NISBET, E. G., Dept. of Geology, Royal Holloway, Univ. of London, Egham TW20 0EX United Kingdom, e.nisbet@gl.rhul.ac.uk.

Oxygenic photosynthesis, coupled tightly with nitrogen fixation, is the manager of the modern atmosphere. When and how did this begin? Carbon isotopes imply that rubisco has controlled the global distribution of carbon in the atmosphere-ocean system for at least 3.5Ga, selectively fractionating carbon into the biosphere from an abundant atmospheric reservoir. Modern biochemical reactions of carbon capture (including nitrogen fixation) already operated very productively by then. Anoxygenic photosynthesis may substantially predate oxygenic. The first life was most likely non-photosynthetic, existing on the redox contrast between the atmosphere-ocean and the mantle.

The biochemistry of key housekeeping enzymes may suggest evolutionary history. Many are metal proteins, especially with Fe-S clusters, including some key proteins in photosynthesis. Nitrogenase uses Fe-Mo, urease uses Ni. The oxygen-evolving complex is Mn-based. Such clues suggest photosynthesis began in and around hydrothermal systems, possibly originally as an accessory, facultative, process. .Support comes from the role of heat shock proteins, essential for assembly of rubisco. A possible speculation is that Haem may have come from an alkaline system, perhaps around ultramafic volcanism.

Perhaps infrared thermotaxis, in a hydrothermally supported organism, allowed the start of anoxygenic photosynthesis, followed by the development of oxygenic photosynthesis in a symbiotic chimaera in a microbial mat. With the evolution of cyanobacteria, capable not only of anoxygenic and oxygenic photosynthesis, but also nitrogen fixation, life could escape the hydrothermal ghetto and occupy the planet. Walker-world intervals (air more reduced than sediment) may have occurred, perhaps many times, but after 3.5Ga, Earth has probably in general had relatively oxidised air, though without abundant free molecular oxygen until the Proterozoic.

Earth System Processes - Global Meeting (June 24-28, 2001)
Session No. G3
Geological Evolution of the Earth System: Precambrian to Early Paleozoic
Edinburgh International Conference Centre: Pentland
10:00 AM-3:00 PM, Monday, June 25, 2001