The interactions of purportedly toxic bacteria with dinoflagellates in the occurrence of Paralytic Shellfish Poisoning (PSP) were investigated. Dinoflagellates of the genus Alexandrium were examined to determine their bacterial population by the use of fluorescently labelled probes. Extracellular associated bacteria were shown to be associated with A. andersonii, A. tamarense and A. lusitanicum. Intracellulae bacteria were found in A. tamarense. Cyanoditolyltetrazolium chloride was used to detect endocytic metabolically active bacteria in these dinoflagellates. Intracellular bacteria were detected in A. tamarense and A. lusitanicum. In field studies, the occurrence of putatively toxic bacteria together with Alexandrium spp. and PSP in mussels was investigated. Lugol's fixed field samples from the Orkney Islands were screened for these bacteria in two sequential years. For the first time, fluorescently labelled probes were successfully applied to Lugol's fixed environmental samples. The bacteria were detected in high numbers in the samples, when both Alexandrium spp. were present and absent in the water column and when mussels contained PSP. The effect of purportedly toxic bacteria on mussel toxicity was examined with feeding experiments. The bacteria were detected in situ in tissue sections of the mussels gut. Mussel flesh did not show toxification after feeding upon the putatively toxic bacteria, although the blue mussels have filtered the bacteria in high numbers. The physical interactions between bacteria and dinoflagellates were investigated by conducting reassociation experiments with purportedly toxic bacteria which were originally isolated from the Alexandrium tamarense clone used in this experiments, or isolated from another related A. tamarense clone, respectively. No intracellular reassociated PTB-bacteria were detected in the dinoflagellates. However, a preferred extracellular attachment of the PTB-1 bacteria to the A. tamarense strain was observed
