Hyperthermophilic Biofilm Mineralization - Implications for Biosignature Detection

Microbial biofilms colonize nearly every perpetually wetted surface in and around silica-depositing hot springs. Biofilms that adhere irreversibly to accreting sinter surfaces ultimately become mineralized in such environments. Mineralization of the cells and extracellular matrices of hyperthermophilic biofilms influences the sedimentological and geochemical characteristics of the corresponding sinter stromatolites known as geyserites. We report here the results of our investigation of biofilm mineralization processes associated with geyserites that form at the high-temperature ends of near-neutral silica-depositing hot springs.

Given that hyperthermophilic microbial biofilms consist primarily of filamentous bacteria in silica-depositing hot springs, these organisms tend to have the greatest impact on geyserite fabrics and stromatolite structures. Hyperthermophilic biofilms, their corresponding sinters, and substratums colonized by such biofilms were characterized by a variety of imaging, diffraction, and spectroscopic methods. We have found that differences in the relative timing of mineralization of the various components of these biofilms are key to preserving evidence of the presence and behavior of microbial populations in the corresponding siliceous sinter deposits.

Key points include, early mineralized microbial filaments act as a scaffolding within any one lamination of the stromatolitic deposits. Though secondary infilling tends to mask microbial input, specimen etching confirms the architectural role of the filamentous structures. The intercalation of abiotic and biogenic laminations in subaerial regimes results from the episodic colonization of sinter surfaces when mineral accretion rates are high. The initial colonization and replication of filamentous cells on geyserite surfaces defines the base of biogenic sinter laminae, whereas the production of copious amounts of EPS define laminae surfaces. The distribution of organics in geyserite laminae reflect these distinct concentration differences in the distribution of microbial cells and extracellular remains in these types of stromatolites.