Microscopic physical biomarkers in carbonate hot springs: Implications in the search for life on Mars

Physical evidence of life (physical biomarkers) from the deposits of carbon ate hot springs were documented at the scale of microorganisms-submillimete r to submicrometer. The four moderate-temperature (57 to 72 degrees C), neu tral pH springs reported on in this study, support diverse communities of b acteria adapted to specific physical and chemical conditions. Some of the m icrobes coexist with travertine deposits in endolithic communities. In othe r cases, the microbes are rapidly coated and destroyed by precipitates but leave distinctive mineral fabrics. Some microbes adapted to carbonate hot s prings produce an extracellular polymeric substance which forms a three-dim ensional matrix with living cells and cell remains, known as a biofilm. Sil icon and iron oxides often coat the biofilm, leading to long-term preservat ion. Submicrometer mineralized spheres composed of calcium fluoride or sili ca are common in carbonate hot spring deposits. Sphere formation is biologi cally mediated, but the spheres themselves are apparently not fossils or mi crobes. Additionally, some microbes selectively weather mineral surfaces in distinctive patterns. Hot spring deposits have been cited as prime locatio ns for exobiological exploration of Mars. The presence of preserved microsc opic physical biomarkers at all four sites supports a strategy of searching for evidence of life in hot spring deposits on Mars. (C) 2000 Academic Pre ss.