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 diat oms (Cylindrotheca fusiformis, Nitzschia laevis, and Thalassiosira weissflo gii) were grown in axenic culture and incubated under controlled lab condit ions with single bacterial isolates. mixed bacterial populations, and seawa ter off Scripps Pier. Growth of both bacteria and algae was significantly h igher when incubated together in f/2 medium or artificial seawater which we re rich in inorganic nutrients, vitamins, and trace metals. In contrast. gr owth of diatoms in a vitamin and trace metal free medium was reduced or eve n negative when incubated together with bacterial isolates or seawater bact eria. In addition, the amount and quality of exopolymeric matter in mixed c ultures were different from those of pure cultures. In general, aminopeptid ase and beta-glucosidase activities of bacteria in mixed cultures strongly increased after 70 h whereas those of a pure culture (F1 7) were slightly h igher until 50 to 70 h of incubation but strongly decreased thereafter. Hig h growth and enhanced hydrolytic ectoenzyme activities of bacteria in the p resence of algae and polymer particles led to high bacterial remineralizati on 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 poll ution can alter bacteria-phytoplankton interactions, which influence the fl ux and cycling of nutrients and carbon at both micro- and global scale.