Lipid Biomarker Preservation in Silica-Depositing Hydrothermal Analogs

The discovery of extensive silica-rich deposits at Home Plate in the Columbia Hills indicates that hydrothermal conditions once existed on Mars (Squyres et al. 2008). Two types of environments could have been responsible for forming these materials: fumaroles or hydrothermal springs. Examples of both types of these thermal features are found throughout Yellowstone National Park (YNP). The discovery of bona fide microfossils in ancient cherts indicates that silica deposition was an excellent mechanism for organic preservation on the early Earth. Given the importance that organic biomarkers have played in identifying some of the earliest microbial life on Earth and the potential for a similar habitable period on Mars, examination of the preservation of organic biomarkers within various hydrothermal, silica-rich modern analogs is essential for future interpretation of Martian organics. Cyanobacterial mats and biofilms are common inhabitants of hydrothermal spring systems worldwide, at temperatures below 73°C and over a broad range of pH, from acidic to alkaline. Bacteriohopanepolyols (BHPs) are the biological precursors for the abundant fossil hopanes recovered from sedimentary rock (Brocks et al. 1999; Summons et al. 1999). The 2-methyl homologs are generally considered a biomarker for cyanobacterial-dominated paleoecosystems. Some evidence exists that the complex molecular structure of BHP is retained upon entombment in hydrothermal silica and that rapid incorporation into the silica matrix may enhance preservation (Gibson et al. 2008). Here we report on the preservation of microbial lipids in several alkaline and acidic, silica-depositing hotsprings in YNP with particular emphasis on the potential for BHP preservation. Brocks JJ et al (1999) Archaean molecular fossils and the early rise of the eukaryotes. Science 185: 1033-1036 Gibson RA et al (2008) Bacteriohopanepolyol signatures of cyanobacterial and methanotrophic bacterial populations recorded in a geothermal vent sinter. Organic Geochemistry 39: 1020-1023 Squyres SW et al (2008) Detection of silica-rich deposits on Mars. Science 320: 1063-1067 Summons RE et al (1999) 2-Methylhopanoids as biomarkers for cyanobacterial oxygenic photosynthesis. Nature 400: 554-557