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The production and composition of the paralytic shellfish toxins (PSTs) of 2 Alexandrium tamarense strains (CI01 and HK9301) in the presence and absence of bacteria were investigated. Attempts were made to produce bacteria-free dinoflagellate cultures using 2 antibiotic treatments. Antibiotic treatment 1 (penicillin-G and streptomycin) partially killed bacteria in 2 dinoflagellate cultures, while antibiotic treatment 2 (penicillin-G, streptomycin, ciprofloxacin, and gentamicin) completely killed bacteria in the 2 dinoflagellate cultures. The toxin production (total toxin yields and cellular toxin content) of the A. tamarense CI01 strain increased after antibiotic treatment 1, but decreased after antibiotic treatment 2. Except for the cellular toxin content after mid-stationary phase, the toxin production of the A. tamarense HK9301 strain after both antibiotic treatments was higher than that of the control cultures. Over the different growth phases, the toxin composition of the A. tamarense HK9301 strain changed when the bacteria were killed by antibiotics, while that of A. tamarense CI01 strain remained unchanged. Our findings suggest that the toxins from both strains are produced by the dinoflagellate cells and not by the bacteria associated with them. However, the increase in growth rates and cell density of the 2 dinoflagellates after partial and complete destruction of the bacteria from the cultures implies that bacteria can affect the level of dinoflagellate toxin production by nutrient competition. The reduction of toxin production by the axenic A. tamarense CI01 strain and the change in toxin composition of the A. tamarense HK9301 strain after antibiotic treatment indicate that bacteria could also affect dinoflagellate toxicity through different mechanisms other than nutrient competition. These potential effects need to be further investigated.

Variations in paralytic shellfish toxin and homolog production in two strains of Alexandrium tamarense after antibiotic treatments