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SLAC National Accelerator Laboratory

Using X-rays to Find an Evolutionary Step in the Origin of Oxygenic Photosynthesis

July 2013 SSRL Science Summary by Manuel Gnida, SLAC Office of Communications


The evolution of oxygenic photosynthesis approximately 2.3 to 2.4 billion years ago revolutionized life on Earth. For most modern-day terrestrial life, oxygen has become indispensable. At the heart of oxygenic photosynthesis is the production of oxygen from water – a process mediated by the water-splitting manganese cluster of Photosystem II. Little is known about how oxygenic photosynthesis originally evolved, although some have hypothesized a manganese-oxidizing photosystem as a precursor step. Researchers from the California Institute of Technology, SSRL, and the Massachusetts Institute of Technology have now found geological evidence of this manganese-oxidizing photosystem occurring before the oxygen-producing system. The study was published in the journal Proceedings of the National Academy of Sciences USA

The researchers examined manganese deposits from a South African drill core, whose deposits took the scientists 2.415 billion years back in time. Soluble manganese (Mn2+) becomes insoluble when being oxidized to high-valent oxides, causing manganese to be deposited and concentrated in marine sediments. However, when analyzing these manganese deposits using EXAFS spectroscopy at SSRL’s Beam Line 4-1 and x-ray imaging at Beam Lines 2-3 and 10-1, the scientists found no sign of manganese oxides. Instead, manganese was present as very fine-grained Mn(II) carbonates. The texture and carbon isotopes of these carbonates indicated that the manganese was originally deposited as oxides and subsequently reduced in the sediments.

In addition to the x-ray studies, the research team examined independent indicators for oxygen (multiple sulfur isotopes and redox-sensitive detrital grains), which revealed that the original manganese oxides were indeed produced in an oxygen-free atmosphere. The compiled evidence thus points to a manganese-oxidizing photosystem that produced manganese oxides and existed before the emergence of water-oxidizing and oxygen-producing photosynthesis. 


Primary Citation

J. E. Johnson, S. M. Webb, K. Thomas, S. Ono, J. L. Kirschvink, and W. W. Fischer, “Manganese-oxidizing Photosynthesis before the Rise of Cyanobacteria”, Proc. Natl. Acad. Sci. USA 110, 11238 (2013); doi: 10.1073/pnas.1305530110

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Jena E. Johnson, California Institute of Technology (

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