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Thiobacillus denitrificans ATCC 25259
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Photo credit: T.E. Letain, S.I. Martin, H.R. Beller

Thiobacillus denitrificans is a widely distributed and well-characterized obligate chemolithoautotrophic bacterium with an unusual metabolic repertoire that is relevant to environmental concerns. T. denitrificans is best known for its ability to couple the oxidation of inorganic sulfur compounds (such as hydrogen sulfide and thiosulfate) to denitrification, although it was recently found to couple the anaerobic oxidation of Fe(II) to denitrification as well. Among the inorganic electron donors that T. denitrificans can utilize are poorly-soluble minerals containing reduced iron and/or sulfur, such as pyrite (FeS2) and FeS. The mechanism by which this species can use solid-phase electron donors that cannot be taken into the cell is of considerable interest but is currently unknown. T. denitrificans differs from many known chemolithotrophic sulfur-oxidizing bacteria (such as Acidithiobacillus ferrooxidans) in that it is a facultative anaerobe (it can respire aerobically or via denitrification) rather than an obligate aerobe and lives at circumneutral rather than acidic pH.

From an environmental perspective, T. denitrificans is a natural agent for intrinsic bioremediation of a major groundwater contaminant - - nitrate. Many recent studies have shown that chemolithoautotrophic denitrification with pyrite or other reduced sulfur minerals as electron donors can be an important means of natural remediation of nitrate-contaminated groundwater. Nitrate contamination of groundwater is a pervasive and high-priority concern in rural and urban areas throughout the United States, at legacy Department of Energy sites, and in many regions worldwide. In addition to its remediation role in natural nitrate-contaminated environments, T. denitrificans has been used in engineered water treatment systems for nitrate removal. Regarding another environmental concern, the ability of T. denitrificans to carry out nitrate-dependent Fe(II) oxidation under anaerobic conditions could influence metal and radionuclide transport in the subsurface, as ferric iron-containing minerals that may be formed, especially iron(III) oxides, are well known for their ability to adsorb heavy metals and radionuclides, such as uranium.

Prior to genome sequencing, much of the molecular work with T. denitrificans had focused on genes associated with CO2 fixation (this species has both form I and form II ribulose 1,5-bisphosphate carboxylase/oxygenase or RubisCO) and to a much lesser extent with sulfur oxidation (such as the gene coding for adenylylsulfate:phosphate adenylyltransferase, which catalyzes one step of AMP-dependent oxidation of sulfite to sulfate via APS). Availability of the whole genome will facilitate study of nitrate-dependent Fe(II) oxidation (the underlying biochemistry and genetics of which are currently not known in any bacteria) and how physical/chemical factors regulate the environmentally-relevant metabolic activities of T. denitrificans, potentially leading to improved methods for reliable prediction and detection of such activities in subsurface environments.

Beller, H. R., P. S. G. Chain, T. E. Letain, A. Chakicherla, F. W. Larimer, P. M. Richardson, M. Coleman, A. P. Wood, and D. P. Kelly. 2006. The genome sequence of the obligately chemolithoautotrophic, facultatively anaerobic bacterium Thiobacillus denitrificans. J. Bacteriol. 188:1473-1488.