Rhodanobacter denitrificans

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Bacteria; Proteobacteria; Gammaproteobacteria; Xanthomonadales; Xanthomonadaceae; Rhodanobacter; denitrificans [8].


2APBS1 and 116-2 [8].


Rhodanobacter denitrificans is a yellow-pigmented, gram-negative, non-sporulating, motile,slow-growing, and facultatively anaerobic rod-shaped bacterium with the ability to perform complete denitrification [8]. For Rhodanobacter species, denitrification was not a characteristic until the discovery of R. denitrificans [6]. Denitrification is the conversion of nitrate to N2 through nitrite, nitric and nitrous oxide intermediates catalyzed by microbial enzymes [11]. Only two species of Rhodanobacter, one of them being Rhodanobacter denitrificans, is capable of this process [8]. In the absence of oxygen, growth is fueled by the use of nitrate, nitrite, and nitrous oxide as electron acceptors [8]. Rhodanobacter denitrificans has a salt tolerance up to 2% NaCl [8]. Growth was observed at pH 4-8 and 10-35ºC, optimally at pH 6.5 and 30ºC [8]. The two strains of R. denitrificans, 2APBS1 and 116-2, were found at Oak Ridge Integrated Field Research Challenge (ORIFRC) [8]. ORIFRC is a model nuclear legacy site with high levels of nitrate and acidity, with uranium and other heavy metal contaminants [8]. At ORIFRC, 2APBS1 was isolated from Area 2 (pH 6-7 and nitrate level <2 mM) and 116-2 was isolated from Area 3 (pH 3-4 and nitrate level 10s to 100s mM) [8]. The two strains are highly abundant and active in acidic, nitrate rich subsurface environments with high metal (e.g. uranium) concentrations [8]. The characteristic of survival in these harsh environments allow R. denitrificans to conduct bioremediation of the contamination sites by denitrification [8].

Ecology and Metabolism

Microbes can reduce contaminants in the subsurface ecosystem, such as in ORIFRC [5]. Nitrate concentration is inversely proportional to pH [11]. High levels of nitrate inhibits most microbes, but R. denitrificans can thrive in a nitrate rich environment conducting denitrification, which is important in subsurface ecology [5]. Low pH, high nitrate, and other co-contaminants cause strong selective pressure on the bacterial community [5]. Selection minimizes microbial diversity and leads to the survival of dominant species such as R. denitrificans [1]. pH is the strongest determination factor of abundance for Rhodanobacter [5]. Rhodanobacter thrives in conditions of high nitrate and uranium, and low pH [5]. Also, electron donors are the limiting factors of denitrification; nitrate levels exceed the number of electron donors [5]. The deaths of other native bacteria in the acidic subsurface led to favoring of horizontal gene transfers, giving complete denitrification capability and acid tolerance to R. denitrificans [5]. R. denitrificans also have a flexible metabolism and able to grow on many organic substrates [5]. They grow extremely well on acetate (great carbon source and electron donor), used to stimulate denitrification and neutralize liquid waste at ORIFRC [5]. All in all, low pH groundwater and high nitrate levels created a specific niche for Rhodanobacter denitrificans [5].