Role of Bacteria in Copepod Fecal Pellet Decomposition: Colonization, Growth Rates and Mineralization

Freshly egested fecal pellets from Calanus pacificus were rapidly colonized by free bacteria found in seawater. In 24 h, bacteria covered 27% of the available area of the fecal pellet. Bacterial concentrations reached 1 × 10⁵ cells per fecal pellet (average volume of pellet 3 × 10⁶ μm³) after 24 h of incubation. The number of bacteria associated with the fecal pellet then steadily declined to 1 × 10⁴ cells per fecal pellet in 4 days. The sinking of fecal pellets through a column of seawater increased the rate of initial colonization. The growth rates of bacteria associated with fecal pellets and those free in the water were not significantly different. Bacteria in 1 μm filtrates of seawater mineralized copepod fecal pellet carbon to CO₂ at 0.5% day^–1. Inclusion of microzooplankton (35 μm filtrate) doubled the rate of mineralization. If our laboratory results can be extrapolated to the natural environment, they would suggest that bacteria colonizing fecal pellets in the euphotic zone may not be the major group responsible for carbon mineralization in the upper 300 m of the water column.