In what was probably one of the most fascinating and entertaining sessions of the meeting (if the press conference was anything to go by) , on Wednesday afternoon Samantha Joye of the University of Georgia presented her audience with the biggest bacteria in the world -- a bug so big you can see it with the naked eye. The bacterium in question, about half a millimetre across, is just one of many wonders that live in and around methane seeps on sea bottoms the world over. This particular one was found in the Gulf of Mexico, at a seep called Bush Hill, and it looks a lot like the current big-bug record holder, Thiomargarita nambiensis, which comes from the seas off Namibia. Like its Namibian relative, the Bush Hills big bug is big because the nitrate it needs to oxidise the suphides it eats is sometimes copious and sometimes very hard to come by, and when it's copious the bacteria like to store up as much as they can to get through the lean times to come.
The bacteria and archaea that live in these strange methane-rich environments are utterly fascinating and still very mysterious. The most intriguing of all are the anaerobic methane-oxidising guys. Most microbes that eat methane do so with the help of oxygen. The anaerobic ones make use of sulphates instead. If you're looking at all this from a martian point of view, that's a potentially rather interesting trick, because Mars has very little oxygen, but as Opportunity has been demonstrating it has a great deal of sulphate to offer. If there are bugs on Mars making methane, then there might well be others turning it into carbon dioxide with the help of sulphates If that's the case the methane that makes it to the atmosphere would be only a fraction of the methane actualy produced, and the martian biosystem might be a fair bit bigger than it would appear at first blush.
Back on earth, what's fascinating about the anaerobic methane-oxidising guys is that they are mixtures of prokaryotic bacteria (what we used to think of as everyday bacteria) and archaea, which look pretty similar to the untutored eye, but are actually very different beasties. As I understand it the denizens of these two separate microbial domains don't normally seem to mix particularly intimately, though studies of rhodopsin, among other things, show that they do swap genes now and then. But when it comes to eating methane they are peculiarly co-dependent. The archaea start of the methane oxidation, and the sulphate-using bacteria finish it off. According to Antje Boetius, one of the discoverers of this process, and someone who seems to be associated with more or less every academic institution the great city of Bremen has to offer, everywhere where there's methane to be eaten you get intimate assemblages of bacteria and archaea cooperating to get the job done.
The archaeans in these partnerships are particularly interesting, because they seem to be pretty closely related to the archaea that make methane in the first place -- the methanogens. Though the idea is still apparently a fairly strongly contested one, Boetius thinks it looks as if, under the right conditions -- such as the presence of an oxidiser such as sulphate -- the chemical pathways with which some archaea make methane can be put into reverse and used to break it down. Under some circumstances making methane produces energy; under other circumstances, or in other company, using that methane up can produce energy: the archaea can play it either way. Without wanting to put too scatological a slant on it, these are organisms whose primordial enzymatic eat-what-you-make cleverness may have entitled them to wear the world's first shit-eating grins -- grins as wide as that of Mr Nye's fox dining from a wire brush.
It's fascinating and mysterious, and just a little bit scary. As Boetius points out, the anaerobic methane eaters are performing what could be a key role in controlling the global methane cycle; 80% of methane produced in ocean sediments is oxidised this way. If there are circumstances in which these bugs that are so helpful in eating methane might start producing it instead, it would be nice to know about them. It's not a big concern -- the idea that the vast amounts of methane that are stored in deep sediments might burst out and unleash greenhouse havoc is a low-probability high-impact climate risk that I don't find particularly worrying, and this seems to be a variant on that. But the idea that there are weird cross-domain microbial collaborations we have barely begun to understand performing key roles in our planet's biogeochemistry would be a sobering one, were it not for the giddy fun involved in trying to learn more about it.