Intracellular cyanobacterial symbionts in the marine diatom Climacodium frauenfeldianum (Bacillariophyceae)

The diatom Climacodium frauenfeldianum Grunow was collected in the tropical Atlantic and Pacific Oceans, Observations with epifluorescence microscopy revealed that this diatom contained coccoid symbionts (2.5-3.5 mu m) with a typical cyanobacterial fluorescence in addition to that of their own chlor oplasts. Mean concentration of C, frauenfeldianum for 28 stations in the SW tropical Pacific Ocean was 530 x 10(3) (SE = 1372) cells(.)m(-2), with hig hest concentration (mean 17.5 cells(.)L(-1)) at 40-m depth. The symbiosis w as only observed at water temperatures between 26.3 and 28.9 degrees C, wit h big-best concentrations at 27.7 degrees C. Three almost complete 16S rDNA sequences from one sample were determined, and they were identical, The ph ylogenetic analysis of this 16S rDNA sequence and those from other cyanobac teria and plastids revealed that it was closely related to the 16S rDNA seq uence from Cyanothece sp, ATCC 51142, Cyanothece sp, ATCC 51142 is a unicel lular nitrogen-fixing cyanobacterium isolated from a coastal marine environ ment and has ultrastructural features similar to the symbionts of C, frauen feldianum. The close relationship between Cyanothece sp, and the cyanobacte rial symbiont in C, frauenfeldianum suggests the potential for nitrogen fix ation in the symbiosis.