Syntrophy

Syntrophy is often used synonymously for mutualistic symbiosis especially between at least two different bacterial species. Syntrophy differs from symbiosis in a way that syntrophic relationship is primarily based on closely linked metabolic interactions to maintain thermodynamically favorable lifestyle in a given environment. Syntrophy plays an important role in a large number of microbial processes especially in oxygen limited environments, methanogenic environments and anaerobic systems. In anoxic or methanogenic environments such as wetlands, swamps, paddy fields, landfills, digestive tract of ruminants, and anerobic digesters syntrophy is employed to overcome the energy constraints as the reactions in these environments proceed close to thermodynamic equilibrium. Mechanism of microbial syntrophy The main mechanism of syntrophy is removing the metabolic end products of one species so as to create an energetically favorable environment for another species. The interspecies electron transfer can be carried out via three ways: interspecies hydrogen transfer, interspecies formate transfer and interspecies direct electron transfer. Reverse electron transport is prominent in syntrophic metabolism. A classical syntrophic relationship can be illustrated by the activity of Methanobacillus omelianskii. It was isolated several times from anaerobic sediments and sewage sludge and was regarded as a pure culture of an anaerobe converting ethanol to acetate and methane. In fact, however, the culture turned out to consist of a methanogenic archaeon "organism M.o.H" and a Gram-negative Bacterium "Organism S" which involves the oxidization of ethanol into acetate and methane mediated by interspecies hydrogen transfer. Individuals of organism S are observed as obligate anaerobic bacteria that use ethanol as an electron donor, whereas M.o.H are methanogens that oxidize hydrogen gas to produce methane. == Examples ==

In ruminants The defining feature of ruminants, such as cows and goats, is a stomach called a rumen. The rumen contains billions of microbes, many of which are syntrophic. Some anaerobic fermenting microbes in the rumen (and other gastrointestinal tracts) are capable of degrading organic matter to short chain fatty acids, and hydrogen. Some fermentation products, such as fatty acids longer than two carbon atoms, alcohols longer than one carbon atom, and branched chain and aromatic fatty acids, cannot directly be used in methanogenesis. In acetogenesis processes, these products are oxidized to acetate and H2 by obligated proton reducing bacteria in syntrophic relationship with methanogenic archaea as low H2 partial pressure is essential for acetogenic reactions to be thermodynamically favorable (ΔG 2 scavengers. These scavengers, like Methanospirillum and Acetobacterium, metabolize the H2 waste produced during amino acid breakdown, preventing a toxic build-up. Another way to improve amino acid breakdown is through interspecies electron transfer mediated by formate. Species like Desulfovibrio employ this method. Amino acid fermenting anaerobes such as Clostridium species, Peptostreptococcus asacchaarolyticus, Acidaminococcus fermentans were known to breakdown amino acids like glutamate with the help of hydrogen scavenging methanogenic partners without going through the usual Stickland fermentation pathway Anaerobic digestion Effective syntrophic cooperation between propionate oxidizing bacteria, acetate oxidizing bacteria and H2/acetate consuming methanogens is necessary to successfully carryout anaerobic digestion to produce biomethane == Syntrophic theories of eukaryogenesis ==

Many symbiogenetic models of eukaryogenesis propose that the first eukaryotic cells were derived from endosymbiosis facilitated by microbial syntrophy between prokaryotic cells. Most of these models involve an archaeon and an alphaproteobacterium, where the dependence of the archaeon on the alphaproteobacterium leads the former to engulf the latter, the alphaproteobacterium then eventually becoming the mitochondria. While these models share the concept of syntrophic interaction as a key driver of endosymbiosis, they often differ on the exact nature of the metabolic interactions involved and the mechanisms by which eukaryogenesis occurred. Hydrogen hypothesis In 1998, William F. Martin and Miklós Müller introduced the hydrogen hypothesis, proposing that eukaryotes arose from syntrophic associations based on the transfer of H2. In this model, an syntrophic association arose where a anaerobic autotrophic methanogenic archaeon was dependent on the H2 made as a byproduct of anaerobic respiration by a facultatively anaerobic alphaproteobacterium. Reverse flow model The reverse flow model was created based on the metabolic analysis of Asgard archaea, which is thought to be the kingdom from which eukaryotes emerged. This model proposes that a syntrophic association arose where anaerobic ancestral Asgard archaea generated and provided reducing equivalents that facultative anaerobic alphaproteobacteria used in the form of H2, small reduced compounds, or by direct electron transfer. Unlike most other symbiogenetic models, the E3 model involves three separate types of microbes: a fermentative archaeon, a facultatively aerobic organotroph (which was acts as the precursor of the mitochondria), and sulfur-reducing bacteria (SRB). Similarly to the E3 model, the syntrophy hypothesis suggests that eukaryogenesis involved three different types of microbes: a complex sulfate-reducing deltaproteobacterium (the precursor to the cytoplasm), an H2-producing Asgard archaeon (the precursor to the nucleus), and a facultatively aerobic sulfide-oxidizing alphaproteobacterium (the precursor to mitochondria). In this model, the deltaproteobacteria forms syntrophic associations with both the Asgard archaeon (based on the transfer of H2) and the alphaproteobacterium (based on the redox of sulfur), leading both to become endosymbionts of the deltaproteobacteria. In this now obligatory symbiosis, organic compounds were degraded in the periplasmic space of the deltaproteobacteria before being moved to the archaeon for further degradation. This interaction drove the periplasm to develop and expand in close proximity with the archaeon to facilitate molecular exchange, resulting in an endomembrane system, transport channels, and the loss of the archaeal membrane. Ultimately, the archaeon became the nucleus while the periplasmic endomembrane system became the endoplasmic reticulum. Meanwhile, the consortium lost the metabolic capability for bacterial sulfate reduction and archaeal energy metabolism as it became more reliant on aerobic respiration performed by the alphaproteobacterium which, ultimately, became the mitochondrion. == Examples of syntrophic organisms ==

• Syntrophomonas wolfei is a gram-negative, anaerobic, fatty-acid oxidizing bacterium that forms syntrophic associations with H2-using bacteria. • Syntrophobacter fumaroxidans is a gram-negative anaerobic bacterium that can oxidize propionate in pure cultures or in syntrophic association with Methanospirillum hungateii. • Pelotomaculum thermopropionicum is a thermophilic, anaerobic, syntrophic propionate-oxidizing bacterium that, in co-culture with Methanothermobacter thermautotrophicus, can grow on propionate, ethanol, lactate, 1-butanol, 1-pentanol, 1,3-propanediol, 1-propanol, and ethylene glycol. • Syntrophus aciditrophicus is a gram-negative, obligately anaerobic, nonmotile, rod-shaped bacterium that, in syntrophic association with hydrogen/formate-using methanogens or sulfate reducers, degrades benzoate and fatty acids. • Syntrophus buswellii is a gram-negative, anaerobic, motile, rod-shaped bacterium that, in syntrophic association with H2-using bacteria, degrades benzoate. • Syntrophus gentianae is a obligately anaerobic bacterium that ferments benzoate in syntrophic association with H2-using bacteria. == References ==