8. Microbial Catabolism
8.6 Lithotrophy
Learning Objectives
- Explain the general process of chemolithotrophy and describe how it differs from chemoheterotrophy
- Identify the most common electron donors and acceptors for chemolithotrophs
- Explain the process of reverse electron flow and how/why is it used by some chemolithoautotrophs
Chemolithotrophy is the oxidation of inorganic chemicals for the generation of energy and is another form of cellular respiration. These organisms are exclusively bacteria and archaea. The electron sources for their electron transport system are inorganic and, as with the heterotrophs, ATP is generated by oxidative phosphorylation. Although lithotrophs may perform aerobic or anaerobic respiration, because of energy constraints, they are typically aerobic.
Inorganic Energy Sources
Chemolithotrophs use a variety of inorganic compounds as electron donors, with the most common substances being hydrogen gas, sulphur compounds (such as sulphide and sulphur), nitrogen compounds (such as ammonium and nitrite), and ferrous iron.
Hydrogen oxidizers
These organisms oxidize hydrogen gas (H2) with the use of a hydrogenase enzyme. Both aerobic and anaerobic hydrogen oxidizers exist, with the aerobic organisms eventually reducing oxygen to water. The sulphate-reducing bacterium Desulfovibrio is an example of a hydrogen oxidizer; these organisms can also use organics as their energy source. Because the very negative standard reduction potential of hydrogen gas, the of anaerobic respiration using sulphate as a terminal electron acceptor is still substantial. This is in contrast to the iron and nitrogen oxidizers (Figure 8.27).
Sulphur oxidizers
As a group, these organisms are capable of oxidizing a wide variety of reduced and partially reduced sulphur compounds such as hydrogen sulphide (H2S), elemental sulphur (S0), thiosulphate (S2O32-), and sulphite (SO32-). Sulphate (SO42-) is frequently a by-product of the oxidation. Often the oxidation occurs in a stepwise fashion with the help of the sulphite oxidase enzyme. Bacteria and archaea associated with acid mine drainage (AMD) typically oxidize reduced sulphur compounds producing sulphuric acid. The resulting acidic pH causes oxidized minerals to precipitate, exacerbating the environmental impact of their activities (Figure 8.28).
Iron oxidizers
These organisms oxidize ferrous iron (Fe2+) to ferric iron (Fe3+). As shown in the Figure 2, Fe2+ has such a positive standard reduction potential, the bioenergetics are not extremely favourable, even using oxygen as a final electron acceptor. Acidithiobacillus ferrooxidans, an AMD bacterium, oxidizes both atoms of the solid mineral FeS2 (also known as iron pyrite or “fool’s gold).
Nitrogen oxidizers: nitrifiers & comammox bacteria
The aerobic oxidation of ammonia (NH3) is generally performed by nitrifying bacteria and archaea in a two-step process, where one group oxidizes ammonia to nitrite (NO2–) and the second group oxidizes the nitrite