Through symbiosis, waste products from Syntrophus aciditrophicus are rapidly turned back into the organism's energy source in harsh, extremely resource-limited environments.

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Syntrophic communities contain two or more species (usually single-celled organisms) that are able to feed off each other's waste products in a relatively efficient cycle. Only a small external energy input is required to maintain the cycle continuously, though cellular reproduction is severely hindered. These communities exist where food and energy sources are extremely limited so some of the species involved have evolved unique biochemical pathways and relationships for their survival. In its oxygen-starved environment, Syntrophus aciditrophicus eats a variety of organic compounds including natural gas (methane) and excretes a carbon-based waste (formic acid) and hydrogen gas. Left on its own, this process would take energy from S. aciditrophicus but its syntrophic partner rapidly consumes these waste products producing more methane for S. aciditrophicus to consume. Overall, this syntrophic relationship produces energy for S. aciditrophicus.

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"Biochemically, the syntrophic bacteria constitute the missing link in our understanding of anaerobic flow of carbon in the biosphere...A distinctive feature of syntrophic metabolism is the need for reverse electron transport; [in Syntrophus aciditrophicus] the presence of a unique Rnf-type ion-translocating electron transfer complex, menaquinone, and membrane-bound Fe-S proteins with associated heterodisulfide reductase domains suggests mechanisms to accomplish this task...Syntrophus aciditrophicus is a rod-shaped, Gram-negative bacterium that degrades fatty acids, coculture with hydrogen/formate-using methanogens or sulfate reducers...anaerobic recycling of organic matter to methane and carbon dioxide [ocurrs]...[They] require symbiotic partners to survive and express their syntrophic life-style...A wide range of organic compounds including alcohols, fatty acids, aromatic acids, organic acids such as lactate and glycolate, many amino acids, sugars, and hydrocarbons including methane are degraded syntrophically under anaerobic conditions. The degradation of these compounds is thermodynamically unfavorable unless hydrogen and/or formate are maintained at low levels by hydrogen/formate-consuming microorganisms such as the methanogens. This thermodynamically based...interaction between microbial species is termed syntrophy...metabolic cooperation among microbial species is required for complete destruction of organic matter where the catalytic unit is a microbial consortium composed of two or more microbial species...the available free energy must be shared by the different organisms. Truly, syntrophy represents an extreme existence, that of a marginal energy economy.

"A distinctive feature of syntrophic metabolism is the need for reverse electron transfer because critical oxidation-reduction reactions result in a negative ΔE', i.e., a thermodynamically unfavorable direction...the membrane components involved in the generation and use of ion gradients are predicted to be key features of syntrophic metabolism...The reliance of S. aciditrophicus on syntrophic fatty and aromatic acid metabolism delineates it from almost all organisms." (McInerney et al. 2007:7600)

"[A] number of the steps involved in the syntrophic benzoate and fatty acid oxidation...benzoate + CoA + ATP -> benzoyl-CoA + AMP + PPi...the membrane potential rather than ATP hydrolysis may drive the electron transfer needed for ring reduction by yet unknown membrane components." (McInerney et al. 2007:7602). 

"...Proteins with 2Fe-2S binding domains. These genes, along with...heterodisulfide reductase gene clusters, indicate a soluble electron transfer machinery that couples substrate metabolism with redox carriers such as ferredoxin and/or NAD+." (McInerney et al. 2007:7603).

"Distinctive features include unique approaches for carbon metabolism and reverse electron transport. The membrane components involved in the generation and use of ion gradients are critical. The genome of S. aciditrophicus contains a membrane-bound, ion-translocating complex (Rnf-like)...Syntrophic metabolism proceeds close to thermodynamic equilibrium and the available free energy of the reactions depends on the terminal electron accepting reaction of the syntrophic partner. Multiple mechanisms are present to create and use ion gradients, which would help modulate the energy status of the cells in response to varying thermodynamic conditions." (McInerney et al. 2007:7604).

"...Ion-translocating, electron transfer complex, proton- and sodium- translocating ATP synthases, proton-translocating pyrophosphatases, sodium-translocating glutaconyl-CoA decarboxylase, and hydrogenase linked with a putative sodium-translocating membrane protein. In addition, the separation of formate synthesis and hydrolysis across the cell membrane could form a proton/sodium gradient." (McInerney et al. 2007:7605).

Journal article
The genome of Syntrophus aciditrophicus: Life at the thermodynamic limit of microbial growthProceedings of the National Academy of SciencesApril 19, 2007
M. J. McInerney, L. Rohlin, H. Mouttaki, U. Kim, R. S. Krupp, L. Rios-Hernandez, J. Sieber, C. G. Struchtemeyer, A. Bhattacharyya, J. W. Campbell, R. P. Gunsalus

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