Influence of phagotrophic processes on the regeneration of nutrients in two-stage continuous culture systems.

Regeneration of nutrients in aquatic ecosystems may be affected by autolysis, bacterial degradation, and activities of phagotrophic organisms. To examine the relative importance of these processes, mineralization of carbon, nitrogen, and phosphorus was studied in a two-stage continuous culture system. In the first stage (production compartment), aChlorella sp. was grown either axenically or in the presence of bacteria under P-limited conditions. In the presence of bacteria, the biomass of algae decreased due to efficient competition for P by bacteria. In the second stage (remineralization compartment), which was kept continuously in the dark, the nutrients incorporated by the organisms in the first stage were remineralized only slightly (0-25%) irrespective of the presence or absence of bacteria. However, remineralization of all nutrients tested was strongly increased (60-80%) after addition of zooflagellates, which grazed on algae and bacteria. These observations suggest that a net regeneration of nutrients was provided by phagotrophic organisms, whereas nutrients were trapped rather than released by bacteria. It is concluded that nutrient cycling could probably not proceed at the high velocities observed in the field without the participation of phagotrophic organisms.