Abstract

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, <i>Alexandrium</i> spp. and <i>Scrippsiella trochoidea</i>, 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 α- and γ-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 <i>Alexandrium</i> 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, <i>Hyphomonas, Caulobacter</i> and some members of the <i>Roseobacter</i> clade including <i>Ruegeria algicola</i>).