Characterisation of bacterial communities associated with toxic and non-toxic dinoflagellates: Alexandrium spp. and Scrippsiella trochoidea

Several dinoflagellate species have been shown to produce potent neurotoxins known as paralytic shellfish toxins. Evidence is also accumulating that marine bacteria associated with dinoflagellates play a role in the accumulation of paralytic shellfish toxins. In this study, the diversity of bacteria in cultures of both toxic and non-toxic dinoflagellates, Alexandrium spp. and Scrippsiella trochoidea, were compared using colony morphology, restriction fragment length polymorphisms, denaturing gradient gel electrophoresis of PCR-amplified 16S rRNA genes and, ultimately, sequence determination of the 16S rRNA genes. The results suggest that a number of different bacterial species are associated with dinoflagellates, some of which are common to each of the dinoflagellate cultures examined, whereas others appear to be unique to a particular dinoflagellate. The phylogenetic diversity of the bacteria observed was limited to two bacterial phyla, the Proteobacteria and the Cytophaga-Flavobacter-Bacteroides (CFB). Although phylum level diversity was limited, many distinct phylogenetic clades were recovered, including members of both the alpha- and gamma-subclasses of the Proteobacteria. Additionally, several of the bacterial phylotypes isolated were not closely related to any published bacterial species but, rather, were identical to isolates characterised from Alexandrium cultures 4 years earlier. Finally, many of the bacteria isolated from the dinoflagellate cultures were related to microorganisms with known surface-associated life histories (e.g. the CFB phylum, Hyphomonas, Caulobacter and some members of the Roseobacter clade including Ruegeria algicola).