Interactions of planktonic algae and bacteria: effects on algal growth and organic matter dynamics

Interactions between phytoplankton and heterotrophic bacteria have been intensively studied, but many aspects of these interactions are still unknown because most of the studies were performed under non-axenic conditions. Therefore, we investigated the growth and release of dissolved organic matter of the marine diatom Thalassiosira rotula in axenic culture in comparison to T. rotula cultures inoculated with either a natural marine bacterial community (German Wadden Sea) or 3 different bacterial isolates. The isolates affiliated to the Roseobacter group (HP50), Hyphomonas (HP48) and Flexibacteriaceae (HP49) and were previously obtained from a T. rotula culture inoculated with a natural marine bacterial community (German Wadden Sea). To test whether the availability of inorganic nutrients, vitamins and trace metals affects algal growth, organic matter release and interactions with heterotrophic bacteria, we performed experiments with either Guillard’s f/2 or f/10 medium, which differ in their inorganic nutrient, vitamin and trace metal concentration by a factor of 5. The bacterial community promoted growth of T. rotula in both media, as shown by the higher algal numbers relative to the axenic cultures, but also led to a rapid decline after the growth phase. Isolate HP50 promoted algal growth in the f/2 medium, but inhibited growth in the f/10 medium, whereas isolate HP48 showed the opposite patterns. Isolate HP49 prevented algal growth in both media. T. rotula exhibited distinct release patterns of dissolved organic carbon (DOC), dissolved amino acids, dissolved neutral carbohydrates, transparent exopolymer particles (TEP) and protein-containing particles (Coumassie Blue-stainable particles, CSP), which were modulated by the bacteria added. TEP and CSP were produced by the growing alga only in the presence of bacteria, indicating that the bacterial modification of algal exudates and/or the bacterial decomposition of the alga are prerequisites for formation of these microparticles. Our results show that the presence of the bacterial community and of specific populations have distinct effects on the growth and organic matter release of T. rotula and presumably also on other algae.