Chemolithotrophy

Chemolithotrophy is found in a limited number of microorganisms. Chemolithotrophs obtain their energy through the oxidation of inorganic substrates and their carbon from CO₂. However, these reactions yield less energy than oxidation of glucose to CO2 so large quantities of substrates have to be metabolized to generate enough energy for sufficient ATP and NADH generation. An example of this is in the process whereby ammonia is oxidized to nitrate and ATP can be generated from this reaction. However, electrons cannot be donated directly for NADH production from ammonia or nitrate because they have a more positive redox potential than NAD+. Instead a process termed ‘reversed electron flow’ allows electrons from the oxidation of nitrite to be used to generate small but sufficient amounts of NADH in Figure 1 for growth. The similar process is used through sulfur-oxidizing bacteria where the oxidation of sulfite to sulfate yields electron reducing power for the    production of NADH and ATP.

                                         Figure  1.  Reversed electron flow.