Abstract

Minipellets, fecal pellets from 3 to 50 μm in diameter, were found on detritus collected by a particle interceptor trap array in the upper 2000 m of the eastern tropical Pacific. The fluxes of minipellets reached 5 × 106 m−2 day−1, and exceeded fluxes of larger (>50 μm diameter) fecal pellets by 3 orders of magnitude. Carbon flux of minipellets was 11–49% that of larger pellets; however, carbon flux of ultrastructurally intact cells (microalgae and bacteria) in minipellets was equal to that of intact cells in the larger pellets. Minipellets also occurred in water samples from similar depths, where they numbered up to 105 m−3, and were usually not associated with particles. Minipellets appear ubiquitous; we have found them in all our samples of particulates from other cruises from surface waters to bathypelagic depths. Minipellet morphologies ranged from Type A, which contained intact, picoplankton-sized cells (cyanobacteria, nitrifying bacteria, morphologically non-descript, Gram-negative bacteria, Chiarella-like cells) in an amorphous matrix surrounded by a boundary, to Type D minipellets, which were identical to previously described olive-green "cells."Minipellets are probably wastes of protozoans and small invertebrates that consume marine snow and larger fecal pellets throughout the water column, thereby maintaining the high numbers of minipellets from the surface to 2000 m. We found several sources of minipellets: two groups of sarcodine protozoans (phaeodarian and spumellarian radiolarians) and small hydromedusae. The minipellet producers reprocess a major portion of surface-derived detritus, and represent important biological intermediates that transform particulate matter settling through the ocean.

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