Interactions between marine bacteria and axenic diatoms (Cylindrotheca fusiformis, Nitzschia laevis, and Thalassiosira weissflogii) incubated under various conditions in the lab

Bacteria-phytoplankton interactions in aquatic systems range from symbiosis to parasitism and are highly variable in space and time. Three marine diatoms (Cylindrotheca fusiformis, Nitzschia laevis, and Thalassiosira weissflogii) were grown in axenic culture and incubated under controlled lab conditions with single bacterial isolates. mixed bacterial populations, and seawater off Scripps Pier. Growth of both bacteria and algae was significantly higher when incubated together in f/2 medium or artificial seawater which were rich in inorganic nutrients, vitamins, and trace metals. In contrast. growth of diatoms in a vitamin and trace metal free medium was reduced or even negative when incubated together with bacterial isolates or seawater bacteria. In addition, the amount and quality of exopolymeric matter in mixed cultures were different from those of pure cultures. In general, aminopeptidase and beta-glucosidase activities of bacteria in mixed cultures strongly increased after 70 h whereas those of a pure culture (F1 7) were slightly higher until 50 to 70 h of incubation but strongly decreased thereafter. High growth and enhanced hydrolytic ectoenzyme activities of bacteria in the presence of algae and polymer particles led to high bacterial remineralization of organic nutrients increasing phytoplankton growth. However, bacteria compete with phytoplankton for nutrients and can inhibit algal growth under certain environmental conditions. Thus, changes in eutrophication and pollution can alter bacteria-phytoplankton interactions, which influence the flux and cycling of nutrients and carbon at both micro- and global scale.