11. The Origin of Oxygenic Photosynthesis and Its Impact on the Atmosphere

The evolutionary history of photosynthesis is complicated by the existence of several pathways and two major types: anoxygenic and oxygenic. The former is thought to have preceded the latter by a highly uncertain time interval. Evidence for photosynthesis is found in both the fossil record (in the form of stomatolites) and in the isotope record (in the form of carbon isotope variations). Stromatolites indicate the presence of bacterial mats capable of photosynthesis as early as 3.5 BYBP, whether anoxygenic or oxygenic has not been (cannot be?) determined. Carbon isotope signatures indicate an antiquity perhaps 200-300 million years greater. Oxygenic photosynthesis very likely began by 2.7-2.8 BYBP based in molecular fossils thought to indicate the presence of cyanobacteria. Sulfur isotopes indicate the general oxidation of Earth’s atmosphere at about 2.3-2.4 BYBP, some few hundred million years after the advent of oxygenic photosynthesis. Given that the probable source of atmospheric oxygen is from cyanobacteria presents a problem in this long delay in oxidizing the Earth’s surface and atmosphere.

The carbon isotope record shows long-term consistency of carbon isotopes in organic sediment and carbonate sediment, implying a long-standing ratio of inorganic (carbonate) carbon to organic carbon. The delay in oxidation of the surface environment is puzzling if fixed (photosynthetic) carbon has been in a more or less constant proportion to carbonate carbon assuming photosynthetic processing of carbonate is responsible for both the organic carbon and the oxygen at the surface. This problem is in addition to the delay in oxidation noted above. At least a partial explanation may lie in nutrient limitations that would have prevented the cyanobacteria from generating the implied amount of both biomass and free oxygen. Examination of such nutrient limitations shows that they are inadequate over the necessary time interval; there is simply too much available nutrient in too short a time to account for the delay in oxidation. It is especially interesting to note that one of the important nutrients, phosphorus, can be utilized by various cyanobacteria in more chemically reduced forms than the simple phosphate (P=+5 oxidation) which is prominent today. This may be a further indication that primitive photosynthesizers may have been especially well adapted to the more reduced conditions on the early Earth. Attempts to explain the delay in oxidation by trapping of CO₂ in rocks are inadequate because of the rapid cycling of CO₂ through the geochemical systems, with a residence time in likely reservoirs of only about 50 million years, compared to 300 million years for the oxidation delay. The likely answer to this problem is longer term storage of carbon in abiotic reduced compounds which only slowly become available to the photosynthetic system

The conversion of the reduced carbon inventory into carbonate/carbon dioxide for photosynthetic processing takes place via subduction of organic compounds into the upper mantle, reaction of same under iron-free magmatic conditions (with production of free hydrogen) and release of the CO₂ and hydrogen as volcanic emissions. This process can easily take hundreds of millions of years since subduction of organic sediments is probably relatively inefficient compared to the size of the total organic carbon pool, most of which is not in the form of deposited sediment. On the other hand, the rate of production of hydrogen (and other hydrogen-carrying compounds such as methane and ammonia) could have been sufficient to produce overall oxidation of the reduced compounds (by ultimate hydrogen loss to space) in perhaps a billion years or so, to provide for the necessary production of the carbonate rock inventory.

Consideration of the implications of an organic compound-rich early Earth suggests a more complicated model for evolution of carbon reservoirs and interpretation of the carbon isotope record. Instead of starting with CO2 as the main (sole) carbon reservoir, with conversion by photosynthesis producing both organic deposits and complementary oxygen, starting with organic carbon (abiotic) as the main carbon reservoir provides a mechanism to avoid problems with the stable isotope record, the delay in oxidation following the advent of oxygenic photosynthesis, the requisite conditions for generating prebiotic compounds in quantity, and the evolution of surface conditions on a suitable time scale. Adding this complicating factor to geochemical and atmospheric modelling has the potential to produce more realistic (if more complex) models that are consistent with the geological and geochemical records