What do swamps and flatulent cows have in common? They're big sources of methane, of course. But while cows seem to let off as much of the potent greenhouse gas as they produce, swamps hold on to most of it. Now researchers say they've solved this methane mystery.
When it comes to climate change, carbon dioxide gets all the press. But the second most abundant greenhouse gas is methane, and molecule-for-molecule it's a far more efficient heat-trapper. The biggest sources of the explosive gas are bacteria that thrive in the acidic muck of peat bogs. But strangely, they seem to produce far more methane than they release into the atmosphere. So where does the excess go?
A clue comes from the fact that most of the carbon in peat bogs ends up in the form of peat, an accumulation of dead sphagnum moss. The methane discrepancy could be explained if the living moss plants gobble up the gas as it's produced. But one snag in this explanation is that plants don't have the biochemical tricks needed to oxidize methane into a form they could use for energy.
Suspecting that the moss may be getting help from a microbe, a team led by Jaap Damsté and Marc Strous, microbiologists at Radboud University in Nijmegen, the Netherlands, and the Royal Netherlands Institute for Sea Research in Den Burg, respectively, gave moss plants a vigorous washing and then probed them for the presence of bacterial RNA. Sure enough, they found telltale sequences of methane-munching bacteria and also found dense clusters of the bacteria within the plant's tissue.
Methane turns out to be a major food item for sphagnum moss, accounting for as much as 15% of the plant's carbon, the team reports 25 August in Nature. Experiments with radioactive methane revealed that bacteria living in the moss rapidly converted the gas into carbon dioxide which was then sucked up by the moss. Stroud argues that the moss-bacteria symbiosis may be as important for global nutrient cycles as the nitrogen fixation of legumes and their root nodule microbes (ScienceNOW, 5 September 2003). "This is elegant and exciting work," says Jeffrey White, a climate scientist at Indiana University in Bloomington. White says the study shows that protecting peat bogs from environmental degradation is crucial because they could release enough green house gas to profoundly disrupt climate, not to mention put the cows to shame.