Chemosynthesis

Chemosynthesis is the biological conversion of one or more carbon molecules (usually carbon dioxide or methane) and nutrients into organic matter using the oxidation of inorganic molecules (e.g. hydrogen gas, hydrogen sulfide) or methane as a source of energy, rather than sunlight, as in photosynthesis. but groups that include conspicuous or biogeochemically-important taxa include the sulfur-oxidizing gamma and epsilon proteobacteria, the Aquificaeles, the Methanogenic archaea and the neutrophilic iron-oxidizing bacteria.

Chemoautotrophs or lithotrophs, organisms that obtain carbon through chemosynthesis, are phylogenetically diverse, united only by their ability to oxidize an inorganic compound as an energy source. Chemosynthesis runs through the Bacteria and the Archaea. Chemoautotrophs are usually organized into "physiological groups" based on their inorganic substrate for energy production and growth (see Table 2 below).

T able 2. Physiological groups of chemoautotrophs

*The overall process of nitrification, conversion of NH₃ to NO₃, requires a consortium of microorganisms. 

The hydrogen bacteria oxidize H₂ (hydrogen gas) as an energy source. The hydrogen bacteria are facultative lithotrophs as evidenced by the pseudomonads that fortuitously possess a hydrogenase enzyme that will oxidize H₂ and put the electrons into their respiratory ETS. They will use H₂ if they find it in their environment even though they are typically heterotrophic. Indeed, most hydrogen bacteria are nutritionally versatile in their ability to use a wide range of carbon and energy sources. 

The methanogens used to be considered a major group of hydrogen bacteria - until it was discovered that they are Archaea. The methanogens are able to oxidize H₂ as a sole source of energy while transferring the electrons from H₂ to CO₂ in its reduction to methane.  Metabolism of the methanogens is absolutely unique, yet methanogens represent the most prevalent and diverse group of Archaea. Methanogens use H₂ and CO₂ to produce cell material and methane. They have unique enzymes and electron transport processes. Their type of energy generating metabolism is never seen in the Bacteria, and their mechanism of autotrophic CO₂ fixation is very rare, except in methanogens.

The carboxydobacteria are able to oxidize CO (carbon monoxide) to CO₂, using an enzyme CODH (carbon monoxide dehydrogenase). The carboxydobacteria are not obligate CO users, i.e., some are also hydrogen bacteria, and some are phototrophic bacteria. Interestingly, the enzyme CODH used by the carboxydobacteria to oxidize CO to CO₂, is used by the methanogens for the reverse reaction - the reduction of CO₂ to CO - in their unique pathway of CO₂ fixation.