Habitability of the early earth: Clues from the physiology of nitrogen fixation and photosynthesis

In the absence of direct evidence concerning the nature of the early Earth environments, it is acceptable under the uniformitarian principle to attempt to define primitive habitats from modern procaryotic physiology. Combining the rock and fossil record with present phylogenetic reconstuctions, application of this paleoecological approach to the evolutionary biochemistry and physiology of nitrogen fixation and photosynthesis leads to several inferences about the nature of Archean environments: 1. To stimulate nitrogenase evolution and avoid its repression, the activity of the NH ion was less than 10, and probably lower. 2. To be consistent with a moderately protective ozone screen, while not also repressing nitrogenase activity, incursions of abiotic dissolved oxygen at levels in the range 10-10 PAL would have been acceptable. 3. To induce the formation and activity of RuBP carboxylase, the pCO was less than 100 PAL. 4. To support Photosystem I activity, sulfide concentrations of at least 10 M were present in the photic zone. 5. To avoid a too-rapid oxidation of sulfide, the pH was probably between 6-7, where HS exceeds HS. Evolutionary 'pressure' to stimulate the later development of oxygenic photosynthesis (Photosystem II), would require several subsequent habitat modifications: 1. Lowering the sulfide to < 10 M to inhibit Photosystem I. 2. Raising the pH above neutral (HS > HS), to mediate more rapid oxidation of HS. 3. Maintaining either an illumination below 300-400 lux (to avoid photosynthetic O self-repression of nitrogen fixation), or an adequate local source of combined nitrogen (aNH > 10) to repress nitrogen fixation entirely. © 1985 D. Reidel Publishing Company.