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> > The first photosystem to evolve, PSI, uses light to convert carbon
> > dioxide (CO2) and hydrogen sulfide (H2S) to glucose. This process
> > releases sulfur as a waste product.
>
> In that case, it's obvious that all the oxygen in the resultant glucose
> came from the CO2, since there isn't any oxygen in H2S in the first
> place. See an analagous question later...

I don't want to monopolize the thread in multiple posts, so briefly this FAQ
appears to be 10+ years outdated on some crucial pieces of information.
This ascending ladder of PS I -> PS II -> oxygen-producing photosynthesis is
not accepted, and the post is incorrect on which organisms use which type of
photosystem (PS I: green sulfur, Gram +; PS II: purple bacteria, green
nonsulfur; cyanobacteria, algae, and plants have both types).  A good recent
reference on this is R.E. Blankenship, Molecular Mechanisms in
Photosynthesis (2002 or 3, I think).

> But sulfur is needed in some amino acids etc. So does that sulfur get
> incorporated, as I presume, by a completely different pathway that
> existed long before photosynthesis evolved? Did it, at that time, come
> from H2S, or from what else?

In general, sulfur assimilation can proceed from a variety of different
sulfur redox states (sulfate, sulfite, sulfur, sulfide, polysulfide) to
sulfide, which is the direct donor onto cysteine.  Cysteine can be
converted, through a variety of different enzymatic pathways, into other
necessary biosynthetic pathways (methionine synthesis, iron-sulfur cluster
assembly, etc.).  Different organisms have vastly different mechanisms of
getting sulfur across the membrane -- and not all organisms can use all
species, i.e. the toxicity of H2S to many organisms -- but this central
cysteine hub is essentially universal.


>
> > About a billion years later, a second photosystem (PS) evolved,
> > probably from a duplication of the first photosystem. Organisms with
> > PSII use both photosystems in conjunction to convert carbon dioxide
> > (CO2) and water (H2O) into glucose.
>
> It would seem reasonable that this variation on the original invention
> would likewise get all the oxygen (for the glucose product) from the
> CO2, not from the H2O. Recent isotope-labeled experiments have
> confirmed this, or so I've heard in the InterNet rumor mill. But I
> haven't been able to find any FAQ or authoritive WebPage that says this
> clearly. Does anybody know of such? This question arose in relation to
> a completely different topic, on sci.space.science
> http://www.google.com/groups?selm=HByrLz.2vJ%40spsystems.net
> where nobody seemed to know the answer for sure, so I thought I'd ask
> the question here and get some good answer(s) then copy the answer(s)
> back there.

Again this misunderstanding arises from colloquialisms in the FAQ.  CO2 and
H2O are not assimilated directly into glucose (this is a bit like saying my
pushing the gas petal in my car is what makes it go; operationally useful,
but a gross technical understatement).  Cyanobacteria, algae, and plants all
use rubisco for CO2 assimilation into phosphoglyceraldehyde, which can be
used in gluconeogenesis among a variety of different things.  In this
process, oxygens are picked up from both CO2 and from H2O.  Importantly
though, photosynthesis in several of the other groups of primitive organisms
mentioned (green sulfur and nonsulfur, Gram +) doesn't drive the fixation
carbon by way of rubisco.  These organisms all do autotrophy via other,
arguably earlier evolving pathways such as reductive TCA and the
3-hydroxypropionate pathway.  Once again, oxygen atoms can be picked up both
from CO2 but also through H2O.
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