TARGETS OF IMPROVEMENT IN BACTERIAL CHROMATE BIOREMEDIATION

Cr(VI) (chromate) is a widespread, toxic and soluble environmental con- taminant. Bacteria can reduce chromate to insoluble and less toxic Cr(III), and thus chromate bioremediation is of interest. Genetic and protein engineering of suitable enzymes can improve bacterial bioremediation. However, many fundamental parameters that define an organism's capacity to remediate chromate have not previously been char- acterized. We have measured the innate ability of a wide range of bacteria to reduce chromate. All bacteria that we tested were able to reduce Cr(VI) through to stable Cr(III) end-products. One of these, Pseudomonas putida MK1, is a promising candidate for chromate bioremediation, and we show that chromate transformation per mass unit increases in these cells as the environmental chromate concentration rises. However, bacterial growth is inhibited by chromate concentrations above 0.8 mM. Furthermore, although cell extracts show increased chromate reductase activity under slow-growth conditions (mimicking nutrient-limited field conditions), the ability of the whole cells to transform chromate is greatly diminished. Chromate reduction by both whole cells and purified ChrR (a P. putida chromate reductase) is also inhibited by co-pollutants of chromate-contaminated sites. Thus, these studies identify several potential areas of improvement for generation of improved chromate-remediating bacteria.