Global maps of sulfate and methane in marine sediment define two realms of subsurface catabolic activity; a sulfate-rich open-ocean region and an ocean-margin region where sulfate is limited to shallow sediments. Methanogenic fermentation occurs in both, but sustains abundant methane only in sulfate-depleted sediments. Catabolic activity is greatest in narrow zones of sulfate-reducing methanotrophy along ocean margins. Although the marine subsurface biosphere has been estimated to constitute 1/10 [1] to 1/3 [2] of Earth's biomass, its metabolic activity is orders of magnitude lower than that of the surface world. Most microbes in subsurface marine sediments are either inactive or adapted for extraordinarily low levels of catabolic activity.

References: [1] Parkes, R.J., B.A. Cragg, and P. Wellsbury (2000), Hydrogeology J., 8, 11-28. [2] Whitman, W.B., D.C. Coleman, and W.J. Wiebe (1998), PNAS, 95, 6578-6583.