A seafloor microbial biome hosted within incipient ferromanganese crusts

Exposed rocks at underwater volcanoes and ridges host complex, abundant and diverse microbial communities. The volcanic glasses associated with these features constitute one of the most geochemically reactive components of the Earths crust. The most commonly held hypothesis is that their oxidation in sea water provides the energy necessary to establish a seafloor biosphere. However, this hypothesis has yet to be directly tested. Here we used synchrotron-based X-ray microprobe mapping, X-ray absorption spectroscopy and high-resolution scanning and transmission electron microscopy techniques to examine the initial chemical changes that occur as the glassy rims of young pillow basalts are colonized by microbial organisms at Loihi seamount, Hawaii. We found little evidence of basalt dissolution. Instead, microbial biofilms were intimately associated with Fe(III)- and Mn(IV)-oxides that had precipitated from sea water onto the fresh basalt surfaces. These accumulations of secondary minerals probably represent the earliest stages of ferromanganese crust formation. We suggest that fluid-derived energy sources, such as dissolved and particulate Fe(II), Mn(II) and organic matter, may support the microbial communities colonizing seafloor rocks to a greater degree than local rock dissolution.