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STScI Astrobiology Lecture Series

Anoxygenic Growth of Cyanobacteria on Fe(II) and their Associated Biosignatures: Implications for Biotic Contributions to Precambrian Iron Formations (and Mars?)

Presented by: Niki Parenteau  (NASA Ames Research Center/SETI)
Category: Science Colloquia   Duration: 2 hours   Broadcast date: October 03, 2014
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The evolution of oxygenic photosynthesis and the resulting oxygenation of the atmosphere and oceans was arguably one of the most important events on the early Earth. In addition to setting the stage for the evolution of higher eukaryotic life forms, oxygen serves as a planetary-scale remotely detectable biosignature when searching for life on other planetary bodies. Cyanobacteria are the most evolutionarily ancient oxygenic phototrophs and use water as an electron donor for photosynthesis, producing oxygen as a waste product. However, it is thought that cyanobacteria didn’t immediately acquire the ability to oxidize water. There is a large difference in the redox potentials between water and hydrogen and sulfide commonly used by the more ancient anoxygenic phototrophs. Members of our group have speculated that an intermediate reductant such as Fe(II) could have bridged the gap and acted as a transitional electron donor before water. The widespread abundance of Fe(II) in Archean and Neoproterozoic ferruginous oceans would have made it particularly suitable as an electron donor for photosynthesis. We have been searching for modern descendants of such an ancestral "missing link" cyanobacterium in the phototrophic mats at Chocolate Pots, a hot spring in Yellowstone National Park with a constant outflow of anoxic Fe(II)-rich thermal water. We present the results of our physiological ecology and complementary biosignature study, which revealed that the cyanobacteria grow anoxygenically using Fe(II) as an electron donor for photosynthesis in situ.