Alexandrium minutum is one of several dinoflagellate species capable of producing paralytic shellfish toxins. Previous work suggests that toxin levels are influenced by a number of parameters, including dinoflagellate-associated bacteria. In the present study, a toxin-producing culture of A. minutum isolated from Anakoha Bay in the Marlborough Sounds of New Zealand was subjected to an antibiotic treatment regimen designed to eliminate the associated bacteria. Antibiotics used included penicillin, streptomycin, kanamycin and tetracycline (Treatment 1); ciprofloxacin and gentamicin (Treatment 2); and penicillin, streptomycin and ciprofloxacin (Treatment 3). Enzyme immunoassay showed that saxitoxin levels in the A. minutum culture fell significantly following the first round of antibiotic treatment, and this coincided with a large reduction in the associated copiotrophic bacterial population. HPLC data indicated that there was also a reduction in gonyautoxins (GTX 1-3 ). The oligotrophic population was more difficult to eliminate and required 2 additional rounds of antibiotic treatment, but saxitoxin levels did not change any further. Scanning laser confocal microscopy following acridine orange staining was used to observe intracellular bacteria-like particles, which were considerably reduced by the end of the treatments, probably due to the inclusion of antibiotics that penetrate eukaryotic cells. Algal mean generation times were not significantly affected by the antibiotic treatments. Qualitative and quantitative changes in toxin production coincided with a reduction in the culturable, copiotrophic and/or intracellular bacteria in the A. minutum Anakoha A culture. The premise that bacteria can exert a strong influence on algal toxicity was supported by this study, although the mechanisms remain unknown.

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