Photosynthetic and Atmospheric Evolution
Questions re Sleep's paper
1. Joe Kirschvink
Please clarify why you think that the Photosystem II oxygen evolving complex developed on land.
Sleep. An organism with Photosystem II combined on land with another organism wth Photosystem I in our sequence.
2. Roger Buick
What about the "Canfield Ocean"?
Sleep. We have ocean that is oxic near surface by anoxic at depth from 2450-1800 Ma. Our sequence differs with that of Canfield in that the deep ocean is suboxic after 1800 Ma.
3. Howard Griffiths
With regard to your argument for the terrestial origin for the microbial state of the ambient photosynthesis, what about UV? Surely microbes in that situation would have been largely susceptible to the UV load that was reaching the earth's surface. Is there any evidence that extant microbes of that ilk have a capacity to tolerate UV?
Sleep. We note that Deinococcus, which has the ability to repair DNA, is in the clade of Terrabacteria. In addition, water and translucent rock provide some UV protect.
4. Euan Nisbet
Jim Lovelock pointed out 20 years ago that a methane smog blocks UV. To get liquid oceans you really have to have a lot of methane in the atmosphere to give you a greenhouse effect and then the likelihood of a methane smog is quite high. This would make the land much more habitable than you would otherwise think.
Sleep. Agreed, thanks.
5. John Allen
With regard to priorities the idea that photosystem I and II are homologous (related) was first proposed to my knowledge by Nitschke and Rutherford in a Trends article in 1992, and then quite explicitly on the basis of the emerging structures by Schubert et al. in the Journal of Molecular Biology in 1998.
Sleep. We cited only one recent work on our slides for brevity. Note that Professor Barber discusses this topic extensively in his paper in this volume.
Allen. I have a question regarding your terminology, looking at this from the point of view of redox chemistry, and particularly when you said that when oxygen evolution first appeared there was no need for an oxygen acceptor. I don't understand 'acceptor' here. And is hydrogen sulphide and oxygen donor?
Sleep. Sorry for the confusion. We use ‘acceptor’ to designate the molecule where the oxygen released by anoxygenic photosynthesis ends up. For example, sulphide is becomes an oxygen acceptor when it is oxidised to sulphate. There is not a good word here and we realise that biochemists use the term differently.
6. Bill Martin
Carl Woese did in fact propose that mitochondria might have arisen from anoxygenic photosynthetic bacteria in the 1978 paper, but that was at a time when the role of oxygen was not as well understood as it is today. His suggestion was that the oxygen respiratory chain actually arose in mitochondria and that it was then exported to bacteria in the environment. So we have to be careful….
Sleep. We use only the functional part of the original hypothesis. That is, the ancestral mitochondria may have acted analogously to modern chloroplasts by performing anoxygenic photosynthesis.
Martin. My comment is that there have been a lot of models in the literature always focusing on the advantages of mitochondria mostly in terms of ATP synthesis of overall yield or oxygen scavenging, etc. The problem is that all the anaerobic eukaryotes we see today that do not have mitochondria should be acquiring them for exactly the same reasons and they don't, which causes us to doubt how compelling these arguments actually are. The origin of mitochondria was a singular event, so it is one thing to acquire an endosymbiont and another thing the transition from an endosymbiont to an organelle. The endosymbionts are a dime a dozen or tens of thousands in modern biota.
Sleep. We discuss mitochondria only before there was much di-oxygen in the air and not their subsequent evolution.
7. Miles Osmaston
From your study of the depositional environment of the 1.9 - 1.8 Ga banded iron-formations have you found that they differ in any way from those BIF deposited at, say, 2.4 Ga, before the rise of atmospheric oxygen?
Sleep. This would be an interesting research topic.
8. Kevin Zahnle
The suppression and disappearance of the MIF sulphur isotopic signal can be accounted for by of three things: and oxygen rise, a disappearance of a reduced gas, or the disappearance of volcanic sulphur in lower emanations from volcanoes. We have written a paper where we argue that it was the lower amounts of a reduced gas, particularly methane. My question is how you calculated the two very specific estimates of oxygen in the atmosphere, 800 - 900 ppm, which was based on mass balance.
How stable would that be? The respiration lifetime of 800 ppm oxygen would be 100,000 years at the most.
Sleep. We gave mass balance between the amount of oxygen carried from the surface mixed layer to the deep ocean with the amount of oxygen consumed by massive sulphide deposits at the ridge axis. We calculated the partial pressure of oxygen in the air (1 bar total pressure) in equilibrium with our concentration in the mixed layer. We needed to assume a reasonable turnover rate for the ocean and a reasonable global seafloor spreading rate for this factor of a few calculation. I apologise for being confusing here by retaining extra digits. Our point is that a small increase in the concentration on oxygen in the surface ocean changed the deep ocean from anoxic to suboxic. We agree this transition could have been complicated.
9. Paul Falkowski
The reason oxygen is stable on those time scales is that once you have a kinetic mechanism for producing it based on a biological process which has unlimited substrate in terms of water, the convective overturn of mantle, which is really controlling the amount of reductant that you are putting into the atmosphere, is much less capable of competing with the kinetics of the production of the oxygen. As the mantle cools fewer and fewer reductants are going into the atmosphere. This is what maintains a stable oxygen once you have got over the hump. The problem is getting over the hump going from an anaerobic to an aerobic world. Once there, there is no geological evidence that it reverts.
Sleep. We agree that the transition needs to be considered in more detail.
10. ?
I was just wondering whether you can make any suitable rate measurements on systems like the Black Sea, which is anaerobic as you go down and full of sulphide.
Sleep. Yes for the reaction of sulphate with organic matter. One could also measure the rank of anoxygenic photosynthesis in modern environments.