Re: Article: Bacterial Evolution Down in the Depths


"Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx> wrote in message
news:ddh9nn$2mhb$1@xxxxxxxxxxxxxxxxxxxxxx
>
> "Larry Moran" <lamoran@xxxxxxxxxxxxxxxxxxxxxxx> wrote in message
> news:ddg4uv$26v6$1@xxxxxxxxxxxxxxxxxxxxxx
>> On Mon, 8 Aug 2005 14:07:33 -0400 (EDT),
>> IRR <iotarhorho@xxxxxxxxxxxxxxxxx> wrote:
>> > Again strictly speaking, this photo-driven H+ transport is termed
>> > phototrophy and not photosynthesis. The former is "harvesting solar
>> > energy"
>> > in the broad sense, the latter only applies to organisms that can use
>> > that
>> > harvested energy to fix carbon (autotrophs, done by way of generating
>> > biological reducing equivalents, e.g. NADH). ...
>>
>> The essence of photosynthesis is the use of light energy to create a
>> proton gradient across a membrane. ...
>
> I found the following online resource which seems to agree with
> Larry on the definitions, and which also has lots of good info on
> photosynthesis and micro-organism biochemistry, including a good
> page about photosynthetic halobacteria.
>
> http://www.science.siu.edu/microbiology/micr425/Index.html#contents
>
> Unfortunately, it didn't tell me whether halobacteria are autotrophic.
> If not, what do they "eat"? It would have to be from something that
> also tolerates a lot of salt, right?

My understanding is that the definition of photosynthesis has been
obfuscated, especially in recent years with the discovery of
bacteriorhodopsin in Archaea and homologous proteorhodopsin in bacteria.
Howard Gest -- one of the premier thinkers in the photosynthesis
community -- recently wrote an article to clarify various misnomers,
archived on Govindjee's website:

http://www.life.uiuc.edu/govindjee/Part1/Part1_Gest.pdf

also, I'd recommend the online textbook of bacteriology for a more
up-to-date definition:
http://textbookofbacteriology.net/metabolism.html
(under the "phototrophic metabolism" section).

To paraphrase, *phototrophs* use light to generate chemical energy in the
form of ATP (photophosphorylation), thereby this includes cyanobacteria,
plants, and algae (all of which produce O2), also non-oxygen evolving
photosynthetic bacteria (several clades), and also halobacteria and some
proteobacteria that all have rhodopsin-based mechanisms for transducing
solar energy into a proton gradient.

*Photosynthesizers* use light energy, by way of generating reducing
equivalents (ferredoxin, NADPH, NADH), to directly assimilate cellular
material. This excludes bacteria that use rhodopsin, such as halobacteria.
I've had several people from this field point out to me that the sloppy
mix-up and misuse of these terms has caused considerable confusion.

I'd respectfully amend to Larry's definition that photosynthesis is also
essentially about generating an electron gradient, e.g. charge separation
across a membrane. In organisms with type I photosynthetic reaction
centers, this is done by "photoexcitation" of an electron through a series
of fast (nanosecond->millisecond) intermediate cofactor carriers, ultimately
to a soluble, intracellular protein, typically ferredoxin.

In type II RC-containing organisms, the electrons are passed to a
hydrophobic quinone carrier which is able to shuttle through the membrane to
various oxidized centers, e.g. on various transmembrane cytochromes and even
to the NADH dehydrogenase complex, where (in a "reversed" reaction from
archetype mitochondrial oxidative phosophorylation) generation of NADH from
oxidized NAD+ can take place, thereby leading to carbon fixation.

Because protons are always associated with these electron transfers (free
electrons are to be avoided in living systems), the proton gradient is
almost invariably a net result of photosynthesis, but at its essence, the
process is about moving electrons.

No halobacteria are known to be autotrophs. They grow mainly by amino acid
fermentation or by oxidation of TCA intermediates and pyruvate.


.

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