Photosynthetic and Atmospheric Evolution
Questions re Martin's paper
1. Norm Sleep
Just to clarify what I said this morning. The point of the hypothesis I mentioned
was that the mitochondrion originally acted as a photosynthetic organism in an anoxic
situation and did anoxygenic photosynthesis and then got called tp for some of these
other things that I did not attempt to discuss. In that case the capture would have to
occur a little bit further back than you put it, but there is no conflict with either of the
presentations that were made after that.
Reply: Yes, anoxic photosynthesis has been suggested a couple of times in the
literature, by Carl Woese, Dennis Searcy and Lynn Margulis. I have gone on the
record in print a few times suggesting that the ancestor of mitochondria might have
had a collection of genes more similar to that of say Rhodobacter or Rhodosprillium
species, which are good facultative anaerobes in addition to being good facultative
photosynthesizers, but what is not found so far in the genomes of heterotrophic
eukaryotes are retained genes that would offer any evidence for the role of
photosynthesis in the association that led to the establishment of the mitochondrial
endosymbiosis. What we do see are many preserved relicts of the facultative
anaerobic lifestyle, which is what the hydrogen hypothesis predicted, and the main
issue with regard to the anerobic, heterotrophic eukaryotes.
2. Euan Nisbet
Two questions:
(i) A small one about the antiquity of the planctomycetes which basically refers
both to your talk and the one before. How old do you think they were and what were
they doing in all of this..?
Reply: The planctomycetes have very diverse and complicated collections of
genes, including genes involved in C1 metabolism that they share with methanogens.
There are some erroneous, I fear, claims in the literature that some planctomycetes
have a structure that is homologous to the eukaryotic nuclear membrane, as I have
pointed out in print elsewhere (Curr. Opin. Microbiol. 8:630-637, 2005). The
planctomycetes are certainly an interesting group of prokaryotes, but I think that if
they, as a groups had had anything to do with the origin of eukaryotes, genome data
will uncover such a link, which has not happened so far.
(ii) Linked to that, molybdenum and nitrogenase (Paul called it Darwin's eye).
How do you evolve nitrogenase?
Reply: How do I evolve nitrogenase? Any easier questions? My view would be
that expressed with Mike Russel in these pages in 2003 and 2007, that the transition
metal sulfide clusters at the active site of Mo-dependent nitrogenase are biologically
portable forms of inorganic catalysts that were doing the job at the origin of life in an
alkaline hydrothermal vent setting, with reducing power for dinitrogen reduction
ultimately stemming from serpentinization.
3. Akiko Tomitani
You mentioned that the closest cyanobacterium to chloroplasts maybe a
filamentous, heterocystous nitrogen fixing organism. Apart from the phylogeny, is
there any evidence to support this idea?
Reply: The phylogenetic and gene distribution evidence to which you refer has
now recently appeared (Mol. Biol. Evol. 25:748-761, 2008). But so far we have found
no obvious trace of ancestral nitogen fixation in plant genomes. There is abundant
evidence for the role in nitrogen metablism in plastids, of course, including the
GOGAT genes in some red algal plastomes, but evidence for the actual process of
nitrogen fixation has not been uncovered to my knowledge. With oxygen in primary
producing marine environments, the environmental availability of nitrate may have
altered the nature of N availability. Your own recent published work in PNAS
(103:5442–5447, 2006) regarding the evolutionary timing of heterocyst formation
during cyanobacterial evolution is highly relevant to our suggestion.
4. John Allen
I think Paul's got a point about serpentinisation not making NH3. I think the
II
Fe /FeIII redox potential is not sufficiently low. But the other point - where did
nitrogenase come from - it is very clearly one of Wolfgang Nitschke's redox
construction kit for the last universal common ancestor, at least based on conserved
redox chemistry binding sites of known proteins.
Reply. Right. I think we all agree that serpentinization does make H2, and that the
reduction of N2 with H2 to NH3 is an exergonic process (Amend JP, Shock EL,
FEMS Microbiol. Rev. 25: 175-243, 2001). I do not know of direct evidence for
ammonium production in hydrothermal vents, I suppose it is a matter of having the
right catalysts there. Schoonen and Xu (Astrobiology 1: 133-142, 2001) reported N2
reduction to about 10 microM NH3 in the presence of 12 mM H2S and Fe2+ under
mild simulated hydrothermal vent conditions (120 °C) in 2-7 days, I’ll have to leave it
at that. And regarding Wolfgang Nitschke’s redox construction kit, I would say that
yes, no doubt, nitrogenase would be one of the genuinely ancient enzymes,
completely agree on that.