MICROBIAL REDUCTION OF CR(VI) BY METHANOTHERMOBACTER THERMAUTOTROPHICUS, A THERMOPHILIC METHANOGEN

Environmental contamination by toxic heavy metals such as chromium has become a major concern recently. Among the two primary phases, Cr(VI) compounds are considered highly mobile and toxic to human health as opposed to Cr(III), which is less mobile and less toxic. Among an array of biogeochemical processes that control the solubility, mobility, and toxicity of these heavy metals in the environment, microbial reduction offers a potential mechanism for immobilizing these contaminants by transforming them to reduced valence states.

Methanothermobacter thermautotrophicus, a strict thermophilic obligate anaerobe was used as a metal reducer in this study. In this experiment, Cr(VI) was provided as potassium dichromate (K₂Cr₂O₇) in two different concentrations (0.2mM and 0.4mM) to observe and compare their reduction rates. Experiments were conducted in growth medium with H₂/CO₂ as substrate. Cr(VI) was measured as a decrease in aqueous Cr(VI) concentration by the diphenylcarbazide method. Total viable cell protein content was determined by using modified Bradford reagent method. Hydrogen consumption and methane production over time was measured by Gas Chromatography. Scanning Electron Microscopy (SEM) was employed to observe any effects of chromium reduction on cell morphology. The results showed the complete reduction of 0.2mM and 0.4mM Cr(VI) within 40 hrs and 120 hrs, respectively. The total viable cell protein showed the inhibition of cell growth until the Cr(VI) was completely reduced and started to grow again after that time point. Very low consumption of hydrogen gas was observed in the case of 0.4mM Cr(VI). However, after the complete reduction of 0.2mM Cr(VI), there was a remarkable decrease in hydrogen concentration and increase in methane production. The presence of chromium nodules on the cell surfaces observed under the SEM were verified by using Energy Dispersive Analysis of X-rays (EDAX).

The potential of this strain to reduce Cr(VI) to Cr(III) could be a promising technique for bioremediation at higher temperature ranges, helping immobilize Cr(VI) in areas such as subsurface disposal of heavy metals and radionuclides where the temperature is expected to be higher.