ABSTRACT: We examined the interactions between 3 hypothesized functional groups of heterotrophic bacteria: POC specialists, which use only particulate organic carbon (POC); free-living DOC specialists, which use only dissolved organic carbon (DOC); and generalists, which use POC when particle-associated and DOC when free-living. We focused on (1) competitive interactions for the use of POC and DOC, and (2) commensal interactions, whereby DOC is supplied by the enzymatic hydrolysis of POC by particle-associated cells. Using a simple model, we found that the quantity and quality of POC and DOC, and the balance between the rate of POC hydrolysis and the release of DOC determine the coexistence between generalists and the specialists in the bacterial community. Surprisingly, a high level of POC hydrolysis, which is an apparently commensal process of generalists, was found to exclude free-living DOC specialists from the community. These factors also determine the community dynamics during a shift in the supply of POC, as may occur during a phytoplankton bloom. This model predicts 3 possible patterns for the order of succession in the community composition, and 2 distinct directions in which the distribution of production within the free-living community could change. These diverse patterns would affect both the biogeochemical processes and the community dynamics at higher trophic levels.