The enzyme nitrate reductase catalyzes the removal of oxygen from nitrate, leaving nitrite and water as products. This reaction is the basis for physiological test used in identifying certain bacteria.
Some species of Pseudomonas and Bacillus possess enzymes that can further reduce nitrite to nitric oxide (NO), nitrous oxide (N2O), and even nitrogen gas (N2). This process, called denitrification, is a very important step in recycling nitrogen in the biosphere. Other oxygen-containing nutrients reduced anaerobically by various bacteria are carbonates and sulfates.
Most obligate anaerobes use the NADH they generate during glycolysis and the Krebs cycle to reduce some compound other than O2 but still use the oxidative phosphorylation pathway. Methanogens (described in chapter 7) reduce CO2 or CO3- (carbonate) to CH4 (methane gas), and certain sulfate bacteria reduce SO42- to S (sulfide) or hydrogen sulfide gas (H2S) some use metals or organic compounds as the final electron acceptor. These microbes live in subterranean habitats such as swamps and deep oceanic vents that are devoid of O2 gas.