Why is microbial metabolism important

Authors Authors and affiliations Aharon Oren. Chapter First Online: 11 January Key Words Prokaryotes metabolic diversity energy generation assimilatory metabolism oxygenic photosynthesis anoxygenic photosynthesis respiration anaerobic respiration fermentation methanogenesis chemoautotrophs biogeochemical cycles.

This is a preview of subscription content, log in to check access. Springer, New York Google Scholar. Thieme, Stuttgart Google Scholar. Gottschalk G Bacterial metabolism, 2nd edn.

Fundamentals and Systematics, Vol. Broda E The evolution of the bioenergetic processes. Pergamon Press, Oxford Google Scholar. Bacteriol Rev — Google Scholar. Schink B Energetics of syntrophic cooperation in methanogenic degradation. Broda E Two kinds of lithotrophs missing in nature.

In these organisms hydrogen is oxidized by a membrane-bound hydrogenase causing proton pumping via electron transfer to various quinones and cytochromes. In many organisms, a second cytoplasmic hydrogenase is used to generate reducing power in the form of NADH , which is subsequently used to fix carbon dioxide via the Calvin cycle. Hydrogen oxidizing organisms, such as Ralstonia eutropha , often inhabit oxic-anoxic interfaces in nature to take advantage of the hydrogen produced by anaerobic fermentative organisms while still maintaining a supply of oxygen.

Sulfur oxidation involves the oxidation of reduced sulfur compounds such as sulfide H 2 S , inorganic sulfur S 0 and thiosulfate S 2 O 2 2- to form sulfuric acid H 2 SO 4.

A classic example of a sulfur oxidizing bacterium is Beggiatoa , a microbe originally described by Sergei Winogradsky , one of the founders of microbiology. Generally, the oxidation of sulfide occurs in stages, with inorganic sulfur being stored either inside or outside of the cell until needed.

This two step process occurs because energetically sulfide is a better electron donor than inorganic sulfur or thiosulfate , allowing for a greater number of protons to be translocated across the membrane.

Sulfur oxidizing organisms generate reducing power for carbon dioxide fixation via the Calvin cycle using reverse electron flow , an energy-requiring process that pushes the electrons against their thermodynamic gradient to produce NADH. Biochemically, reduced sulfur compounds are converted to sulfite SO 3 2- and subsequently converted to sulfate by the enzyme sulfite oxidase. Some organisms, however, accomplish the same oxidation using a reversal of the APS reductase system used by sulfate-reducing bacteria see above.

In addition to aerobic sulfur oxidation , some organisms e. Thiobacillus denitrificans use nitrate NO 3 2- as a terminal electron acceptor and therefore grow anaerobically. Ferrous iron is a soluble form of iron that is stable at extremely low pHs or under anaerobic conditions. There exists, therefore, three distinct types of ferrous iron-oxidizing microbes.

The first are acidophiles , such as the bacteria Acidithiobacillus ferooxidans and Leptospirrillum ferrooxidans , as well as the archaeon Ferroplasma. These microbes oxidize iron in environments that have a very low pH and are important in acid mine drainage.

The second type of microbes oxidize ferrous iron at neutral pH along oxic-anoxic interfaces. Both these bacteria , such as Gallionella ferruginea and Sphaerotilus natans , and the acidophilic iron oxidizing-bacteria are aerobes.

The third type of iron-oxidizing microbes are anaerobic photosynthetic bacteria such as Chlorobium , which use ferrous iron to produce NADH for autotrophic carbon dioxide fixation.

Biochemically, aerobic iron reduction is a very energetically poor process which therefore requires large amounts of iron to be oxidized by the enzyme rusticyanin to facilitate the formation of proton motive force. Like during sulfur oxidation reverse electron flow must be used to form the NADH used for carbon dioxide fixation via the Calvin cycle.

Nitrification is the process by which ammonia NH 3 is converted to nitrate NO 3 -. Nitrification is actually the net result of two distinct processes: oxidation of ammonia to nitrite NO 2 - by nitrosifying bacteria e. Nitrosomonas and oxidation of nitrite to nitrate by the nitrite-oxidizing bacteria e. Both of these processes are extremely poor energetically leading to very slow growth rates for both types of organisms.

Biochemically, ammonia oxidation occurs by the stepwise oxidation of ammonia to hydroxylamine NH 2 OH by the enzyme ammonia monooxygenase in the cytoplasm followed by the oxidation of hydroxylamine to nitrite by the enzyme hydroxylamine oxidoreductase in the periplasm. Electron and proton cycling are very complex but as a net result only one proton is translocated across the membrane per molecule of ammonia oxidized.

Nitrite reduction is much simpler, with nitrite being oxidized by the enzyme nitrite oxidoreductase coupled to proton translocation by a very short electron transport chain , again leading to very low growth rates for these organisms. In both ammonia - and nitrite - oxidation oxygen is required, meaning that both nitrosifying and nitrite-oxidizing bacteria are aerobes. As in sulfur and iron oxidation, NADH for carbon dioxide fixation using the Calvin cycle is generated by reverse electron flow , thereby placing a further metabolic burden on an already energy-poor process.

It occurs in members of the Planctomycetes e. Candidatus Brocadia anammoxidans and involves the coupling of ammonia oxidation to nitrite reduction. As oxygen is not required for this process these organisms are strict anaerobes. Amazingly, hydrazine N 2 H 4 -rocket fuel is produced as an intermediate during anammox metabolism. To deal with the high toxicity of hydrazine , anammox bacteria contain an hydrazine -containing intracellular organelle called the anammoxasome surrounded by highly compact and unusual ladderane lipid membrane.

These lipids are unique in nature, as is the use of hydrazine as a metabolic intermediate. Anammox organisms are autotrophs although the mechanism for carbon dioxide fixation is unclear. Because of this property, these organisms have been applied industrially to remove nitrogen in wastewater treatment processes. Many microbes are capable of using light as a source of energy phototrophy. Of these, algae are particularly significant because they are oxygenic, using water as an electron donor for electron transfer during photosynthesis.

Because chloroplasts were derived from a lineage of the Cyanobacteria, the general principles of metabolism in these endosymbionts can also be applied to chloroplasts.

In addition to oxygenic photosynthesis , many bacteria can also photosynthesize anaerobically, typically using sulfide H 2 S as an electron donor to produce sulfate. Phylogenetically, all oxygenic photosynthetic bacteria are Cyanobacteria , while anoxygenic photosynthetic bacteria belong to the purple bacteria Proteobacteria , Green sulfur bacteria e. Chlorobium , Green non-sulfur bacteria e. In addition to these organisms, some microbes e.

This type of metabolism is not considered to be photosynthesis but rather photophosphorylation , since it generates energy, but does not directly fix carbon. As befits the large diversity of photosynthetic bacteria , there exist many different mechanisms by which light is converted into energy for metabolism.

All photosynthetic organisms locate their photosynthetic reaction centers within a membrane, which may be invaginations of the cytoplasmic membrane purple bacteria , thylakoid membranes Cyanobacteria , specialized antenna structures called chlorosomes Green sulfur and non-sulfur bacteria or the cytoplasmic membrane itself heliobacteria. Different photosynthetic bacteria also contain different photosynthetic pigments such as chlorophylls and carotenoids allowing them to take advantage of different portions of the electromagnetic spectrum and thereby inhabit different niches.

Some groups of organisms contain more specialized light-harvesting structures e. Biochemically, anoxygenic photosynthesis is very different from oxygenic photosynthesis. Cyanobacteria and by extension chloroplasts use the Z scheme of electron flow in which electrons eventually are used to form NADH.

Two different reaction centers photosystems are used and proton motive force is generated both by using cyclic electron flow and the quinone pool. In anoxygenic photosynthetic bacteria electron flow is cyclic, with all electrons used in photosynthesis eventually being transferred back to the single reaction center. A proton motive force is generated using only the quinone pool.

In heliobacteria , Green sulfur and non-sulfur bacteria NADH is formed using the protein ferredoxin , an energetically favorable reaction. In purple bacteria NADH is formed by reverse electron flow due to the lower chemical potential of this reaction centre. Most photosynthetic microbes are autotrophic , fixing carbon dioxide via the Calvin cycle. Some photosynthetic bacteria e. Chloroflexus are photoheterotrophs, meaning that they use organic carbon compounds as a carbon source for growth.

Some photosynthetic organisms also fix nitrogen see below. Nitrogen is an element required for growth by all biological systems. Throughout all of nature, only specialized bacteria are capable of nitrogen fixation, converting dinitrogen gas into ammonia NH 3 , which is easily assimilated by all organisms. These bacteria , therefore are very important ecologically and are often essential for the survival entire ecosystems. This is especially true in the ocean, where nitrogen-fixing cyanobacteria are often the only sources or fixed nitrogen and in soils where specialized symbioses exist between legumes and their nitrogen-fixing partners to provide the nitrogen needed by these plants for growth.

Nitrogen fixation can be found distributed throughout nearly all bacterial lineages and physiological classes but is not a universal property. By metabolizing such substances, microbes chemically convert them to other forms. In some cases, microbial metabolism produces chemicals that can be harmful to other organisms; in others, it produces substances that are essential to the metabolism and survival of other life forms Figure 1.

Figure 1. Prokaryotes have great metabolic diversity with important consequences to other forms of life.