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doi:10.2204/iodp.sp.331.2010

Introduction

Microbiologists participating in the Ocean Drilling Program (ODP) and IODP for the past decade have demonstrated that marine subsurface sediments consistently contain more than 10⁵ cells/cm³ even at depths as great as 1000 meters below seafloor (mbsf). On the basis of these findings, it has been proposed that the subseafloor environment has the largest biomass potential on Earth, exceeding that found in terrestrial and oceanic biospheres. Recent geochemical and microbiological data from sediments cored by ODP and IODP provide a picture of the subseafloor biosphere wherein prominent microbial metabolic activity is limited to relatively narrow interface zones between seawater and seafloor sediments and between these sediments and oceanic basement. The deep subseafloor biosphere appears to be, for the most part, composed of inactive or viable but extraordinarily slowly metabolizing microorganisms. Although these hypotheses have not been confirmed, if they are true and low metabolic activity of subseafloor life is ubiquitous and typical, it would likely be explained by a combination of in situ conditions of low temperature and low porosity coupled with extremely low fluxes of inorganic and organic nutrients.

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