What is it about?

Anaerobic green sulfur bacteria actually became more prolific and widespread as the oceans of the planet became oxygenated.

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Why is it important?

Counterintuitively, a photosynthetic organism that is a strict anaerobe expanded its ecological niche as oxygen increased in the atmosphere and oceans. This is because they require hydrogen sulfide for photosynthesis. Today, hydrogen sulfide is most commonly generated when sulfate reducing bacteria respire organic matter using sulfate as an electron acceptor. So it is sulfate that provides the sulfide that is used by the GSB. Geochemists think that the inventory sulfate in the oceans increased as the atmosphere gained more oxygen. More sulfate --> more places to form sulfide --> more opportunities for green sulfur bacteria to proliferate. I study fossil molecules preserved in ancient sedimentary rocks and some of the most distinctive chemical fossils come from photosynthetic pigments. Green sulfur bacteria make carotenoids that are not quite unique but almost. By studying how the relative abundances of these molecules have changed through time we can see a marked uptick at the end of the Proterozoic Eon. These studies help us understand how the composition of the ocean plankton, and the interconnections between the biogeochemical cycles of carbon and sulfur, have evolved over geological time.


I am excited to see how one can combine some relatively simple organic chemistry, with the knowledge and skills of colleagues who study microbiology and geology to illuminate the history of our planet. It's all about creative collaboration.

Roger Summons
Massachusetts Institute of Technology

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This page is a summary of: Niche expansion for phototrophic sulfur bacteria at the Proterozoic–Phanerozoic transition, Proceedings of the National Academy of Sciences, July 2020, Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2006379117.
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