GEOCHEMICAL DIFFERENCES IN HYDROTHERMAL MICROBIAL COMMUNITY BIOMASS

Trace element enrichments in geologic materials may be used as biosignatures if those enrichments are unique to specific microbial communities. Owing to gradients in temperatures, pH, and chemical composition, hydrothermal ecosystems host diverse microbial communities, ranging from strictly chemolithotrophic hyperthermophiles (e.g., pink filaments) to complex photosynthetic mats, which provide numerous opportunities for differentiation of trace element signatures. We have collected pink filaments, semi-solid white gel material, photosynthetic stromatolite-like microbial structures (“stromatotherms”), and microbial black mats, together with water and rock samples at Yellowstone National Park during 2003 and 2004. We chose hot springs that are separated by tens to thousands of meters at temperatures ranging from 41° to 91°C, and pH values from 3.4 to 8.4. Water and biomass samples were analyzed by High Resolution Inductively Coupled Mass Spectrometry for trace elements including transition metals, rare earths, and group IIIB and VB elements. Results indicate that microbial biomass accumulates many trace and rare earth elements at concentrations that greatly exceed those in the water in which the organisms live, when comparing enrichment factors calculated as the ratio of the concentration in dry biomass (mol/g) to concentration in the water (mol/kg). These enrichment factors range from 10 to 100,000. Large enrichments are observed for Al, Ti, Cr, Mn, Fe, Cu, Zn, Ba, and La. Some elements, such as B, As, and W, are enriched relatively little. As an example of possible community signatures, pink filaments from several locations exhibit similar concentrations of Be, Sb, W, and Cu, while in contrast the stromatotherms exhibit similar concentrations of Al, V, Ni, and Zn. Most if not all of these communities are either being actively encased in silica precipitating from the hydrothermal water, or buried in sediments. Either of these scenarios could lead to the trapping of elements in the biomass in the forming rock, creating what could then be a trace element biosignature. Preliminary results for biomass indicate that, barring major differences in diagenesis, it should be possible to differentiate biosignatures of chemolithotrophic from photosynthetic communities preserved in the geologic record.