Relationship between induced phytoplankton blooms and the structure and dynamics of the free-living heterotrophic bacterial community

Bacterial community activity and structure are thought to be directly or indirectly related to phytoplankton development and, in particular, to the phytoplankton species dominating specific algal blooms. To test this hypothesis, we performed a mesocosm experiment designed to generate blooms of different types of phytoplankton through the additions of silicate, urea and phosphorus to oligotrophic water from the Blanes Bay Microbial Observatory (NW Mediterranean). Over 10 d of incubation bacterial activity, bacterial abundance, nutrient composition and free-living bacterial community structure were monitored, as well as phytoplankton composition and the fluorescence characteristics of dissolved organic matter (DOM). While we found clear effects of the different nutrient additions on chlorophyll levels, bacterial production and the type of dominant DOM, bacterial abundance followed a similar pattern across different nutrient treatments, which deviated from that observed in the control. While phytoplankton composition in the treatment with added silicate evolved differently with respect to the other treatments, free-living bacterial community structure (as determined with DGGE) did not show conspicuous differences between treatments. Our results reveal that the changes in bacterial community composition were mostly due to the variation in grazing pressure with time, with a small contribution from changes in bottom-up nutrient supply mediated by the shifts in phytoplankton composition.