Quoted message said:Do you have a reference for the "several proteins shy of its modern composition" statement?
An in press paper by De Las Rivas et al. (Trends in Plant Science, Jan 2004) highlights a few
examples of this (specifically the cyanobacterial and chloroplast/plastid oxygen evolving complex
proteins). Also any of several great papers by William Martin (google: Bill Martin chloroplast) on
inferring the protein content of the cyanobacterial/chloroplast ancestor, and other evolutionary
themes in general.
Quoted message said:I was aware of the atmospheric O2 difference, thats what I was fishing
for.
Quoted message said:But I have not read anything on the subject for a quarter century, but the cyanobacteria did not
live in the atmosphere, AFAIK. Chloroplasts live inside cells, so they have their distinct niche,
obviously. I studied mycorhizal/plant root bacteria briefly, but not the genetics.
Definitely cyanobacteria did not live in the atmosphere, sensu stricto, but they did have to live in
the photic zone, within the reach of sunlight. So this implies perhaps some shallow water habitat
that was in equilibrium with the atmosphere and so probably had plenty of oxygen around. There are
some nice current ideas in geology about a Precambrian (well, neoproterozoic) ocean that was
probably something like the modern Black Sea -- oxygenated within the upper layer/photic zone but
still highly sulfidic (anoxic and reduced) below this level. Cyanobacteria and early algae would
have occupied the surface layers and more primitive, obligately anaerobic bacteria would have been
dominant further down in the depths.
Quoted message said:I'm speaking of the niche of the "Archean cyanobacteria". Is there a
living
Quoted message said:cyanobacterium that lives in the same niche? What about the thermos?? Is there any habitat (or
microhabitat) that exists on earth today that is indistinguishable, qualitatively, from that of 2
billion YA? that is,
with
Quoted message said:the same temp, light, chemicals, anaerobic, etc. If so, then why would
those
Quoted message said:successful organisms change? The only answer I can think of is mutation drift.
These are all good questions, and certainly microbiologists have spent countless hours looking for
such living fossils. So there are modern cyanobacteria that can live anaerobically, shutting off
oxygenic photosynthesis and oxidizing hydrogen sulfide (instead of water). These have even been
compared to what an early Earth cyano would have been like
(e.g. by Schopf in several of his books, though his evidence for early Archean cyanobacteria is much
more controverted now than it was just a few years ago). But the main problem is, all known
cyanobacteria have exquisite and very complex systems for dealing with molecular oxygen at the
levels produced within the cell (ground zero of O2 production, right?). These adaptations
would not have been required until after the development of oxygenic photosynthesis, and so
even if Schopf is right and his microfossils are cyanos that simply haven't learned to oxidize
water yet, they are going to be completely different beasts than what we see today.
There are also thermophilic cyanobacteria (IIRC, 72 deg. C is the max known temperature for
photosynthesis) e.g. the emerald green color in many of the hot springs of Yellowstone, but these
look more or less just like any other modern cyano that have figured out how to function at high
temperatures.
Quoted message said:Finally, if you are a geneticist, which species living today is "least different" genetically from
the presumed 2 billion year old version?
Another good question. Among cyanos, members of the genus Gloeobacter exhibit many "primitive"
characteristics and are almost invariably early branchers on phylogenetic trees, and so this was
once argued to be the "oldest living cyano". But based on a recently completed genome, these guys
are really not much different (based on protein content and ultrastructure) from other
cyanobacteria, and so we're still left hunting for a more primitive organism that, as you are
absolutely correct in suggesting, may be hiding out in one of these niches.
An interesting aside, there has been several recent attempts to date the appearance of modern groups
of bacteria based on genetic analyses (and using geological calibration points). Though there are a
lot of arguments why these molecular clock techniques will fail when extrapolating so far back in
the Earth's history, its worth mentioning that most of these have the modern groups of cyanobacteria
appearing only around 1.5 billion years ago.
Assuming that date is correct, either our sampling sucks so far, or none of the earliest cyanos have
survived to modern times.
