Photosynthetic and Atmospheric Evolution
Questions re Nisbet's paper
Joe Kirschvink
I'm still a bit sceptical about your carbon isotope arguments for photosystem II this goes way back in the geochemical literature to the 50s when various people in
Chicago worried about the fixation of carbon, rubisco and other inorganic means. The
problem comes in that the Fischer-Tropsch reactions, when you take hydrogen and
CO2 with metal catalysts like Ni, Fe or anything that will donate electrons nicely for
it, to yield organic products that are fractionated about the same as rubisco - rubisco is
simply doing its thing. It has equal access to the CO2 that is bouncing around and the
C12 goes through simply because the bond energies are not as strong as the C13
bonds, and so to say that this is rubisco and it's dependent on this and that - that's just
the world doing the way it should do.
Miles Osmaston
Re your reference to 2.8 to 2.3 Ga or thereabouts as the danger period when things
were varying a lot - I would contend that the danger period was primarily because the
interior of Mother Earth was going through a change of life in middle age.
Bob Tabita
This issue about rubisco being perfectly optimized by evolution - Yes, in the
organisms in which it evolves, but genes move through other organisms and they can
evolve differently, so I wouldn't take that as a static point.
Response by E.G, Nisbet:
The optimisation of Rubisco. Agreed: but biochemistry seems to be remarkably
conservative. The same basic reactions that support the biosphere today probably
supported it 2.7 Ga ago.
Bob Tabita
What I don't understand as a biochemist is how many samples do you look at,
what is statistically relevant, and when do you stop and when do you say this is the
way things happened. Or do you look at other samples? Is equilibration the same in
every location, at different temperatures and pressures?
Response by E.G. Nisbet:
Very detailed sampling of a suite of samples from an organic-rich rock unit should
give a reasonable understanding of what is preserved. The problems are in scale of
sample – a bulk sample homogenises all the microbial detail that may have
survived – and in getting a sample population that reflects the record. Nathalie
Grassineau’s techniques are very effective in dealing with this. Pressure is set by
nitrogen and hence nitrogenase. The problem is how you mobilize molybdenum .
Under oxic conditions, yes, under anoxic maybe no.
Paul Falkowski
That's a misconception. There are three nitrogenases. There's an iron-iron
nitrogenase, a vanadium nitrogenase and a molybdenum nitrogenase. Under perfectly
anaerobic conditions when molybdenum is a sulphide and is really inaccessible in the
water column, for the most part, you don't have to say - we didn't have molybdenum
so therefore nitrogen fixation didn't occur.
Tony Larkum
I want to make the point again about the timescale over which oxygen developed.
To me it seems that that took a very long time - there was a great experimental
development over millions of years. I see nothing in what you have said to dispute
with that. So that your have the experiment starting on stromatolites and then you get
the optimization of the oxygen evolution mechanism giving rise to the great
oxygenation at the end of that period of about 2.3 Ga.
Mark van der Giezen
Is it not possible to do a rough calculation of how much oxygen a bacterium
produces nowadays and the time you would need to fill the whole atmosphere, or
have a good stab at how many reducing equivalents there were around the planet to be
oxidized before oxygen levels could rise?
Joe Kirschvink
There is a big debate about whether many of these Archaean stromatolites might
be direct precipitates from the sea water. I wanted to make the point that if so it seems
that the ocean chemistry was much more towards saturation of the carbonate system
than it is today. There are very few experiments of what anaerobic photosynthetic
bugs do in that environment. We know that photosynthesis enhances carbonate
precipitation, which is why if you go to Shark Bay these things are concrete - it's a
process that's involved. One of our former students did an experiment as part of her
thesis where she actually grew these photosynthetic purple bacteria in ocean water
chemistry that mimicked the Archaean, or what the Archaean might have been, and
the carbonate precipitation went up by a factor of 10.
Howard Griffiths
A point about stromatolites from the living plant perspective. I think you got a
change of organic 13C from -10 ‰ at 2.2.8 Ga to -29 ‰ at 2.6 Ga or thereabouts. A
shift in composition like that in modern day organisms or whatever could be brought
about by three things:
(i) diffusion limitation, entirely consistent with a reduction in discrimination;
(ii) a different inherent fractionation factor being expressed by rubisco
(cyanobacteria 18 ‰ and higher plants 28 ‰);
(iii) the operation of a carbon concentrating mechanism.