The length of the transition period from the reducing to the neutral biosphere

Nitrogen fixation as evidence for the reducing nature of the early biosphere

Probably the first nitrogen fixers were anaerobic, non-photosynthetic, bacteria, i.e. fermenters. During the evolution of N2 fixation they still needed nitrogen on the oxidation level of ammonia. Because of the complexities in structure and function of nitrogenase this evolution must have required a long time. The photosynthetic and later the respiring bacteria inherited the capacity for N2 fixation from the fermenters, but the process did not change a great deal when it was taken over. Because of the long need for NH3, which is unstable in a redoxneutral atmosphere, a long-persisting reducing atmosphere was needed. The transition to a redoxneutral atmosphere, dominated by CO2, H2O and N2, cannot have been rapid, and the NH3 in it was recycled. Probably the atmosphere contained for a long time, as was suggested by Urey but is often denied now, a great deal of methane as a reductant. The recycling occurred with participation of intermediates like cyanide, through energy input as UV radiation or as electric discharges. A stationary state was set up. The hypothesis is recalled that coloured, photosynthetic, NH3 bacteria, analogous to coloured sulphur bacteria, may have existed, or may still exist, in reducing conditions. A few remarks are made about the origin of nitrification in the later, oxidizing atmosphere.

The synthesis of some lipid-like structures in simulated primeval Earth conditions

Starting from a simple gas mixture rich in methane, suggested as being a model of the primeval atmosphere for a certain time period, we succeeded in synthesizing lipid-like compounds. The hydrophobic nature of the products was demonstrated by IR, NMR, MS, elementary analysis, gel filtration measurements and solubility tests. The hydrophobic products self-assemble in stable, uniformly sized microspheres possessing membrane-like properties. They can selectively retain biologically active compounds. The wall-membrane of the microspheres shows a supramolecular order.

Evolutionary considerations on the thermodynamics of nitrogen fixation

An evolutionary explanation is sought for the fact that ATP is needed for N2 fixation in spite of the exergonicity of the process. After a survey of the state of knowledge about the thermodynamics of N2 fixation in fermenters, photosynthesizers and respirers it is suggested that nitrogenase, which still shows ATP-dependent hydrogenase activity, evolved from an ATP-requiring hydrogenase that lacked nitrogenase activity. The hydrogenase action in the Archaean, reducing, biosphere may have needed ATP to ensure expulsion of H2. Extant non-nitrogenase hydrogenases have lost the dependence on ATP. Because of its complexity, nitrogenase could not rid itself of the ATP dependence or of hydrogenase activity, both wasteful. Presumably all hydrogenases evoled from ferredoxin-like Fe-S proteins.

Model experiments on nitrite and nitrate in simulated primeval conditions

In experiments on the prebiotic formation of nitric oxides, anoxic mixtures of N2 and water vapour were sparked in contact with phosphate buffer solutions at various pH values. Nitrite was found in the aqueous phase, and nitrate grew from it, presumably by reaction with H2O2. In acid solutions, these anions were reduced and destroyed by Fe2+, and the same was true of nitrite in solutions kept at a pH value similar to that of the contemporary ocean (8.2) with HEPES buffer. Nitrate was not destroyed in short-term experiments, but as in sparking nitrate is formed only vianitrite, neither anion could accumulate. In further sparking experiments with alkaline sulphide, both nitrite and nitrate were reduced entirely. It is concluded that it is unlikely that the primeval ocean contained appreciable concentrations of nitrite or nitrate either at the reducing or at the redox-neutral stage.