The geomicrobiology of chromium (VI) pollution: Microbial diversity and its bioremediation potential

The role and significance of microorganisms in environmental recycling activities marks geomicrobiology one of the essential branches within the environmental biotechnology field. Naturally occurring microbes also play geo-active roles in rocks,leading to biomineralization or biomobilization of minerals and metals. Heavy metals,such as chromium (Cr),are essential micronutrients at very low concentrations,but are very toxic at higher concentrations. Generally,heavy metals are leached to the environment through natural processes or anthropogenic activities such as industrial processes,leading to pollution with serious consequences. The presence of potentially toxic heavy metals,including Cr,in soils does not necessarily result in toxicity because not all forms of metals are toxic. Microbial interaction with Cr by different mechanisms leads to its oxidation or reduction,where its toxicity could be increased or decreased. Chromite contains both Cr(III) and Fe(II) and microbial utilization of Fe(II)- Fe(III) conversion or Cr (III) - Cr (VI) could lead to the break-down of this mineral. Therefore,the extraction of chromium from its mineral as Cr (III) form increases the possibility of its oxidation and conversion to the more toxic form (Cr (VI)),either biologically or geochemically. Cr (VI) is quite toxic to plants,animals and microbes,thus its levels in the environment need to be studied and controlled properly. Several bacterial and fungal isolates showed high tolerance and resistance to toxic Cr species and they also demonstrated transformation to less toxic form Cr (III),and precipitation. The current review highlights toxicity issues associated with Cr species and environmental friendly bioremediation mediated by microorganisms.