Hydrogen is considered a clean source of energy because its combustion "waste product" is water. Hydrogen must nevertheless be obtained from renewable sources, or its "green" moniker is somewhat of a sham.
Energy in the future might be obtained in part from what may be the ultimate renewable resource: cells. Bacteria have recently been studied as a source of electrical energy, and even as a source of hydrogen.
Jung-Hyun Lee, Sung Gyun Kang (Korean Ocean Research and Development Institute, and University of Science and Technology, Korea) and coworkers have found a microorganism which produces hydrogen, adenosine triphosphate, and bicarbonate from formate and water. They even grow under these conditions, generally considered insufficient for microbial growth.
What's unusual about this reaction?
Under standard conditions, the Gibbs free energy of the biochemical conversion of formate and water to bicarbonate and hydrogen is positive. In other words, more energy is put into the reaction than is obtained from it.
This reaction does happen in cells, especially in oxygen-deprived conditions. However, it's generally not enough to sustain microbial growth.
Two species of microorganisms can cooperate to support growth under such conditions, e.g. when one of the species consumes the hydrogen and therefore reduces the overall energetic cost of the reaction. Nevertheless, no single microbial species is known to grow on formate, e.g. produce high-energy molecules, via this particular reaction.
Rugged microbes.
The scientists studied archaea from the genus Thermococcus in their research. These cells live at unusually high temperatures (up to 80°C, i.e. 176°F), conditions which destroy membranes and enzymes from most other organisms.
It's reasonable to expect that these microbes feature some unusual biochemistry. Some of it has already proven itself commercially useful, e.g. in high-temperature biotechnology applications.
Furthermore, the genome of Thermococcus onnurineus NA1 possesses many genes which encode for formate-utilizing and hydrogen-producing enzymes. Some of these genes are located near one another, suggesting that they may be functionally linked.
Hydrogen and energy production.
The scientists cultured their microbes with formate, under oxygen-deprived conditions at 80°C, and monitored both hydrogen production and cell growth. Neither growth nor hydrogen production was observed in the absence of formate, but in its presence, both hydrogen and bicarbonate were produced.
Furthermore, the change in Gibbs free energy ranged from -20 to -8 kilojoules per mole (i.e. the reaction was energetically favorable), with the reaction becoming less favorable over the course of roughly 11 hours. However, the reaction was still favorable until at least 21 hours, and likely longer than that.
Therefore, in theory, this reaction could yield energy for adenosine triphosphate production. This is the main energy molecule for most cells.
When formate was added to a microbial suspension, adenosine triphosphate was indeed produced as a function of the extent of bicarbonate and hydrogen production, with a concurrent consumption of formate. Neither hydrogen sulfide, carbon monoxide, nor methane (other gases common to oxygen-deprived energy production) were observed, and adenosine triphosphate was not produced in the absence of formate.
How common is this ability among Thermococcus microbes? The scientists found four other Thermococcus species (out of 15 others studied) which were able to grown under the previously-discussed conditions observed for Thermococcus onnurineus NA1, likely a result of certain genetic similarities among them.
To determine the genetic basis behind these results, the scientists measured activity across the genome of Thermococcus onnurineus. They found the largest changes in one gene cluster relevant to formate and hydrogen production; seventy-two percent (13 of 18) of the genes were expressed at least two times above baseline levels, and disrupting suspected genes hindered microbial growth.
Overall evaluation.
The energy production system of Thermococcus onnurineus is exceedingly simple and rugged. It furthermore produces an extremely useful energy-storage molecule (hydrogen).
Future efforts should focus on harvesting the hydrogen it generates, genetically modifying the microorganisms to enhance hydrogen production to the maximum possible extent, and scaling up production to a level of maximum commercial utility.
NOTE: The scientists' research was funded by the Korean Ocean Research and Development Institute; the Ministry of Land, Transport, and Maritime Affairs; the Russian Academy of Sciences; and the Russian Foundation of Basic Research.
for more information:
Kim, Y. J., Lee, H. S., Kim, E. S., Bae, S. S., Lim, J. K., Matsumi, R., Lebedinsky, A. V., Sokolova, T. G., Kozhevnikova, D. A., Cha, S.-S., Kim, S.-J., Kwon, K. K., Imanaka, T., Atomi, H., Bonch-Osmolovskaya, E. A., Lee, J.-H., & Kang, S. G. (2010). Formate-driven growth coupled with H2 production Nature, 467 (7313), 352-355 DOI: 10.1038/nature09375