In both aerobic and anaerobic respiration, ATP is formed as a result of electron transport chain activity. Electrons for the chain can be obtained from inorganic nutrients, and it is possible to derive energy from the oxidation of inorganic molecules rather than from organic nutrients. This ability is confined to a group of bacteria called chemolithotrophs. Each species is rather specific in its preferences for electron donors and acceptors. The acceptor is usually O2, but sulfate and
nitrate are also used. The most common electron donors are hydrogen, reduced nitrogen compounds, reduced sulfur compounds, and ferrous iron.
Chemolithotrophic bacteria are usually autotrophic and employ the Calvin cycle to fix CO2 as their carbon source. However, some chemolithotrophs can function as heterotrophs if reduced organic compounds are available. Because the yield of ATP is so low, chemolithotrophs must oxidize a large quantity of inorganic material to grow and reproduce, and this magnifies their ecological impact.
Several bacterial genera(ex:- Alcaligenes, Hydrogenophaga, and Pseudomonas spp. can oxidize hydrogen gas to produce energy because they possess a hydrogenase enzyme that catalyzes the oxidation of hydrogen
H2 →2H+ + 2e–
The electrons are donated either to an electron transport chain or to NAD+, depending on the hydrogenase. If NADH is produced, it can be used in ATP synthesis by electron transport and oxidative phosphorylation, with O2 as the terminal electron acceptor.
These hydrogen-oxidizing microorganisms often will use organic compounds as energy sources when such nutrients are available. The best-studied nitrogen-oxidizing chemolithotrophs are
the nitrifying bacteria. Ammonia oxidation to nitrate depends on the activity of at least two differentgenera. For example, Nitrosomonas and Nitrosospira oxidize ammonia to nitrite.
NH4+ + 11⁄2 O2 →NO2– + H2O + 2H+
The nitrite can then be further oxidized by Nitrobacter and Nitrococcus to yield nitrate.
NO2– + 1⁄2 O2→ NO3–
When two genera work together, ammonia in the soil is oxidized to nitrate in a process called nitrification. Energy released upon the oxidation of both ammonia and nitrite is used to make ATP by oxidative phosphorylation. However, microorganisms need a source of electrons (reducing power) as well as a source of ATP in order to reduce CO2 and other molecules.
Since molecules like ammonia and nitrite have more positive reduction potentials than NAD+ ,they cannot directly donate their electrons to form the required NADH and NADPH. This is
because electrons spontaneously move only from donors with more negative reduction potentials to acceptors with more positive potentials.
Sulfur-oxidizing bacteria face the same difficulty. Both types of chemolithotrophs solve
this problem by using proton motive force to reverse the flow of electrons in their electron transport chains and reduce NAD+ with electrons from nitrogen and sulfur donors. Because
energy is used to generate NADH as well as ATP, the net yield of ATP is fairly low. Chemolithotrophs can afford this inefficiency as they have no serious competitors for their unique energy sources. The sulfur-oxidizing bacteria are the third major group of chemolithotrophs. The metabolism of Thiobacillus has been best studied. These bacteria oxidize sulfur, hydrogen sulfide, thiosulfate, and other reduced sulfur compounds to sulfuric acid; therefore they have a significant ecological impact. Interestingly they generate ATP by both oxidative phosphorylation and substrate-level phosphorylation involving adenosine5′-phosphosulfate (APS).APS is a high-energy molecule formed from sulfite and adenosine monophosphate.
