Interactions between marine osmo- and phagotrophs in different physicochemical environments

Abstract

The interaction between the marine phototrophic dinoflagellate Prorocentrum minimum and associated bacteria was studied in different physico-chemical environments in batch cultures. In stagnant cultures the dinoflagellates aggregated at the surface, while with low shear induced by gentle stirring and turbulent mixing induced by bubbling, the cells were dispersed. Both pH and O2 concentration eventually increased in the stagnant and stirred cultures as a result of photosynthesis. Bacterial community dynamics, productivity and composition was monitored by direct cell counts, plate counts, 3H-leucine incorporation, richness of genomes estimated by PCR-DGGE and frequency of growth rate types on agar plates. Bacterial productivity and diversity was restrained in both stagnant and stirred cultures, compared with bubbled ones. This pattern was observed early, before bulk water was affected by metabolic activity, which suggests that algal aggregation was not a prerequisite for inhibition of bacterial growth. Increased bacterial production during stationary growth phase of the algae in stagnant and stirred cultures suggests that high pH or O2 concentration were not the most important inhibiting factors. Rather, the need for turbulent mixing suggests that the structure of the extracellular organic material must be modified to make it available to bacteria. One growth rate type in the bacterial community was not inhibited in stagnant and stirred cultures and developed similarly in all cultures.