wrote in message
news:c75ot3$2q6a$1{at}darwin.ediacara.org...
> http://www.google.com/groups?selm=bvksnf%241pli%241%40darwin.ediacara.org
> > From: jamenegay{at}ra.rockwell.com (Jim Menegay)
> > 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.
>
> The Earth was formed from many planetesimals that merged by
> gravitational attraction. Over time, the iron settled to the center,
> taking some sulfur etc. with it, and the rest seeped toward the top.
> For a long time hydrogen was probably seeping from the iron upward.
> With the very active geothermal and asteroid-impact stuff going on for
> a long time, I can easily imagine various places where more or less
> hydrogen was seeping out, probably enough in some places to supply the
> methanogens quite well. But I'm just speculating. Would an expert on
> planetary formation please comment? (Should I cross-post to
> sci.astro.something to try to find an expert?)
>
> > 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.
>
> Yeah, how to remove hydrogen from carbon monoxide. What does that
> enzyme really cause to occur anyway?
>
> > 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.

It's feasible CO was around in trace quantities from the very beginning, but
the real attractiveness of coupled CO dehydrogenase/acetyl-CoA synthesis is
the fact that CODH can operate reversibly -- coupling H2 oxidation to CO2
reduction to give CO, which is then used in acetyl-CoA synthesis (carbon
fixation), as Jim Menegay describes elsewhere.  I'm impressed by the rich
diversity of 1 carbon biochemistry, which can utilize simple compounds such
as CO2, H2, CO, and formaldehyde to do a variety of metabolic tricks.  It
may be more than coincidence that this chemistry is found in some of the
most ancient organisms on our tree of life.

> Speculating again: Heating of carbonaceous asteroid/comet material to
> produce mostly carbon charcoal/soot/sludge, then later partial
> oxidization of it, such as by heating in CO2 atmosphere, to yield lots
> of CO.
>
> > We know that Fe was the main final electron donor around 2.3Gy ago -
> > hence the red beds.
>
> Different from banded red iron layers, or basically part of same?

Same idea (oxidizing iron), but red beds are oxidized, iron rich ancient
soils and are different from banded iron formations.

>
> > Perhaps the Fe was reacting with biogenic O2 or whatever.  But I'm
> > guessing that it was oxidized directly by organisms.
>
> Well the banded layers we caused by alternating surplus and lack of
> free O2 to convert Fe++ to Fe+++, the latter which precipates out. I'm
> not sure whether the theory is that the oxygen from photosynthesis
> dissolved in the ocean water and freely reacted with Fe++, or whether
> Fe++ was used within the cells as a detoxifying mechamism for the O2
> being produced by photosynthesis. But even the latter seems different
> from what you are suggesting.

Free O2 is certainly the dominant idea; precipitating an excess of reduced
iron (Fe++) inside the cell during oxidation is not a good thing.
Detoxifying O2 (particularly superoxide) became a formidable problem during
this period, and you're absolutely right that Fe++ was a primitive solution.
Interestingly, Fe++ by itself does a reasonably poor job of detoxifying
superoxides, Fe complexed inside a heme moiety is about 1000-fold more
effective than that, and Fe complexed inside heme within the enzyme
superoxide dismutase is yet another 1000-fold better.

> > Otherwise, I doubt that we would have so much use of Fe in various
> > stages of modern electron transport chains.
>
> Fe++ was so common in the ancient ocean, and so useful complexed with
> organic molecules such as in heme, that I see no reason why there
> wouldn't have been several independent uses of Fe in early life, so
> that the two you mention could be independent rather than related as
> you suggest. Dumping O2 to Fe++ could be almost an inorganic process
> completely unrelated to Fe in various electron transport chains. Do we
> have an expert who actually knows?

Not claiming to be an expert by any means, but certainly the widescale
oxidation of iron around 2.4-2.2 billion years ago was inorganic as much as
anything (due to increasing atmospheric O2).  This is also the reason for
extensive red bed deposits at the same time.  There are different ideas on
the mechanism for banded iron formation (BIF) depending on when in history
you're talking about.  Early Archean BIFs (pre-O2) are thought to have been
formed by anoxygenic photosynthetic (and possibly other) organisms able to
use reduced iron as an electron donor.  Extensive BIFs are also seen around
2.2 billion years ago, when oxygenic photosynthesis "turned on", biomass
significantly expanded, and the Huronian glaciations occurred.  Yet another
series of BIFs are associated with the more recent snowball Earth episodes
around 800 million years ago.
---
þ RIMEGate(tm)/RGXPost V1.14 at BBSWORLD * Info{at}bbsworld.com

---
 * RIMEGate(tm)V10.2áÿ* RelayNet(tm) NNTP Gateway * MoonDog BBS
 * RgateImp.MoonDog.BBS at 5/8/04 12:00:16 PM