Photosynthesis in the Archean Era
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The earliest reductant for photosynthesis may have been H2. The carbon isotope composition measured in graphite from the 3.8-Ga Isua Supercrustal Belt in Greenland is attributed to H2-driven photosynthesis, rather than to oxygenic photosynthesis as there would have been no evolutionary pressure for oxygenic photosynthesis in the presence of H2. Anoxygenic photosynthesis may also be responsible for the filamentous mats found in the 3.4-Ga Buck Reef Chert in South Africa. Another early reductant was probably H2S. Eventually the supply of H2 in the atmosphere was likely to have been attenuated by the production of CH4 by methanogens, and the supply of H2S was likely to have been restricted to special environments near volcanos. Evaporites, possible stromatolites, and possible microfossils found in the 3.5-Ga Warrawoona Megasequence in Australia are attributed to sulfur-driven photosynthesis. Proteobacteria and protocyanobacteria are assumed to have evolved to use ferrous iron as reductant sometime around 3.0 Ga or earlier. This type of photosynthesis could have produced banded iron formations similar to those produced by oxygenic photosynthesis. Microfossils, stromatolites, and chemical biomarkers in Australia and South Africa show that cyanobacteria containing chlorophyll a and carrying out oxygenic photosynthesis appeared by 2.8 Ga, but the oxygen level in the atmosphere did not begin to increase until about 2.3 Ga.
Keywordsanoxygenic photosynthesis bacteriochlorophyll banded iron formation Buck Reef Chert chemical biomarkers chlorophyll cyanobacteria Fortesque Group evaporites ferrous iron hydrogen isotope fractionation Isua Supercrustal Belt microfossils oxygenic photosynthesis proteobacteria reaction center reductants for CO2 fixation stromatolites Swaziland Sequence Warrawoona Megasequence
banded iron formation
Peedee belemnite standard
ribulose bisphosphate carboxylase
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I thank Minik Rosing (Geology Museum, Copenhagen) for showing me his rock samples and for teaching me some basic geology. I also thank Mette Miller (University of Southern Denmark) and Don Bryant (Pennsylvania State University) for critical comments on earlier versions of this article. Thanks are also due to two anonymous reviewers for additional valuable comments.
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