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 neurotoxin s known as paralytic shellfish toxins. Evidence is also accumulating that m arine bacteria associated with dinoflagellates play a role in the accumulat ion 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, restric tion 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 bacteri al 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 ba cteria observed was limited to two bacterial phyla, the Proteobacteria and the Cytophaga-Flavobacter-Bacteroides (CFB). Although phylum level diversit y was limited, many distinct phylogenetic clades were recovered, including members of both the alpha- and gamma -subclasses of the Proteobacteria. Add itionally, several of the bacterial phylotypes isolated were not closely re lated to any published bacterial species but, rather, were identical to iso lates characterised from Alexandrium cultures 4 years earlier. Finally, man y 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 i ncluding Ruegeria algicola). (C) 2001 Federation of European Microbiologica l Societies. Published by Elsevier Science B.V. All rights reserved.