Hexavalent chromium (Cr(VI)) is found together with a variety of aromatic compounds in a number of contaminated sites, including groundwater aquifers, lake and river sediments, and soils. Cr(VI) and its organic copollutants often originate from industrial sources such as leather tanning, photographic-film making, wood preservation, car manufacturing, petroleum refining, and agricultural activity (Nkhalambayausi-Chirwa and Wang, 2001).
A common pathway for Cr(VI) reduction to the less toxic Cr(III) is done through the unstable Cr(V) intermediate (Cervantes et al, 2001). This reduction is carried out by many different chromium resistant microorganisms.
A less common Cr(VI) reduction involves simultaneous Cr(VI) reduction and phenol degradation, which was observed in a fixed-film bioreactor consisting of a coculture of Pseudomonas putida DMP-1 and Escherichia coli ATCC 33456 (Nkhalambayausi-Chirwa and Wang, 2001). E. coli reduces Cr(VI) under both aerobic and anaerobic conditions (Shen and Wang, 1993), while P. putida is an obligate aerobe capable of degrading phenol (Wang Y. T. and Qu M. Substrate interactions during biodegradation of phenols by a Pseudomonas sp. Proceeding of the 65th Annual Water Environment Federation Conference and Technical Exposition (Alexandria, VA) (1992) 63.). Nkhalambayausi-Chirwa and Wang (2001) believe the spatial and physiological heterogeneity introduced within microbial communities by the formation of biofilms may enhance Cr(VI) reduction by E. coli within the quasi-anaerobic interior of the biofilm while supporting maximum growth of P. putida along the more aerobic surface layers.
The following is a text-format chromium (VI) pathway map. Organisms which are known to carry out these reactions are given, however other organisms may also carry out the steps of the left path. Follow the links for more information on compounds or reactions. This map is also available in graphic (16k) format.
Chromate Chromate Pseudomonas sp. G-1 Escherichia coli ATCC 33456 Desulfovibrio vulgaris | | | | | | | A | | v | [Chromium (V)] v | Chromium (III) | Cr(VI) reductase | cytochrome-c3 hydrogenase | v Chromium (III)
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