Influence of phytoplankton lysis or grazing on bacterial metabolism and trophic relationships.

Experimental microcosms were used to study the dynamics of heterotrophic bacterial populations with respect to phytoplankton loss. In a two-stage linked culture system, we artificially separated production and loss processes of a diatom Phaeodactylum tricornutum. In the first (productive) stage, the algae developed axenically and continuously. The outflow was fluxed in two degradation stages, where phytoplankton-derived detritus resulted respectively from: (1) excretion and by-products of phagotrophic organisms (protozoans), and (2) bacterial degradation through bacterial attachment and lysis. According to the phytoplankton decay mode, i.e., lysis or grazing, bacterial adaptations were different. The study of bacterial productivity and aminopeptidase activity showed specific bacterial evolution during the succession of different prey-predator relationships. The occurrence of aggregates allowed nanoflagellates to develop an alternative diet; they fed not only on bacteria, but also on partially degraded phytoplankton detritus, inducing a strong short-cut in the food chain. Sources and controls of extracellular proteolytic activity are discussed. Such experimental approaches are interesting because they separate bacterial lysis and protozoan grazing of phytoplankton, as well as the fates of their corresponding phytoplankton detritus in the microbial food web.