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