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

Abstract

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. KEY WORDS: Nutrients · Phytoplankton · Bacterial diversity · MesocosmsResale or republication not permitted without written consent of the publisher

Shifts in phytoplankton community structure modify bacterial production, abundance and community composition

“…The DIATOM treatment was dominated by Chaetoceros sp. This taxon releases higher concentrations of DOM, measured as percentage of extracellular release (Wetz & Wheeler 2007), and also produces more extracellular polysaccharides than other diatom species (Myklestad 1995), enhancing bacterial activity (Lekunberri et al 2012, Sarmento & Gasol 2012. The DOM released in the DIATOM treatment was likely translated into 2-fold higher BP, since diatoms can produce highly labile substrates, such as polysaccharides (Myklestad 2000).…”

The transcriptional response of prokaryotes to phytoplankton‐derived dissolved organic matter in seawater

“…Our results accordingly imply that two phytoplankton derived DOM sources (Exp_CyDi) triggered more similar transcriptional responses by prokaryotes than did phytoplankton derived DOM compared with unamended seawater (control, Exp_SwDi). Similarly, a study focusing on taxonomic data showed that different phytoplankton blooms induced the development of similar prokaryotic communities that were nonetheless quite distinct from the unamended control community (Lekunberri et al ., ).…”

“…The majority of prokaryotes growing in our continuous cultures were Gammaproteobacteria (Landa et al ., 2013a,b) which, based on transcript abundance, were also the most active community members. Members of Alphaproteobacteria and Bacteroidetes are often detected in association with the occurrence of marine phytoplankton blooms (Lekunberri et al ., ; Sarmento and Gasol, ; Teeling et al ., ) but were not abundant here. The continuous cultures possibly favoured Gammaproteobacteria by selecting for faster growing organisms.…”

“…Thus, the question of whether prokaryotic community composition changes in response to phytoplankton DOM of different origins has been investigated. Outcomes of these studies have differed, however, with some indicating pronounced compositional responses and others only weak effects (Van Hannen et al ., ; Lekunberri et al ., ; Sarmento and Gasol, ; Landa et al ., ). One explanation for these contrasting results could be that both specialist and generalist strategies exist among prokaryotic taxa (Langenheder et al ., ; Gómez‐Consarnau et al ., ; Dinasquet et al ., ).…”

Structuring of bacterioplankton communities by specific dissolved organic carbon compounds

“…Still, ecosystem processes may also remain stable despite changes in community composition, indicating functional redundancy among bacterial populations (Fernandez et al ., 2000; Comte and del Giorgio, 2011). Moreover, the development of similar communities in response to different environmental conditions argues that bacterial generalists can also be important players in the cycling of DOC (Langenheder et al ., 2005; Mou et al ., 2008; Lekunberri et al ., 2012). So far, no clear patterns have emerged as to when and where populations responsive to disturbance are either resource specialists or generalists and how this affects ecosystem functioning.…”