Bacterium-dinoflagellate interactions: investigative microscopy of Alexandrium spp. (Gonyaulacales, Dinophyceae)

The association of bacteria With dinoflagellates has been a neglected field of study, which has gained prominence in recent years because of the possi ble role of bacteria in toxin synthesis. A number of dinoflagellates underg o sexual reproduction, passing through various life-cycle stages in additio n to the vegetative form. The presence of bacteria within dinoflagellates h as been well established, but their presence throughout the dinoflagellate life-cycle has not been investigated. Using cultures of Alexandrium (A. tam arense, A. fundyense), We investigated the association of bacteria With var ious vegetative growth phases (lag, log, stationary) and sexual life-cycle stages (planozygote, planomeiocyte, hypnozygote), using scanning electron m icroscopy, transmission electron microscopy (TEM) and epifluorescence micro scopy. Bacteria were found to be associated with the surfaces of vegetative cells, planozygotes, hypnozygotes and planomeiocytes. TEM showed bacteria to be present within all vegetative growth phases, as well as in the sexual planozygote, cyst and planomeiocyte. The presence of intracellular bacteri a in vegetative cells was also confirmed using DAPI staining combined with epifluorescence microscopy, and lipopolysaccharide staining combined with T EM.