New energy source for microbes

Microorganisms living in deep sea hydrothermal vents can grow off of energy derived from one of the simplest forms of anaerobic respiration ever described, according to a study published this week in Nature.

A deep-sea vent projects hot vent
fluid into the frigid water.
Image: Wikimedia commons, National
Oceanic and Atmospheric Administration

The reaction -- in which a chemical called formate is broken down into hydrogen and carbon dioxide -- was previously thought to be too energy poor to support the growth of even the smallest organisms. Finding single-celled microbes from Domain Arcahaea that can glean energy from it may point the way towards efficient hydrogen fuel production while providing clues as to how our planet's earliest inhabitants survived the harsh environment of a young Earth. "The cleavage of formate into hydrogen and CO₂ -- that's a process which is already known for a very long time," said microbial physiologist Fons Stams of linkurl:Wageningen University;http://www.wageningenuniversity.nl/UK/ in the Netherlands, who was not involved in the research. "What is not [well] known is that such a process can be coupled to growth and energy conservation." But these new findings show that "there's energy produced which [these archaea] are able to catch" -- adding to recent evidence that this reaction can support microbe growth after all. Formate is the simplest carboxylate anion, consisting of just one carbon, one hydrogen, and two oxygen atoms, and is an important player in fermentation reactions and anaerobic digestion. Some microbes are known to convert formate to methane and CO₂ to produce energy, but the conversion of formate into hydrogen and CO₂ wasn't thought to release enough energy to support microbial growth. The problem, it seemed, was that the accumulation of hydrogen inhibits the reaction from continuing to occur. But in special circumstances, this may not be the case. In 2008, for example, Stams and his colleagues showed that in microbial communities where methanogens -- archaea that produce methane as a byproduct of metabolism -- live in partnership with fermentative bacteria that convert formate to hydrogen and CO₂, the methanogens consume the hydrogen product of the reaction, allowing the reaction to continue. In doing so, enough energy is generated to support the growth of the bacteria. The new study provides the first evidence that a microbe can derive energy from this conversion without the help of any partners. Sung Gyun Kang of the linkurl:Korea Ocean Research and Development Institute;http://www.moeri.re.kr/eng/ and his colleagues were sequencing the genome of a strain of the deep-sea hydrothermal vent archaea Thermococcus onnurineus when they noticed it contained many copies of formate dehydrogenases and hydrogenases thought to be involved in formate conversion reactions. To see if the strain could derive energy from breaking down formate, the team incubated the archaea in suspension at 80 degrees C with no energy source except formate. Indeed, they found that ATP was synthesized when formate was added, and that the strain was able to grow efficiently under the Spartan conditions. Control cultures lacking formate did not grow, and no ATP was detected. "[We were] very surprised," Kang told The Scientist in an email. "Despite my belief that there are a bunch of microbes still unknown to us, the finding of microbes [that] thrive [on] formate conversion to hydrogen did not seem easy. We feel vey lucky." The team tested other Thermococcus species and identified a handful that also possessed the ability to grow on formate. Most of the strains were isolated from deep-sea hydrothermal environments, suggesting that the extreme environment may in some way be contributing to the microbes' ability to derive energy from the reaction. "One of the characteristics of the hydrothermal vents is that they have a high temperature," Stams said. "At a higher temperature, higher hydrogen concentration can be tolerated. That may be what's happening in this paper -- there is not a strict need of efficiently removing the hydrogen." But the details of how the higher temperatures are contributing to the reaction's energy production are unclear, said Kang, adding that there is much to be learned from further study of these Thermococcus strains. "If this formate-metabolizing potential is found [to be more] ubiquitous in deep-sea vent area than we thought, then it might provide another player [that may have been able to] survive or thrive in the harsh condition resembling the early Earth condition," Kang said. In addition, the discovery of a microbe that can survive on formate while producing hydrogen may "provide more options to develop technology for biohydrogen production," he said. And perhaps most importantly, "this finding may allow the conceptual advance in understanding the contribution of formate-utilizing hyperthermophiles in the thermal vent environment." Y.J. Kim, et al., "Formate-driven growth coupled with H2 production," Nature, 467:352-5, 2010.
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[30th May 1994]