Using X-rays to Find an Evolutionary Step in the Origin of Oxygenic Photosynthesis
|
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
Related Links
- Science Highlight – HTML / PDF
- SSRL Science Highlights Archive
- SSRL Beam Lines
- News Release, California Institute of Technology
Contact
Jena E. Johnson, California Institute of Technology (jena@caltech.edu)
