P32B-05
Global Surface Photosynthetic Biosignatures Prior to the Rise of Oxygen

Wednesday, 16 December 2015: 12:08
2012 (Moscone West)
Mary Nichole Parenteau, SETI Institute Mountain View, Mountain View, CA, United States, Nancy Y Kiang, NASA Goddard Institute for Space Studies, New York, NY, United States, Robert E. Blankenship, Washington University in St Louis, Departments of Biology and Chemistry, St. Louis, MO, United States, Esther Sanromá, Instituto de Astrofísica de Canarias, Tenerife, Spain, Enric Palle Bago, Universidad de La Laguna, Departamento de Astrofísica, La Laguna, Spain, Tori M Hoehler, NASA Ames Research Center, Moffett Field, CA, United States, Beverly K. Pierson, University of Puget Sound, Biology Department, Tacoma, WA, United States and Victoria Suzanne Meadows, University of Washington, Seattle, WA, United States
Abstract:
The study of potential exoplanet biosignatures -- the global impact of life on a planetary environment -- has been informed primarily by the modern Earth, with little yet explored beyond atmospheric O2 from oxygenic photosynthesis out of chemical equilibrium, and its accompanying planetary surface reflectance feature, the vegetation “red edge” reflectance. However, these biosignatures have only been present for less than half the Earth’s history, and recent geochemical evidence suggests that atmospheric O2 may have been at very low - likely undetectable - levels, until 0.8 Ga (Planavsky et al., 2014, Science 346:635-638). Given that our planet was inhabited for very long periods prior to the rise of oxygen, and that a similar period of anoxygenic life may occur on exoplanets, more studies are needed to characterize remotely detectable biosignatures associated with more evolutionarily ancient anoxygenic phototrophs.

Our measurements of the surface reflectance spectra of pure cultures of anoxygenic phototrophs revealed “NIR edge(s)” due to absorption of light by bacteriochlorophyll (Bchl) pigments. We used the pure culture spectra to deconvolve complex spectra of environmental samples of microbial mats. We observed multiple NIR edges associated with multiple pigments in the mats. We initially expected only to detect the absorption of light by the pigments in the surface layer of the mat. Surprisingly, we detected cyanobacterial Chl a in the surface layer, as well as Bchl c and Bchl a in the anoxygenic underlayers. This suggests that it does not matter “who’s on top,” as we were able to observe pigments through all mat layers due to their different absorption maxima. The presence of multiple pigments and thus multiple “NIR edges” could signify layered phototrophic communities and possibly strengthen support for the detection of a surface exoplanet biosignature. In general, the proposed work will inform the search for life on exoplanets at a similar stage of evolution or biogeochemical state as the pre-oxic Earth.

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