Ocean Sediments—an Enormous but Underappreciated Microbial Habitat: Modeling proves essential in efforts to understand the estimated 3×1029 microbial cells that comprise this unusual ecosystem

Approximately 70% of the Earth's surface is covered by ocean—on average, under 3,700 m of water. At the seafloor is a blanket of unconsolidated sediment consisting of continental detritus; particulate organic matter; silica- and carbonate-rich, biologically produced hard materials; and void spaces filled with saline fluids of wide-ranging chemistries. Near the continents, especially where relief is high and physical weathering is prominent, the sediment thickness can measure more than 10 km. However, underneath the oligotrophic open ocean gyres, it can be less than 0.1 km, even on seafloor that is tens of millions of years old, and parts of the ocean floor, especially the mid-ocean ridges, young ridge flanks, and ubiquitous seamounts, are naked or covered by only a thin veneer of sediment.