Bacteria nitrifying chemoautotrophs

Recall from Section 2.4.2 that nitrification is a multistep process in which ammonium (NUt ) is oxidized to nitrate using O2. Nitrification is carried out by various nitrifiers, chemoautotrophic bacteria that occur in oxic environments where ammonia is present (Ha)mes, 1986). Nitrifiers use chemical energy released by ammonium oxidation to reduce CO2 to organic matter, much as green plants use energy captured from light to reduce CO2. The intermediate species of nitrification are shown in Eq. (4.55). Note that N2O can be produced by side reactions from either of two intermediates by chemical rearrangement of the short-lived nitroxyl molecule (HNO) or by the reduction of nitrite (N02 ) within the cells of the nitrifiers. 

Photosynthetic organisms plants, algae, bacteria Chemoautotrophic organisms nitrifying bacteria, some sulfur oxidizers, iron oxidizers, hydrogen oxidizers... 

Symbiotic prokaryotes Rhizobium spp. and others Chemoautotrophic nitrifying bacteria... 

These are the eukaryotic assimilatory nitrate reductases and three distinct bacterial enzymes, comprising the cytoplasmic assimilatory (Nas), the membrane-bound (Nar), and the periplasmic dissimilatory (Nap) nitrate reductases [11], Nitrite oxidoreductase, a membrane-bound enzyme from nitrifying bacteria also exhibits nitrate reductase activity. This enzyme shows high sequence similarity to the membrane-bound Nar, and catalyzes the nitrite oxidation to nitrate, to allow chemoautotrophic growth [75]. Many bacteria have more than one of the four types of nitrate reductases [9]. The functional, biochemical, and structural properties of prokaryotic and eukaryotic nitrate reductases have been recently reviewed [3,4,76,77]. Protein sequence data have been used to determine phylogenetic relationships and to examine similarities in structure and function of nitrate reductases. Three distinct clades of nitrate reductase evolved the eukaryotic assimilatory Nas, the membrane-associated prokaryotic Nar, and a clade that included both the periplasmatic Nap and prokaryotic assimilatory Nas [78]. 

Nitrification is the biological oxidation of ammonium (NH4) to nitrate (NOJ), with nitrite (NOJ) as an intermediate under aerobic conditions (NHj NOJ NOJ). Under oxygen-limited conditions nitrifiers can use NOJ as a terminal electron acceptor to avoid accumulation of the toxic NOJ, whereby N2O and NO are produced. During that process, because separate bacteria (Meiklejohn 2006) oxidize NHj into NOJ and NOJ into NOJ, the process can lead to the temporary accumulation of NOJ in soil and water. These bacteria are generally chemoautotrophic, requiring only CO2, H2O and O2. The nitrifying bacteria Nitro-somas, which converts NHj to NOJ, is also able to reduce NOJ. This nitrite can decompose abiotically, yielding NO or NO2, substantially favoured in acidic soils... 

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