The subsurface biosphere at mid-ocean ridges: Issues and challenges

A recent growth of interest in subsurface microbiology has been fueled by the recognition that the subsurface may have played an important role in the origin and early evolution of life, and may presently sustain a substantial fraction of Earth's biomass. The uppermost igneous oceanic crust is likely to be one of the most habitable subsurface environments because it is porous and the locus of extensive hydrothermal circulation. This circulation is most vigorous at spreading centers where it is driven by the volcanic accretion of oceanic crust. Hot reduced hydrothermal fluids created by water-rock reactions above magma bodies mix with cold seawater in the subsurface and the resulting chemical disequilibria provide energy and carbon sources that support diverse microbial communities. These communities can be sampled in chronic low-temperature hydrothermal vents and in the hydrothermal fluids released following volcanic eruptions. Investigations of the subseafloor environment at mid-ocean ridges integrate biological and geological approaches to understand the characteristics of hydrothermal circulation and how they are modulated by geological events; the sources of carbon, nutrients and energy; and the types and functions of subsurface organisms. They also utilize analogies with accessible sulfide edifices and comparisons with similar subsurface environments elsewhere. Future studies will combine increasingly sophisticated shore-based studies with data from long-term observatories comprising networks of instruments for measuring key physical, chemical and microbial parameters. They will require the development of technology to drill bare rock mid-ocean ridge sites, collect uncontaminated subsurface samples and deploy instruments at different depths in the crust.