jamenegay{at}ra.rockwell.com (Jim Menegay) wrote in
news:bvksnf$1pli$1{at}darwin.ediacara.org: 

> "irr"  wrote in message
> news:... 

I was orginally going to respond to "irr" - but then Jim followed and I 
thought I would try to kill two birds with one stone.
 
>> How much simpler can it get than to run biochemistry on
>> H2 and CO2?  But contrarily, aren't there a lot of ideas floating
>> around that methanogens are really a fairly late-emerging group of
>> organisms, and so the LUCA probably wasn't doing methanogenesis?

You may well be right about the emergence question. My familiarity with 
the subject dates from the 70's when I learned about nitrification, 
denitrification, and anaerobic digestion while studying agricultural 
waste management.  Just to clarify, there are methanogens that produce 
methane from H2 and CO2, and there are methanogens that produce it 
methane from  various carboxylic acids (typically formic,acetic or 
butyric). I was thinking of the latter as being more ancient - but 
checking my soil microbiology text (and some brief searching on the net)
indicated that they mostly use the same pathways: CH3COOH + 2 H2O -> 2 
CO2 + 8 H, then 8 H + CO2 -> CH4 + 2 H20. Although there is a 
Methanosarcina species that converts CH3COOH directly to CH4 and CO2 by 
decarboxylation. I could not easily find any discussion as to whether 
that mechanism might be more ancient than the hydrogenase pathway.

 
> One problem with methanogens is that the ~P energy yield is so low.
> Plus, the H2 probably wasn't available in sufficient quantities
> outside very specialized niches.  Modern methanogens are mostly
> dependent on biogenic H2 from fermenters.  However, it wouldn't
> surprise me if the strangely named enzyme "carbon monoxide
> dehydrogenase" of the methanogens and the acetogens was at the center
> of the LUCA's carbon fixation technology.  I've got to figure that a
> technique of fixing carbon that generates acetyl CoA directly is
> probably pretty ancient. Plus, I like the fact that it could have used
> CO as a carbon source originally and later switched to the more
> difficult and energy expensive use of CO2.

One site I checked indicated that the methanogens produce ATP via a 
different pathway than that used in fermentations - but this would 
indicate that if the methanogens were late-emerging, then that carbon 
fixing technology was not used by the LUCA. What I may try to follow up 
on is the anaerobic photosynthetic bacteria - the green sulfur bacteria 
may have been the first photosynthesizers.

> My guess is that the LUCA was oxidizing Fe++ to Fe+++.  We know that
> Fe was the main final electron donor around 2.3Gy ago - hence the red
> beds.  Perhaps the Fe was reacting with biogenic O2 or whatever.  But
> I'm guessing that it was oxidized directly by organisms.  Otherwise, I
> doubt that we would have so much use of Fe in various stages of modern
> electron transport chains.

> Anyone know what modern iron oxidizers are like?  I presume that they
> are anaerobes.  Are they strict?

No, in iron oxidation the iron is serving as an electron donor, with 
oxygen the electron acceptor - so it doesn't take place under anaerobic 
conditions. At the time my soil microbiology book was written, there was 
a considerable question as to whether most iron oxidizing bacteria got 
much energy from the reaction. In fact the difference in delta G is not 
enough to form ATP from ADP, except at very low pH. At that pH you do 
have the very interesting Ferrobacillus ferrooxidans and Thiobacillus 
ferrooxidans, which are responsible for acid mine drainage in 
Pennsylvania. But under anaerobic conditions, iron is used as an electron 
acceptor and Fe+++ is reduced to Fe++. Soil bacteria also solubilize 
metallic iron under anaerobic conditions. This would indicate that it was 
the production of biogenic O2 that led to the red beds, although bacteria 
probably helped the process along.

Yours,

Bill Morse
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