There’s an interesting paper in Proceedings of the National Academy of Sciences this week which adds a small new twist to a great double-headed mystery concerning the beginning the history of photosynthesis on earth (which thus comes pretty near the middle of Eating the Sun: it’s a book with a rather arcane relation to chronology.) When did photosynthesis of the oxygen producing persuasion start, and why did the oxygen produced not immediately turn up in the atmosphere?
There’s pretty hard-to-doubt evidence that the earth’s atmosphere did not have a significant amount of oxygen in it on a permanent basis before about 2.45 billion years ago. (Some people do manage to doubt this, but they have to work fairly hard in order to do so — most people in the field accept it.) At the same time, shales dug out of a borehole in Australia contain various chemicals that suggest a) that there were cyanobacteria, which produce oxygen, living 2.7 billion years ago and b) that some other creatures were using this oxygen for their own metabolic needs. There are persistent rumours of similar results from even older shales, but as far as I know they haven’t actually made it to publication.
This creates a seeming paradox: how can it be possible to have a biosphere that produces oxygen through photosynthesis and at the same time see no oxygen in the atmosphere. And how can such a state of affairs go on for at least 250 million years, and quite possibly a lot longer. The bottomline of most of the answers to this conundrum is that there was enough other stuff — organic carbon and reduced gases from the mantle — for the oxygen to react with that it could never build up stably in the atmosphere. Only when the supply of this “other stuff” was changed, through some mixture of an increased rate of burial of organic carbon, a lessened flux of reduced gases from the mantle, and an increased oxidation of the atmosphere as a whole due to the escape of hydrogen into space, did it become possible for oxygen levels to rise, at least a little. This fascinating interplay between life and the planet it enlivens is gone into in some detail in the book: David Catling has provided a helpful pdf update on the state of the debate for those who want more.
The new paper, Biosynthesis of 2-methylbaceriohopanepolyols by an anoxygenic phototroph (PNAS doi/10.1073/pnas.0704912104), comes out of the labs of Alex Sessions and Dianne Newman at CalTech, of and from which many good things are heard. The lead author is Sky Rashby, for more on whom see this post; also on the team is Roger Summons of MIT, who with colleagues published the evidence for cyanobacteria at 2.7 billion years ago (Newman has just moved to MIT, too). To some extent this new paper challenges that evidence, in that it removes one strand from it. Cyanobacteria make chemicals called 2-methyhopanes, and until now nothing else has seemed to do so, and so the presence of those chemicals in the ancient shales seemed good evidence for cyanobacteria. The new paper shows that a bacterium called Rhodopseudomonas palustris also makes the molecules in question. R. palustris is a photosynthesiser, too — but not an oxygen producing one (there is a wide range of bacteria that use sunlight this way without producing oxygen). So the idea that there were cyanobacteria present long before there was atmospheric oxygen can no longer rely on the methyhopanes for support.
At first blush, this might seem to give comfort to those who, like Joe Kirschvink, also at Caltech, want to argue that oxygen-producing photosynthesis led to the oxygenation of the atmosphere directly — that an evolutionary transition led directly to a geological one, as outlined in this PNAS paper. But the methyhopanes aren’t the only evidence for an earlier origin. What was particularly interesting about the 2.7 billion year old shales Summons and his colleagues studied was that they contained both what seemed to be evidence of oxygen producing cyanobacteria and what seemed to be evidence of oxygen consumption, in the form of molecules called steranes. Producing steranes is thought by most people to require oxygen. And so it’s not special pleading to say that while cyanobacteria are no longer the only possible source for the 2.7 billion year old 2-methyhopanes, the oxygen requiring steranes in the same sample make them the most likely source, and pretty much require that something photosynthetic somewhere was producing oxygen at the time. (There is also, as I understand it, evidence elsewhere that oxygen-requiring methane eating bugs were around before the oxygen hit the atmosphere. Not all methane eaters require oxygen — but some do, and apparently there are molecular markers that can tell you which sort was present where.)
So the idea of a lag between the evolution of oxygen production and the arrival of free oxygen in the atmosphere has at worst only taken a minor knock; from what I can gather most people in the field are still fairly convinced on the matter. It would take a second dramatic finding — for instance evidence that the steranes in the 2.7 billion year old samples are contaminants — to really throw things into doubt. As far as I know there is no indication that any such second shoe is about to drop.
Meanwhile, the molecules involved in this identity parade may well prove interesting in and of themselves. The fact that they are present in two different photosynthetic bacteria suggests that they might have a photosynthetic purpose of some sort. What that might be remains to be seen, though there are already some theories. If the genes for the pathway that makes the stuff can be found, then it might be possibleto be more definitive about which bugs use it and under what conditions.
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