title: Microscopic physical biomarkers in carbonate hot springs: implications in the search for life on Mars.
abstract: Physical evidence of life (physical biomarkers) from the deposits of carbonate hot springs were documented at the scale of microorganisms--submillimeter to submicrometer. The four moderate-temperature (57 to 72 degrees C), neutral pH springs reported on in this study, support diverse communities of bacteria adapted to specific physical and chemical conditions. Some of the microbes coexist with travertine deposits in endolithic communities. In other cases, the microbes are rapidly coated and destroyed by precipitates but leave distinctive mineral fabrics. Some microbes adapted to carbonate hot springs produce an extracellular polymeric substance which forms a three-dimensional matrix with living cells and cell remains, known as a biofilm. Silicon and iron oxides often coat the biofilm, leading to long-term preservation. Submicrometer mineralized spheres composed of calcium fluoride or silica are common in carbonate hot spring deposits. Sphere formation is biologically mediated, but the spheres themselves are apparently not fossils or microbes. Additionally, some microbes selectively weather mineral surfaces in distinctive patterns. Hot spring deposits have been cited as prime locations for exobiological exploration of Mars. The presence of preserved microscopic physical biomarkers at all four sites supports a strategy of searching for evidence of life in hot spring deposits on Mars.
authors.affiliation: Lockheed Martin Space Operations, Houston, Texas 77058, USA. carlton.c.allen1@jsc.nasa.gov
authors.forename: C C
authors.initials: CC
authors.lastname: Allen
authors.forename: F G
authors.initials: FG
authors.lastname: Albert
authors.forename: H S
authors.initials: HS
authors.lastname: Chafetz
authors.forename: J
authors.initials: J
authors.lastname: Combie
authors.forename: C R
authors.initials: CR
authors.lastname: Graham
authors.forename: T L
authors.initials: TL
authors.lastname: Kieft
authors.forename: S J
authors.initials: SJ
authors.lastname: Kivett
authors.forename: D S
authors.initials: DS
authors.lastname: McKay
authors.forename: A
authors.initials: A
authors.lastname: Steele
authors.forename: A E
authors.initials: AE
authors.lastname: Taunton
authors.forename: M R
authors.initials: MR
authors.lastname: Taylor
authors.forename: K L
authors.initials: KL
authors.lastname: Thomas-Keprta
authors.forename: F
authors.initials: F
authors.lastname: Westall
mesh_terms: D001131:Arkansas; D018441:Biofilms; D015415:Biomarkers; D002254:Carbonates / Q000032:analysis*; D000458:Cyanobacteria; D018559:Exobiology; D005618:Fresh Water / Q000737:chemistry / Q000382:microbiology*; D006358:Hot Temperature; D007558:Italy; D018535:Mars; D008854:Microscopy, Electron; D008855:Microscopy, Electron, Scanning; D009516:New Mexico; D013824:Thermus; D014871:Water Microbiology; D014959:Wyoming
chemical_list: D015415:Biomarkers; D002254:Carbonates