Extremophile's genome sequenced, may improve biohydrogen production

The bacterium Syntrophus aciditrophicus, one of the most extreme-survival organisms ever discovered, has had its genome sequenced. Microbiologists think the findings on the extremophile's unique metabolism may be used in the production of biohydrogen.

Syntrophus lives on a diet so austere that it exists on the brink of energetic death. The genes now discovered making up its genome are providing clues as to how it survives, and might even improve the efficiency by which we can make hydrogen from waste materials, the researchers say. They published the results of their study [*abstract] in the April 18 edition of the Proceedings of the National Academy of Sciences.

Robert Gunsalus at the University of California, Los Angeles, and colleagues (image), identified 3169 genes in Syntrophus. The bacterium performs a key part of the global carbon cycle by breaking down fatty acids in organic matter – a very limited diet consumed by almost no other organisms. To do this it needs genes that can participate in thermodynamically unfavourable reactions known as reverse electron transport.

Most organisms use oxygen to help breakdown organic compounds for energy use. In this process, organic compounds are chemically oxidised, and the electrons produced in the reaction are used to drive the production of the energy-storage compound ATP.

Syntrophus lives in an anaerobic (non-oxygen) environment, where such a key reaction is impossible. Instead, the flow of electrons occurs in the opposite direction – reverse electron transport – through a reaction that produces hydrogen and formate, which actually requires energy. Without the "help" of other types of bacteria, which consume the hydrogen and formate and provide energy in return, Syntrophus could not survive:
bioenergy :: biofuels :: energy :: sustainability :: bacterium :: microbe :: genome :: biohydrogen :: biomass ::

Gunsalus's team found several genes that appear to participate in this process, and they hope to gain a better understanding of the mechanism. “If we can understand such 'syntrophic metabolism', we may be able to increase the amount of hydrogen that can be made from waste materials, and hopefully make biohydrogen production a reality,” says Gunsalus.

Biohydrogen is the most competitve way to produce the gas without relying on fossil fuels (earlier post).

There are roughly three main ways of obtaining the gas from biological sources: (1) biochemical conversion: chemotrophic or phototrophic micro-organisms are allowed to ferment sugars, under anaerobic or aerobic conditions (depending on the micro-organism) during which hydrogenase or nitrogenase enzymes produce hydrogen directly (on H2 production from cyanobacteria and micro-algae see the last section of our post on biofuels from algae), (2) thermochemical conversion: biomass in solid form (wood, straw, etc) is transformed through gasification into a hydrogen-rich gas, from which the H2 is then separated, or (3) indirectly from biogas: biomass is anaerobically fermented into biogas, the methane of which is further converted into hydrogen (similar to H2 production from natural gas); combinations between biohydrogen and biomethane production are being researched as well.

The unique metabolic pathways used by Syntrophus makes the scientist think it can play a role in anaerobic hydrogen production from biomass.

More information:
Robert P. Gunsalus, et al., "The genome of Syntrophus aciditrophicus: Life at the thermodynamic limit of microbial growth" [*abstract], Published online before print April 18, 2007, Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0610456104.

New Scientist: Extreme-living bacteria has genome sequenced - April 16, 2007.

Kegg pathway maps for Syntrophus aciditrophicus.