Bacterial community associated with Pfiesteria-like dinoflagellate cultures

Dinoflagellates (Eukaryota; Alveolata; Dinophyceae) are single-cell eukaryo tic microorganisms implicated in many toxic outbreaks in the marine and est uarine environment. Co-existing with dinoflagellate communities are bacteri al assemblages that undergo changes in species composition, compete for nut rients and produce bioactive compounds, including toxins. As part of an inv estigation to understand the role of the bacteria in dinoflagellate physiol ogy and toxigenesis, we have characterized the bacterial community associat ed with laboratory cultures of four 'Pfiesteria-like' dinoflagellates isola ted from 1997 fish killing events in Chesapeake Bay. A polymerase chain rea ction with oligonucleotide primers specific to prokaryotic 16S rDNA gene se quences was used to characterize the total bacterial population, including culturable and non-culturable species, as well as possible endosymbiotic ba cteria. The results indicate a diverse group of over 30 bacteria species co -existing in the dinoflagellate cultures. The broad phylogenetic types of d inoflagellate-associated bacteria were generally similar, although not iden tical, to those bacterial types found in association with other harmful alg al species. Dinoflagellates were made axenic, and the culturable bacteria w ere added back to determine the contribution of the bacteria to dinoflagell ate growth. Confocal scanning laser fluorescence microscopy with 16S rDNA p robes was used to demonstrate a physical association of a subset of the bac teria and the dinoflagellate cells. These data point to a key component in the bacterial community being species in the marine alpha-proteobacteria gr oup, most closely associated with the alpha -3 or SAR83 cluster.