatalysis.

However, due to the high cost and lack of environmentally friendly solutions, bioremediation by using bacterial biosorbents, potential microbial biomass, biosurfactants, and enzymes produced from microbes is being greatly used. It was also found that using bio-enzymes and biosurfactants was more advantageous than using the microbes as a whole. This is due to the drawback that the short life span of microbes leads to the production of dead biomass which limits their feasibility in the large-scale application.

Some useful bacterial biosorbents:

Certain bacterial groups that contribute to HM removal include- Bacillus sp., Pseudomonas sp., Arthobacter sp., Azotobacter sp., and Rhodococcus sp.

• B.thuringiensis OSM269, tolerant to various concentrations (25-150 mg/L) of HM like Cd, Cr, Ni, Pb, Cu.
• Pseudomonas sp. are resistant to AS, Cd, Cr, Co, Cu, Hg, Ni and Pb.
• Recently an exopolysaccharide produced by Lactiplantibacillus plantarium BGAN8 strain has been discovered to have high Cd-binding capacity and prevent Cd-induced toxicity.

Methods in the bioremediation process:

Various microbial biomass has different bioremediation abilities. HMs may disrupt microbial cell membranes but they possess characteristic enzymes to overcome toxic effects. The process of bioremediation takes place via various methods like—

1. Alkylation and redox reactions to transform the HMs.
2. Passive adsorption.
3. Active adsorption.
4. HM converted to non-bioavailable forms by binding to metallothioneins, low molecular weight cysteine-rich protein, and metallo-chaperons.
5. In Bacillus cereus KMS31, Azotobacter chroocoum, metal ion uptake is carried out by a mechanism of releasing EPS on the outside of the cell wall.

Biosorption: It is a passive process in which HM is bound to the surface of either living or dead microbes (These microbes come to be known as bacterial biosorbents). It is a simple, rapid process with no energy requirements. The biomass used can be regenerated and reused. This process can occur at a wide range of pH and temperature. It is also capable of multiple HM uptake which is dependent on the diversity of cell wall structures of various microbes.

Bioaccumulation: It is an active process in which HMs are accumulated within the cytoplasm of living cells. It takes a longer time, more energy, and live biomass and is inhibited by the lack of nutrients, varied temperature, and pH changes.

However, living cells are more efficient in HM removal than dead biomass.

Factors affecting biosorption and bioaccumulation:

• Environmental conditions.
• The concentration of pollutants.
• Characteristics of the bacterial biosorbent (type of bacterial species used).
• Aeration conditions.
• Type of substrate.

Extremophiles in heavy metal(HM) removal:

Indigenous sp. of extremophiles isolated from contaminated sites have been reported to demonstrate exceptional resistance and biosorption efficiency towards HMs. Extremophiles synthesize certain siderophores for metal scavenging. They can also detoxify the toxic pollutants through special cellular metabolism. It is evident that extremophiles are potent bioremediation agents for metal chelating.

Thermophilic Geobacillus thermantarcticus and Anoxybacillus amylolyticus were used for HM biosorption at an extremely high temperature of 80°C. Acidocella aromatica and Acidothiobacillus ferroxidans are also used for the bioremediation process.

Future prospects:

Microbes are being genetically engineered to enhance their capacity for tolerance and accumulation of HMs. Along with this, several novel techniques are being developed for the isolation of multifunctional bacterial candidates which can utilize a large number of pollutants as nutrients for their growth.

Also read: What is your brain up to when you’re just walking?

Reference:

Pham, V., Kim, J., Chang, S., & Chung, W. (2022). Bacterial Biosorbents, an Efficient Heavy Metals Green Clean-Up Strategy: Prospects, Challenges, and Opportunities. Microorganisms, 10(3), 610. https://doi.org/10.3390/microorganisms10030610

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