"Jim Menegay"  wrote in message
news:c8114v$2bio$1{at}darwin.ediacara.org...
> RobertMaas{at}YahooGroups.Com wrote in message
news:...
> > > From: jimmenegay{at}sbcglobal.net (Jim Menegay)
> > > My speculation (that early major Fe+++ deposits were directly
biogenic,
> > > rather than indirect via biogenic O2) is based on two observations:
> > > 1.  The biochemistry of oxidizing H2O to O2 is very complex.  The
> > > biochemistry of oxidizing Fe++ to Fe+++ is much simpler, and seems
> > > to be integral to all electron transport chains.  While Fe++ was
widely
> > > available, there seems to be little incentive for life to develop the
> > > complex machinery for oxidizing H2O.
> >
> > Are you suggesting that when there was plenty of Fe++ dissolved in the
> > ocean, life used the simpler biochemistry to directly convert it to
> > Fe+++, which slowly precipated out of the ocean? But until it was
> > seriously depleted, there was enough of it to go around, so some other
> > resource was consumed to the limit, leaving none of it availble as
> > surplus for other forms of life that didn't need Fe++ to use. But when
> > enough of the Fe++ was gone and volcanic activity had reduced to the
> > point where it wasn't being re-supplied enough, Fe++ became the
> > limiting nutrient in the ocean, which caused all life dependent on
> > consuming it to decline in abundance, leaving some of other nutrients
> > available for other forms of life to grow on?
> >
> > Some of these other forms of life first chanced upon a way to convert
> > sunlight into useful energy, using H2S and other sulfur compounds as
> > the hydrogen donor, which led the way to eventual discovery of a
> > variant of that method to use H2O as the hydrogen donor, which suddenly
> > filled the atmosphere with O2 and filled the ocean with dissolved O2
> > which suddenly eliminated the already low level of Fe++ in the ocean,
> > changing the previous longterm slow problem into a sudden crisis?
>
> Whoops.  "I didn't read the confession before I signed, your honor"!
> My story is that the organisms in the first paragraph were
> photosynthetic too.  Fe++ to Fe+++ oxidation is not an energy source
> in an anaerobic environment.  Also, the Fe+++ may have been directly
> deposited into minerals, rather than being released into solution to
> precipitate independently.
>

Google: "anaerobic iron oxidation".  Can be coupled to nitrate reduction or
phototrophy under anoxic conditions (in fact this is exactly when you would
expect this to be most useful -- under oxic conditions, aqueous & O2-driven
ferrous iron oxidation/precipitation is going to be a serious competitor).

But would nitrate have been present to any appreciable degree prior to the
appearance of O2 in the atmosphere (and subsequent abiotic nitrogen fixation
by lightning)?  IIRC, isotopic data suggest it was not, but my "IIRC" here
is a few years dated -- anyone have an update?

One more note to Robert Maas -- this idea of a transition or "variant" from
H2S utilization to H2O utilization by proxy is completely incorrect;
biochemically, these two mechanisms are totally unrelated.  I think Larry
Moran posted a well-written and informative reply to you on this
misassumption in another thread.
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