Abstract
While several studies have suggested that bacterium-phytoplankton interactions have the potential to dramatically influence harmful algal bloom dynamics, little is known about how bacteria and phytoplankton communities interact at the species composition level. The objective of the current study was to determine whether there are specific associations between diverse phytoplankton and the bacteria that co-occur with them. We determined the phylogenetic diversity of bacterial assemblages associated with 10 Alexandrium strains and representatives of the major taxonomic groups of phytoplankton in the Gulf of Maine. For this analysis we chose xenic phytoplankton cultures that (i) represented a broad taxonomic range, (ii) represented a broad geographic range for Alexandrium spp. isolates, (iii) grew under similar cultivation conditions, (iv) had a minimal length of time since the original isolation, and (v) had been isolated from a vegetative phytoplankton cell. 16S rRNA gene fragments of most Bacteria were amplified from DNA extracted from cultures and were analyzed by denaturing gradient gel electrophoresis and sequencing. A greater number of bacterial species were shared by different Alexandrium cultures, regardless of the geographic origin, than by Alexandrium species and nontoxic phytoplankton from the Gulf of Maine. In particular, members of the Roseobacter clade showed a higher degree of association with Alexandrium than with other bacterial groups, and many sequences matched sequences reported to be associated with other toxic dinoflagellates. These results provide evidence for specificity in bacterium-phytoplankton associations.
Highlights
Bacteria and phytoplankton dynamics are thought to be closely linked, and the “phycosphere,” or region immediately surrounding and influenced by phytoplankton cells, is an important bacterial habitat that is distinct from the surrounding water
We investigated the possibility of specific bacteriumphytoplankton associations by analyzing the phylogenetic diversity of bacteria associated with cultures of diverse nontoxic phytoplankton, as well as members of the toxic dinoflagellate genus Alexandrium, isolated from several regions of the world
There was substantial variability in the number of bands associated with each culture; as few as one nonchloroplast denaturing gradient gel electrophoresis (DGGE) band was associated with the Chlorarchnion sp. culture, and as many as seven nonchloroplast DGGE bands were associated with the Tetraselmis sp. culture (Fig. 3)
Summary
Bacteria and phytoplankton dynamics are thought to be closely linked, and the “phycosphere,” or region immediately surrounding and influenced by phytoplankton cells, is an important bacterial habitat that is distinct from the surrounding water. Strain 5N-3, which was found to have algicidal activity against G. nagasakiense but to have no effect on Chattonella antiqua, Heterosigma akashiwo, or S. costatum These findings have important implications, especially if they hold true for interactions between bacteria and members of the general phytoplankton community that form the basis for carbon cycling in coastal marine environments [32]. Rooney-Varga et al [49] recently reported a link between the community dynamics of phytoplankton and particle-associated bacteria in the Bay of Fundy, suggesting that species-specific associations occur in the phycosphere. Given the complexity of coastal temperate phytoplankton communities, it has not yet been possible to determine whether links between dynamics at the community level are the result of specific interactions between particular bacteria and phytoplankton and, if so, which bacterial species are associated with which phytoplankton. The results reveal a pattern of specific bacterial associations with toxic Alexandrium species, regardless of the location of isolation
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