Chromium is a first row transition metal, but its interactions with biological systems are very different from that of its neighbors in the Periodic Table such as manganese and iron. The latter metals often serve as enzyme cofactors. Chromium, however, is largely substitution inert and serves poorly in enzyme catalysis. Chromium will replace other metals in biological systems with toxic effects (ToxFAQs: Chromium). Chromium(VI) is a known human carcinogen. In this context, most studies on prokaryotes and chromium involve reduction of chromium(VI) to less toxic and less mobile forms such as chromium(III). Chromium(III) is an essential trace element in humans, where it enhances insulin function. However, no beneficial function of chromium has been described in microorganisms.

Microbial chromium(VI) reduction has been observed most often under aerobic conditions, but can also occur under anaerobic conditions (reviewed by Wang and Shen, 1995). Cytochromes of type c and b have been implicated in chromium reduction, but the mechanisms of chromium bioreduction have not been completely determined (reviewed by Lovley, 1993). Chromium(VI) reduction is likely a detoxification mechanism, but bacterial chromium resistance is also mediated by non-reductive plasmid-based systems that appear to exclude chromium (as chromate ions) from the cell (reviewed by Silver, 1998). Chromium resistance has also been described in fungi and algae, and some of these organisms can immobilize soluble chromium via bioaccumulation and biosorption (reviewed by Cervantes et al, 2001). Chromate influx is known to occur via sulfate uptake systems in some microorganisms (reviewed by Nies, 1999).

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Nkhalambayausi-Chirwa EM, Wang YT. Simultaneous chromium(VI) reduction and phenol degradation in a fixed-film coculture bioreactor: reactor performance. Water Res. 2001 Jun;35(8):1921-32.