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As far as I understand, cyanobacteria began performing photosynthesis long before plant cells as we know them arrived on the scene. But cyanobacteria do not seem to use polysaccharides in the same way as plant cells do (building materials, for example). So what evolutionary benefit did cyanobacteria gain from photosynthesis, that could have made them continue to produce oxygen generation after generation?

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But cyanobacteria do not seem to use polysaccharides in the same way as plant cells do (building materials, for example)

  1. The Calvin-Benson cycle produces glucose which is the starting material for a lot of biosynthetic pathways including that of the nucleotides (ribose from the pentose-phosphate pathway). Glycolytic intermediates are also involved in the synthesis of amino acid, isoprenoids etc. Essentially all the carbon that cyanobacteria contains comes from fixation of carbon (from CO2) via photosynthesis.
  2. Cyanobacteria also store starch as an energy reserve (Suzuki et al., 2013). They also produce other polysaccharides (Phillipis and Vincenzini, 1998). Moreover, the peptidoglycan cell wall has a "glycan" part which is a glucose derivative (a polysaccharide).
  3. Moreover, cyanobacteria cannot produce ATP via the photosynthetic electron transport chain in the dark and ATP in dark conditions is generated (though significantly less) via substrate level phosphorylation in glycolysis.
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Why do you think that the only benefit from photosynthesis is polysaccharide synthesis? Photosynthesis allows an organism to convert photons into chemical energy. That chemical energy can be stored as polysaccharides and used as a building material, but it can also just be converted into some other compound, or just used to run the organisms metabolism, which is the real significance of photosynthesis and the reason it supports most of the world's ecosystems.

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  • $\begingroup$ I just know polysaccharides are a big deal. I am asking what other things cyanobacteria would use photosynthesis for that made it powerful enough to keep doing it at that scale. $\endgroup$
    – Henry Stone
    Commented Apr 27, 2016 at 16:30
  • $\begingroup$ Literally everything an organism does requires energy. Photosynthesis provides an electrochemical gradient that can be used to make ATP - which is then used in many, many chemical reactions throughout the cell as an energy source. DNA synthesis requires ATP. RNA synthesis requires ATP. Protein synthesis requires ATP. $\endgroup$
    – Dermot Harnett
    Commented Apr 27, 2016 at 16:35
  • $\begingroup$ So the ATP proces did exist in early cyanobacteria? Was it mitochindrial? $\endgroup$
    – Henry Stone
    Commented Apr 27, 2016 at 16:53
  • $\begingroup$ ATP is found in all known organisms, so there's every reason to think it would have been. Bacteria don't have mitochondria, so no, it wouldn't have been mitochondrial. $\endgroup$
    – Dermot Harnett
    Commented Apr 27, 2016 at 17:51
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    $\begingroup$ Mitochondrial oxidative phosphorylation and cyanobacterial photosynthesis are really two special cases of electron transport chains, which come in endless forms. They all follow the same principle, namely the transfer of high energy electrons redox acceptors, favourable reactions which release energy that can be harnessed. The donor of high energy electrons can be anything (NADH in oxphos or P700 for photosynthesis) and the electron acceptor just has to have a more negative redox potential than the donor. As long as the transfer of the electron from the donor to the acceptor releases.. $\endgroup$
    – Jory
    Commented Apr 28, 2016 at 6:07

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