Diversity of endocytic bacteria in the dinoflagellate Noctiluca scintillans

Noctiluca scintillans is one of the largest species of marine dinoflagellat es. A small fraction of these bloom forming algae was reported to be turbid due to endocytic bacteria. The diversity of these endocytic bacteria livin g in turbid Noctiluca cells was investigated by denaturing gradient gel ele ctrophoresis (DGGE). The results indicate the occurrence of 1 dominant grou p of endocytic bacteria and some other groups of less dominance. DGGE profi les were compared between the endocytic bacterial populations of cultivated and non-cultivated turbid Noctiluca cells; the latter were directly collec ted from the North Sea. DGGE profiles displayed no differences between them . In contrast, the comparison of band patterns of endocytic bacteria and fr ee-living marine bacteria were different, indicating the development of a s pecific bacterial population within N. scintillans. The DGGE bands identifi ed by DNA sequencing were assigned to the species Marinobacter PCOB-2, to t he Pseudoalteromonas group, and the Vibrio group, all members of the gamma subdivisions of Proteobacteria. Another DGGE band was identical to the 18S ribosomal gene of N. scintillans itself. Furthermore, 16 bacterial isolates derived from single Noctiluca cells were characterized by 16S rRNA phyloge netic analysis. Data revealed that these bacteria belong to several differe nt phylogenetic groups. Most of the isolates (14 strains) belong to several groups of the gamma subdivision of Proteobacteria; 2 isolates are related to the Vibrio group and 1 isolate to Moraxella. The other isolates were ass igned to the following groups of the gamma subdivision: Colwellia group, St enotrophomonas and Pseudoalteromonas group. Two of them were closely relate d to sequences obtained from DGGE bands (Pseudoalteromonas group, and Marin obacter PCOB-2). Two isolates were assigned to the phylum of Grampositive b acteria.