Cyanobacteria mediated heavy metal removal: a review on mechanism, biosynthesis, and removal capability

Heavy metal in wastewater is a crucial concern due to its toxicological manifestations on human health, particularly in developing countries. Compared to different conventional heavy metal removal methods, cyanobacteria mediated heavy metal removal is a potential method, as it is a cost-effective, in situ operable, and green chemistry approach. They are excellent tools for multidirectional metal sequestration operations as they can simultaneously sequester metal through biosorption and bioaccumulation. Biosorption is a cell surface phenomenon, whereas bioaccumulation occurs inside the cell. This study reviewed deeply how cyanobacteria sequester heavy metal ions by these two processes from an ambient water body and the defense mechanism of cyanobacteria against metal-induced toxicity. Further, among the different components of the cyanobacteria's cell wall, this blue–green algae biosorb the metal ion mainly through Exopolysaccharide (EPS). The article discusses several pathways of EPS biosynthesis to know the potential engineering approach for producing this incredible polymer, which facilitates its metal ion adsorption property. Furthermore, we compare different cyanobacterial species’ ability to sequester heavy metals from water in different environmental conditions. Limnococcussp, Nostocmuscorum, and Synechococcus sp. PCC 7942 shows optimum efficiency based on heavy metal removal, multi-metal removal by biosorption and bioaccumulation, and contact time. Finally we for the first time, discussed the circadian clock application in the cyanobacterial metal ion sequestration process, which might disclose the molecular-level mechanisms of cyanobacteria to sequester metal ions and different defense mechanisms.