ABSTRACT: Noctiluca scintillans is one of the largest species of marine dinoflagellates. A small fraction of these bloom forming algae was reported to be turbid due to endocytic bacteria. The diversity of these endocytic bacteria living in turbid Noctiluca cells was investigated by denaturing gradient gel electrophoresis (DGGE). The results indicate the occurrence of 1 dominant group of endocytic bacteria and some other groups of less dominance. DGGE profiles were compared between the endocytic bacterial populations of cultivated and non-cultivated turbid Noctiluca cells; the latter were directly collected from the North Sea. DGGE profiles displayed no differences between them. In contrast, the comparison of band patterns of endocytic bacteria and free-living marine bacteria were different, indicating the development of a specific bacterial population within N. scintillans. The DGGE bands identified by DNA sequencing were assigned to the species Marinobacter PCOB-2, to the Pseudoalteromonas group, and the Vibrio group, all members of the g 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 phylogenetic analysis. Data revealed that these bacteria belong to several different phylogenetic groups. Most of the isolates (14 strains) belong to several groups of the g subdivision of Proteobacteria; 2 isolates are related to the Vibrio group and 1 isolate to Moraxella. The other isolates were assigned to the following groups of the g subdivision: Colwellia group, Stenotrophomonas and Pseudoalteromonas group. Two of them were closely related to sequences obtained from DGGE bands (Pseudoalteromonas group, and Marinobacter PCOB-2). Two isolates were assigned to the phylum of Gram-positive bacteria.

KEY WORDS: Endocytic bacteria · Noctiluca scintillans · DGGE · Diversity

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Published in AME Vol. 25, No. 3 (2001) on September 28
Print ISSN: 0948-3055; Online ISSN: 1616-1564. Copyright © Inter-Research, Oldendorf/Luhe, 2001