Phototrophy, diazotrophy and palaeoatmospheres: biological catalysis and the H,. C, N and O cycles

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Abstract
Photolithotrophy (use of light as energy source, CO 2 , as carbon source) and then diazotrophy (use of N 2 , as N source), became important as prebiotically-synthesized organic compounds were consumed. 13 C/ 12 C data suggest that ribulose bisphosphate carboxylase-oxygenase has dominated photolithotrophic CO 2 , fixation over the last 3.5 Ga. The long-term decline in atmospheric CO 2 , can be related to carbon isotope evidence for a CO, accumulating mechanism in aquatic plants over the last 100 Ma and of C 4 , metabolism in terrestrial plants over 10 Ma. Increasing biogenic O 2 , levels permitted, via an O 3 , UV screen, photolithotrophs to grow in high-light environments and, via O 2 , 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 CO 2 , fixation. Symbiotic N 2 , fixation in land plants partially effects these two couplings, and permits novel methods of dealing with O 2 , inactivation of nitrogenase.

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