Photolithotrophy (use of light as energy source, CO2, as carbon source) and then diazotrophy (use of N2, as N source), became important as prebiotically-synthesized organic compounds were consumed. 13C/12C data suggest that ribulose bisphosphate carboxylase-oxygenase has dominated photolithotrophic CO2, fixation over the last 3.5 Ga. The long-term decline in atmospheric CO2, can be related to carbon isotope evidence for a CO, accumulating mechanism in aquatic plants over the last 100 Ma and of C4, metabolism in terrestrial plants over 10 Ma. Increasing biogenic O2, levels permitted, via an O3, UV screen, photolithotrophs to grow in high-light environments and, via O2, availability for respiration and biosyntheses, to produce large and (on land) homoiohydric plants. This greatly increased the productivity and diversity of photolithotrophy. Diazotrophy is less strongly coupled to homoiohydry and to photochemistry than is CO2, fixation. Symbiotic N2, fixation in land plants partially effects these two couplings, and permits novel methods of dealing with O2, inactivation of nitrogenase.

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