Microbe Finds Hydrogen Yummy

Scope Correspondent

Deep in swampy salt marshes, among the knotted roots of the cord grass, lives a special microbe—the first living organism that scientists have ever directly shown to be attracted to hydrogen gas.

“It is a new finding that there is [attraction] to hydrogen,” said Reinhard Wirth of the University of Regensburg, who was not involved in the recently published research. “This is the first time this has been reported.”

In the low oxygen environment where this microbe, Methanococcus maripaludis, resides, “hydrogen is a currency exchanged between cells,” explained Barny Whitman of the University of Georgia, who isolated and cultured the first laboratory strain of the microbe but was not part of the recent study. Understanding how organisms respond to that environmental currency could ultimately allow them to be genetically manipulated for human use, perhaps as sources of biofuel.

This particular microbe is one of several microscopic swamp denizens that eat hydrogen to make methane, making swamps the largest source of methane on the planet. Already thirty or forty years ago, scientists speculated that methane-making microbes might be seeking out and swimming towards the hydrogen made by other organisms in their environment as a means of feeding themselves, said Matthew Fields of Montana State University, the senior author of the paper describing the research.

Until now, however, no one had created the system necessary to test the hypothesis: an oxygen-free glass capillary with a trickle of hydrogen gas above the liquid home of the hydrogen-starved bug. In the thin tube, the hydrogen-deprived microbes swam faster towards the source of hydrogen than their hydrogen-sated cousins.

That seems consistent with earlier studies by John Leigh at the University of Washington and Bishwarup Mukhodpadhyay at Virginia Tech which showed that both this microbe and a related species ramp up the production of their swimming organs when put in a low hydrogen environment.

Given the long-held assumption that the microbes were attracted to hydrogen, the study’s outcome was not really a surprise. “Of course it might be a good idea for a bug to swim towards its energy source,” said Wirth.

Whitman and Leigh, who was also not part of Fields’ study, found the data for the attraction to hydrogen convincing but were surprised by the relatively low affinity for its life-giving gas. “It suggests they’re waiting for hydrogen levels to be extremely high,” said Whitman.

Wirth, who has studied the swimming behavior of M. maripaludis and similar organisms, was surprised for another reason. “I was astonished that maripaludis swims rather slowly,” said Wirth, who found that under his conditions the microbe swam about ten times faster. But laboratory studies of the organism may not fully represent the complexity of its behavior in its home environment, the salt marshes of South Carolina, where Whitman said it is a minority among the hydrogen-eating microorganisms.

Since hydrogen “mediates a lot of interactions between microbes,” Fields hopes that knowing more about both individual microbes and their relationships with others could one day lead to a new method of biofuel production. The methane made by the microbe and its swamp neighbors is essentially equivalent to natural gas, said Leigh.

But before these microbial systems become a viable source of fuels for human use, M. maripaludis and similar organisms would likely have to be genetically altered to be more efficient at making methane. That requires a better biological understanding than scientists have at present. Leigh and his collaborators have done some of the necessary genetic work by sequencing the microbe’s entire genome. In the process, they identified some genes likely to be involved in movement towards a chemical attractant.

Given the new finding that M. maripaludis swims towards its hydrogen food source, Leigh now wonders whether these genes might be important for the hydrogen response demonstrated in Fields’ study. That could easily be tested by putting microbes with defects in these genes in the setup designed by Fields to measure their swimming speed towards hydrogen.

Using swamp microbes as a source of biofuel may be far away, but Fields’ experiment shows there is plenty left to discover about their biology and environment. “Even the fundamentals of understanding novel biology are quite interesting,” said Fields.