Electrons derived from sugars and other organic molecules are usually donated either to endogenous organic electron acceptors or to molecular O2 by way of an electron transport chain.
However, many bacteria have electron transport chains that can operate with exogenous electron acceptors other than O2. As noted earlier, this energy-yielding process is called anaerobic respiration. The major electron acceptors are nitrate, sulfate, and CO2, but metals and a few organic molecules can also be reduced. Some bacteria can use nitrate as the electron acceptor at the end of their electron transport chain and still produce ATP. Often this process is called dissimilatory nitrate reduction. Nitrate may be reduced to nitrite by nitrate reductase.
NO3–+2e– + 2H+ —————> NO2– + H2O
However, reduction of nitrate to nitrite is not a particularly effective way of making ATP, because a large amount of nitrate is required for growth .The nitrite formed is also quite toxic. Therefore nitrate often is further reduced all the way to nitrogen gas, a process known as denitrification. Each nitrate will then accept five electrons, and the product will be nontoxic.
There is considerable evidence that denitrification is a multistep process with four enzymes participating: nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase.
Denitrification is carried out by some members of the genera Pseudomonas, Paracoccus, and Bacillus. They use this route as an alternative to normal aerobic respiration and may be considered facultative anaerobes. If O2 is present, these bacteria use aerobic respiration (the synthesis of nitrate reductase is repressed by O2). Denitrification in anaerobic soil results in the loss of soil nitrogen and adversely affects soil fertility.
Two other major groups of bacteria employing anaerobic respiration are obligate anaerobes. Those using CO2 or carbonate as a terminal electron acceptor is called methanogens because they reduce CO2 to methane. Sulfate also can act as the final acceptor in bacteria such as Desulfovibrio. It is reduced to sulfide (S2–or H2S), and eight electrons are accepted.
SO42–+8e– +8H+ ———-> S2–+4H2O
Anaerobic respiration is not as efficient in ATP synthesis as aerobic respiration—that is, not as much ATP is produced by oxidative phosphorylation with nitrate, sulfate, or CO2 as the terminal
acceptors. Reduction in ATP yield arises from the fact that these alternate electron acceptors have less positive reduction potentials than O2. The reduction potential difference between
a donor like NADH and nitrate is smaller than the difference between NADH and O2. Because energy yield is directly related to the magnitude of the reduction potential difference, less energy is available to make ATP in anaerobic respiration. Nevertheless, anaerobic respiration is useful because it is more efficient than fermentation and allows ATP synthesis by electron transport and oxidative phosphorylation in the absence of O2. Anaerobic respiration is very prevalent in oxygen-depleted soils and sediments.
