Ciliated protozoa associated with oceanic sinking detritus

In the past decade, protozoa have been recognized as ubiquitous residents of the upper waters of the sea, where they have key roles in detrital and producer food webs1–4. In contrast, protozoa are almost unknown from aphotic depths, where food chains presumably are supported by detritus. We show here that ciliate protozoans do accompany sinking detritus to bathypelagic depths. In the productive northeastern Pacific Ocean, the biomass of ciliates on particles settling between the surface and 2,000 m far exceeds the biomass of bacteria, which are the only widely recognized detrital microorganisms in deep pelagic communities. Although absolute numbers of ciliates decline below the euphotic zone, their concentrations (number per g detritus) drop only slightly by 2,000 m. Taxonomic data and patterns of abundance5 indicate that the ciliates are endemics and not remnants of surface colonists, and that the deep sea is a favourable environment for them. Their high concentrations, possibly together with other smaller protozoa, indicate that protists may control bacterial mineralization rates on sinking particles, thereby regulating the vertical distribution of many biologically active elements in the sea.