Interactions between bacteria and members of the marine diatom genus Pseudonitzschia may enhance production of the toxin domoic acid (DA) by toxigenic strains of Pseudonitzschia. To gain a broader understanding of relationships between bacteria and Pseudo-nitzschia species, we used automated ribosomal intergenic spacer analysis (ARISA) to assess the composition of the bacterial communities coexisting with 18 Pseudo-nitzschia strains representing 6 species. For cultures surveyed across multiple time points and size fractions, the attached and free-living bacterial communities were not significantly distinct from one another, and bacterial composition was stable across diatom growth phases (exponential versus stationary) and approximately 1 yr in culture. Among all cultures, bacterial communities differed significantly with Pseudo-nitzschia species and toxigenicity. Toxigenic strains of Pseudo-nitzschia hosted fewer bacterial ARISA operational taxonomic units (OTUs), in comparison to nontoxigenic strains. We constructed two 16S rDNA clone libraries to identify bacteria coexisting with 1 P. multiseries (toxigenic) and 1 P. delicatissima (nontoxigenic) culture. Both cultures hosted members of the Roseobacter clade, Gammaproteobacteria, and Flavobacteria, yet the specific bacteria coexisting with each Pseudo-nitzschia strain differed at the genus level or above. Our findings support the hypothesis that bacterial communities respond to DA or other species-specific differences in the environments created by Pseudo-nitzschia strains.KEY WORDS: Pseudo-nitzschia · Domoic acid · Phytoplankton-bacteria interactions · ARISA Resale or republication not permitted without written consent of the publisher

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