Add this one to the “things we didn’t realize about living creatures” file. Everyone’s heard of methane, the simplest hydrocarbon, as a component of natural gas and as a potent greenhouse gas when it’s floating around in the atmosphere. It’s long been known that there are “methanogenic” organisms, which are all single-celled Archaea, that produce the gas as part of their own metabolic cycles. This is “swamp gas” methane, from decomposition of organic matter, and these same species are found in mammalian digestive tracts where they’re responsible for (often inadvertant) emissions of methane as well.
It’s long been considered as a marker of life – thus the controversies over methane detection in the Martian atmosphere. There’s definitely methane on Mars, but how it’s produced (and when) and where it goes are all the subject of great speculation. It seems that there’s a very low constant amount in the atmosphere, but with occasional bursts whose origin is yet unexplained. The readings from ground measurements from the various rovers versus orbital detection have not lined up well, which had led to further complications. And just because there’s methane being produced doesn’t mean that it absolutely has to come from microorganisms, either: over the decades it’s become clear that there are several abiotic routes to methane formation. Some of these occur under volcanic conditions, which are probably not relevant to present-day Mars, but there are lower-temperature nonvolcanic routes as well. One of these is thought to be a low-temperature Sabatier reaction (hydrogenation of carbon dioxide), and you can’t rule out Fischer-Tropsch type reactions (hydrogenation of carbon monoxide), either. Both of these are of course run industrially at high temperatures with metal catalysts, but there are a lot of ways to get those reactions to occur, and slow production in huge masses of rock could add up to quite a bit of methane.
You can settle the biotic-versus-abiotic methane origin question by careful isotope measurements – the source of the hydrogen atoms in the resulting methane matters a great deal, for one thing, because in enzymatic production it comes from CH and SH bonds and in the abiotic routes it comes from metal-hydrogen bonds, with very different kinetics. Deuterium atoms and plain hydrogens will have subtle rate differences that show up in the eventual methane product (there can be isotopic signatures in the carbons, 12C and 13C as well). Unfortunately, we don’t have any instruments on Mars that are capable of these sorts of measurements, and it looks now like we’ll have to have a successful sample return mission to Earth to resolve the question. Knowing the complexity of that plan, I fully expect its timeline to slip and I will consider myself lucky to live to see the results!
But here on Earth, it turns out that methane is even more of a signature of life than we realized. This recent paper presents evidence that it’s produced in all sorts of organisms, not just those weirdo Archaea, and via oxidative pathways. There have been many reports over the years of bacteria and other cells producing small amounts of methane, but this seems to be a unifying hypothesis for how this happens. All sorts of common cellular constituents (such as glucose, pyruvate, etc.) can be turned into S-methylated or N-methylated intermediates, and these react with iron(II) ions and hydrogen peroxide (oxidative stress conditions) to produce methyl radicals via Fenton chemistry. Those methyl radicals can grab a hydrogen from the first thing they see, and there’s your methane. The Fenton reaction might not be something that you usually think of occurring in your cells, but it certainly does (as a source of hydroxyl radicals for apoptosis processes and more). The new paper does indeed show that human cells (and plant cells and everything else) are capable of methane production, especially under high metabolic stress.
That’s a different route entirely than the (anaerobic) methanogen organisms use – they handle the single-carbon as a formyl group off some only-in-those-species compounds called methanofurans. If there is indeed biotic methane leaking out on Mars, I would pick that over the aerobic metabolism route, just because there’s not much oxygen floating around there, but who knows? Let’s hope that someday we find out. . .
