Bioalteration of the seafloor and subseafloor: A molecular and spectroscopic investigation into microbe-mineral interactions

Abstract

The ocean crust comprises 70% of the Earth's crust and has a habitable surface area of 600,000 km². When it erupts onto the seafloor, it is chemically reduced. Upon exposure to the oxygenated seawater, it begins altering via oxidative weathering and dissolution. Though thermodynamically favorable, many of the chemical reactions involved in seafloor weathering are kinetically inhibited under the temperature and pressure conditions at the seafloor and are therefore hypothesized to be microbially mediated. Recent advances in deep sea microbiology reveal an elaborate web of microbe-mineral and microbe-microbe interactions. The so-called "deep biosphere" can be extended down though 200m of oceanic crust because it is highly permeable to circulating seawater. Elucidating specific microbe-mineral interactions occurring at and below the seafloor requires understanding how microbial communities change with host mineralogy as well as how the mineral itself transforms. Furthermore, _{in situ} experimentation is required to constrain the temporal scale at which these interactions occur. Fe-bearing minerals were exposed to seafloor conditions for up to 6 years and allow for investigation into seafloor bioalteration processes. Bacterial 16S rRNA sequencing revealed differences in the endolithic (rock-hosted) communities with changes in mineralogy. Synchrotron-based X-ray analyses (fluorescence and absorption spectroscopy) were employed to characterize secondary alteration products which are indistinguishable amongst the different mineralogies. Implications for this work include carbon sequestration, global biogeochemical cycles, limits and origin of life and astrobiology.