Comparison of the performance of Chlorococcum ellipsoideum and Tetradesmus obliquus as a carbon source for reduction of Cr(VI) with bacteria

Abstract

Chromium (Cr) is used in a variety of metallurgical, refractory and chemical processes, as well as in industrial activities such as the refining of ore, the production of steel and alloys, metal plating, and tanneries. Due to its widespread use in anthropogenic processes Cr(VI) is commonly released into the environment. Cr(VI) is known to be carcinogenic and mutagenic to living organisms, however some bacteria species have evolved a detoxification mechanism through which they reduce Cr(VI) to Cr(III), which is 100 times less toxic than Cr(VI). In previous studies of the bioreduction of Cr(VI), glucose and Luria-Bertani broth was used as the primary carbon source. In this study, an indigenous mixed culture of bacteria (Escherichia coli, Bacillus thermoamylovorans and Citrobacter sedlakii) is utilised to reduce Cr(VI) while consuming carbon sources produced by various algae species (Chlorococcum ellipsoideum and Tetradesmus obliquus). The different algae species were compared as possible carbon sources for the bioreduction process. Batch studies show that locally isolated bacteria cocultured with algae achieved 100 % removal of Cr(VI) within 24 h. The performance of the different algae was very similar however bacteria utilizing Tetradesmus obliquus algae as a carbon source achieved the 100 % Cr(VI) reduction the fastest. Algae is sensitive to Cr(VI) toxicity and the algae growth is inhibited, therefore the algae must be cultured beforehand. SEM results indicate that the algae cells were adversely affected by the Cr(VI). The Cr(VI) destroyed the algae cell walls, allowing the bacteria to utilise the internal metabolites. Utilizing carbon sources produced by algae would be more practical to implement in the real world than adding glucose. This study demonstrates the potential of combining locally isolated Cr(VI) reducing bacteria and green algae to decontaminate Cr(VI) polluted sites in South Africa.