Dynamics of bacterial community composition during degradation of copepod fecal pellets

The community of bacteria associated with the fecal pellets of planktonic copepods and those free living in surrounding seawater were investigated by denaturing gradient gel electrophoresis (DGGE) during a 10-day laboratory fecal pellet degradation experiment. Generally, fecal pellets containing bacteria were composed of different phylogenetic groups compared with those living in seawater. Bacteria in fecal pellets were dominated by g-Proteobacteria and Sulfitobacter (a-Proteobacteria), whereas bacteria in seawater had higher species richness and mainly consisted of a-Proteobacteria. Remarkable bacterial community shifts occurred in the first 2 days of the experiment along with the apparent increase in dissolved organic carbon and decrease of dissolved oxygen in the incubation bottles. Throughout the incubation, bacteria that were initially unique to fecal pellets were never found in surrounding seawater, while the communities of bacteria in fecal pellets and seawater became more similar, indicating colonization of bacteria from seawater to fecal pellets during the degradation process. These results suggest that the colonization of free-living bacteria took place rapidly and that they might contribute significantly to the degradation of planktonic copepod fecal pellets.