Metagenomic profiling of cyanide-degrading microbial communities in steel industry wastewater with an implication for bioremediation

Abstract

Extremely toxic cyanide-contaminated wastewater discharged from steel industries poses serious environmental and health risks. Cyanide removal through physical and chemical treatments has cost-intensive operational challenges. Microbial bioremediation is cost-effective and environment friendly. As microorganisms can degrade cyanide by utilizing it as a source of nitrogen and converting it into less toxic compounds such as ammonia, this study was planned to get metagenomic data first and then potential microbial strains from the contaminated samples. In this study, wastewater samples from the equalizer and the sludge thickener tank of a steel wastewater treatment plant showed high cyanide concentration (mg/L) of 21.6 and 27.59, respectively. The free form of cyanide was predominant as the wastewater pH was ⩾ 9. The contents of anions (F^-, Cl^-, NO₂^-, Br^-, NO₃^-, SO₄^2-) and elements (Na, Al, K, Fe) were also on the higher side. The metagenomic study revealed microbial community structure and taxonomic abundance with dominance of cyanide-degrading bacterial genera in the wastewaters, viz., Bordetella, Achromobacter, Pseudomonas, and Burkholderia, providing an opportunity to screen and obtain the efficient CN-degrading strains. Bacteria belonging to the phylum Deinococcus and the genus Mesorhizobium were reported for the first time from cyanide-contaminated water. Also, functional analysis showed high prevalence of genes encoding enzymes critical to cyanide degradation pathways e.g., rhodanese, nitrilase, nitrile hydratase, amidase, cyanide-insensitive terminal oxidase, and malate:quinone oxidoreductase. Eight distinct alkaliphilic cyanotropic microorganisms were successfully isolated and shown to degrade cyanide effectively at pH 9.5, indicating their metabolic adaptation to cyanide toxicity and alkaline stress. These findings can lead to a microbial technology against cyanide contamination.