Re: Is there any non-chlorophyl/non-cyanobacteria photosynthesis?


"Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx> wrote in message
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"William Morse" <wdmorse@xxxxxxxxxxxx> wrote in message
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"Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx> wrote in
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[Re: article by Falkowski in the Mar 24, 2006 Science and Morse's
claim that the article supports an early infra-red based form of
photosynthesis.]


Fell over this thread a few days late (as usual), but thought I might add
$.02...

Apparently I didn't read the article very carefully, because I didn't
notice that claim ... As far as I know, the single-photosystem
H2S-splitting variant of photosynthesis in the green sulfur bacteria
based on bacteriochlorophyll uses visible light. I had never heard
of a kind of photosynthesis predecessor using infra-red.

(Also relevant to Bill Morse's posts...) There are a number of anoxygenic
phototrophs that absorb quite strongly in the (near) infrared. IIRC the
"record" is out at around 980nm, though the more typical range is 800-900nm.
Note that this is absorption of the reaction centers, not antenna
pigments -- most of these bugs can also absorb higher energy photons, which
are then energetically "funnelled" down to the lower energy/higher
wavelength absorbing reaction center (i.e. reaction centers are always
"redshifted" from antennas so that this energetic funnel functions). The
main point though is that this long wavelength/infrared energy is
substantial enough to drive photochemistry, and is still found in quite a
few microbes.

There was an article 10 or so years ago in Nature (Euan Nisbet?) arguing
that photosynthesis could have evolved in close proximity to a hydrothermal
vent, based on a notable overlap between bacteriochlorophyll absorption and
the water-attenuated blackbody radiation curve (sort of a nontrivial point
of maximal returns -- moving from visible -> near IR -> IR, water absorption
increases, energy per photon decreases, but blackbody photon flux increases.

But, Bill's point is right -- seems like phototrophs would be much happier
near the surface, no? Something to do with UV damage?


You may be right and the article wrong, but the statement is "These
electrons, extracted either with the release of energy or with the aid of
low-energy (infrared) solar photons" referring to electron transfer from
the electron donors H2, H2S, and CH4 to the acceptors CO2 and SO4. The
statement is reinforced with the later statement that "oxygenic
photosynthesis was confined to a thin film of sunlit water on the surface
of the planet"

Ok. I interpret that as a speculation based on a mistaken argument
from thermodynamics. At modern-day global concentrations of CO2, there
is no plausible concentration of H2 or H2S which makes reduction of
CO2 to biomass energetically favorable. So, there must have been
some supplementary energy source, and infra-red photons are a somewhat
plausible possibility.

Certainly CO2 and H2 were much more concentrated in ancient environments.
CO2 fixation energetics also become favorable (exergonic for something like
14/20 standard amino acids, IIRC) as temperature increases towards 100 deg.
C. I might be misreading, but I'm not so sure a supplementary energy source
must have been present -- not that any self-respecting microbe should turn
down free photons :).

But the argument must be flawed because modern methanogens make a
(precarious) living precisely by reducing CO2 to biomass (and beyond)
using H2 and no supplementary infra-red or geochemical energy source.
At least they seem to! It appears that they circumvent the thermodynamic
argument by using high local concentrations of CO2 - much higher than
the atmospheric level.

Ah I see... I think in fact we're making the same argument! (I swear I read
the post before I started typing, though apparently I'm too tired to
process!?)

My own viewpoint is that early pre-photochemical life probably
either used geochemical sources of energetic electrons such as
Fe(m) -> Fe++, or high local concentrations of H2 or H2S at vents,
or a source of easy to eat CO instead of CO2, or else used high
global levels of CO2 (as high as 0.5 bar). If living organisms
had ever developed the ability to extract useful energy from infra-red
photons, I would expect that we would still see this technology in
use today in certain niches. And, as far as I know, we don't.

Also re: the oxygen/evolution article. My take was that there are actually
many parallel adaptations that occurred when oxygen became available, but
that the "genetic signature"(?) for these suggested they were probably
horizontally transferred (as opposed to inherited from e.g. the last common
ancestor).. But I wonder if this isn't to be expected, given what's known
about the timing of the appearance of oxygen in the atmosphere versus the
divergence of the major domains of life?


.

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