Comparative study of Thermotolerant Hexavalent Cr Bioremediating Bacteria from Dharavi in India

Introduction

Anthropogenic activities are a source of heavy metal pollution nefarious to life on earth¹. About 5 million sites are contaminated by heavy metals and metalloids with concentrations higher than the regulatory limits in the world^{2, 3}. Contamination of the soil with heavy metals affects the functions of the microbial communities of the soil that contribute to the degradation of organic pollutants and can lead to shifts in microbial population⁴. Hexavalent Cr pollution is of worldwide concern because of its long persistence and highly toxic effect on the environment and human life. It is carcinogenic, genotoxic, and mutagenic. Industries including leather tanning, chrome plating, alloying, textile, dye and, pigment manufacturing, aircraft, and wood preservation extensively utilize Cr^{5, 6}.Tanning industries in India release an estimated 2000-3000 tons of Cr into the environment each year, with effluent containing up to 5000 mg/L, compared with a safe limit of 2 mg/L and 0.05 mg/L in drinking water^{7, 8}. As per the reports of Vyawahare Malavika (2017)⁹, there are more than 300 hazardous waste dump sites across the country. Among these identified sites of pollutants, the most often detected heavy metal was Cr followed by lead, cadmium, mercury, and pesticides.

Bioremediation, a cost-effective technology that does not harm the ecosystem is the need of the hour to restore such sites^{10, 11}. The use of potential microorganisms for this purpose is emerging as an effective technique¹².

Thermophiles grow at 55°C to 80°C, while Thermo-tolerant microorganisms grow better at 25°C to 40°C but can tolerate 60°C to 80°C, showing adaptability to extreme temperatures¹³. Thermophiles use mechanisms like horizontal gene transfer, gene reduction, higher Guanine and Cytosine (G and C) content, and heat shock protein synthesis to survive high temperatures¹⁴. Temperature significantly affects heavy metal adsorption by microorganisms, influencing their actions, metabolism, and enzyme activity for faster bioremediation¹⁵. However, the potential of thermophiles in reducing toxic metals remains underexplored¹⁶. Microorganisms thriving in high-temperature conditions emerge as promising candidates for bioremediation, offering increased process efficiency¹⁷.These organisms, with their extremozymes, are valuable biocatalysts for industrial processes, crucial for challenging conditions like high temperature, pH, or salinity....