The Evolution of Photosynthesis

The accumulation of small heritable changes within populations over time.

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The Evolution of Photosynthesis

#1  Postby lpetrich » Dec 07, 2017 4:52 am

Photosynthesis has evolved at least twice on our planet, and I like to call those instances retinal photosynthesis and chlorophyll photosynthesis.

Retinal photosynthesis evolved among halobacteria, some Archaea that live in very salty conditions. It pumps hydrogen ions out of the cell membrane, thus doing a part of its cell's chemiosmotic energy metabolism. It now has very limited distribution, though it may have been more common some 3 - 2 billion years ago: the "purple Earth hypothesis".

But the best-known kind is chlorophyll photosynthesis, well-known for being done by chloroplasts. These organelles are descended from long-ago cyanobacteria, so it is there that we must turn for its origins. It has this form: electron transfer along a chain of:

Water splitting -> Photosystem II -> Photosystem I -> biosynthesis

where the two photosystems have antenna complexes with chlorophyll and various other photosynthetic pigments, like carotenoids. But some other bacteria can do photosynthesis, though they use only one of the two photosystems.
  • Purple nonsulfur bacteria -- Ps II -- Hydrobacteria > Proteobacteria > Alpha-proteobacteria > Rhodospirillaceae
  • Purple sulfur bacteria -- Ps II -- Hydrobacteria > Proteobacteria > Gamma-proteobacteria > Chromatiales
  • Green sulfur bacteria -- Ps I -- Hydrobacteria > Chlorobi > Chlorobiaceae
  • Green nonsulfur bacteria -- Ps II -- Terrabacteria > Chloroflexi > Chloroflexus aurantiacus
  • Heliobacteria -- Ps I -- Terrabacteria > (Gram-positive) > Firmicutes > Clostridia > Heliobacteriaceae
Cyanobacteria themselves are in Terrabacteria.

I've found this paper from 2000: se360001724p - Xiong2000.pdf attempting to untangle the evolution of photosynthesis by finding the family tree of the proteins involved in it. They find three main groups:

(cyanobacteria-chloroplasts, heliobacteria), (green sulfur bacteria, green nonsulfur bacteria), (purple nonsulfur bacteria)

with no clear hint as to how they are related. With the superphyla and photosystems, this is

(Ter I-II, Ter I), (Hyd I, Ter II), (Hyd II)

So either photosynthesis was lost numerous times or else it was spread by lateral gene transfer. Working from the latter hypothesis, we can try to guess which were donors and which were recipients. According to the paper, heliobacteria and purple bacteria have big clusters of photosynthesis-related genes, something convenient for being a donor. But neither Chlorobium nor Chloroflexus have big clusters, though they have small clusters of closely-related photosynthesis-related genes. So those two organisms were likely recipients. This leaves us with:

(Cyanobactera: Ter I-II, Heliobacteria: Ter I), (Purple Bacteria: Hyd II)

Carbon Fixation in Phototrophs -- both cyanobacteria and purple bacteria use the Calvin cycle, and other photosynthetic bacteria use other CO2-fixing pathways.

Tracking molecular evolution of photosynthesis by characterization of a major photosynthesis gene cluster from Heliobacillus mobilis (a heliobacterium), The cyanobacterial genome core and the origin of photosynthesis

The second paper's authors propose that the first chlorophyll photosynthesizer was some ancestor of cyanobacteria, a "procyanobacterium". It used only photosystem I and it did not release oxygen.

Some procyanobacteria then donated photosynthesis genes to an ancestral heliobacterium and to some proteobacterium. The latter organism modified the photosystem to II and it preferred to use the Calvin cycle for carbon fixation. Thus becoming a purple bacterium.

Later, some purple bacterium donated its Photosystem-II and its Calvin-cycle genes to some procyanobacterium, making it more like present-day cyanobacteria. The final step was to add water splitting and feed its electrons into PS II.
lpetrich
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