Biology
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p53d..08d&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P53D-08
Biology
0406 Astrobiology And Extraterrestrial Materials, 0424 Biosignatures And Proxies, 0463 Microbe/Mineral Interactions, 5225 Early Environment Of Earth, 6225 Mars
Scientific paper
A biosignature is an object, substance and/or pattern whose origin specifically requires a biological agent. The usefulness of a biosignature is determined, not only by the probability of life creating it, but also by the improbability of nonbiological processes producing it. So what sets life apart from the rest? Life as we know it is the harnessing of free energy to sustain and perpetuate, by molecular replication and evolution, a high density of information in the form of functional complex molecules and functionally-related larger structures. Accordingly, biosignatures can arise from key attributes such as converting solar to chemical energy, exploiting the versatility of organic chemistry to sustain metabolic processes and preserve information, and maintaining microenvironments that enhance these functions. The external environment affects such functions and so it must be defined in order to interpret effectively the biosignatures that emerge from them. Hypersaline benthic cyanobacterial communities at Guerrero Negro, Baja California Sur, Mexico provide illustrative examples of biosignatures research that is relevant to our early biosphere and to Mars. Where brines are undersaturated with respect to gypsum, filamentous cyanobacteria dominate over unicellular cyanobacteria and can trap and bind sedimentary grains more effectively, thus altering their response to sedimentary processes and creating laminated fabrics. Biofilms in gypsiferrous sediments also can alter the response of the clastic or crystal matrix to chemical and physical sedimentary processes such as erosion or precipitate accumulation. Gypsum precipitating within biofilms offers compelling evidence of biological influences on crystal textures and habits. Such gypsum exhibits dissolution textures, accessory mineral precipitation and unique crystal form aspect ratios. Irregular textures include conchoidal and globular features associated with both dissolution and nucleation that are likely affected by biofilm pore water compositions. The accessory phases forming in association with gypsum-hosted biofilms (Sº, Ca-carbonate, and Sr/Ca-sulfate) are known byproducts of bacterially mediated sulfate reduction. Light penetrates the relatively transparent gypsum to sustain discretely layered successions of orange-, green-, purple-, pink-, and black-pigmented endoevaporitic biofilms. Lipid biosignatures include carotenoids, tricyclic terpenoids, benzothiophenes, thiacycloalkanes and methylhopanoids. These represent the aggregate effects of light regimes and hypersaline conditions. Features that could be preserved over geological timescales therefore include sedimentary textures, minerals, crystal forms, and lipids. Collectively these features can serve both as biosignatures and paleoenvironmental indicators on early Earth and on Mars.
Des Marais David J.
Vogel Marilyn B.
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