Antibiotic treatment enhances C2 toxin production by Alexandrium tamarense in batch cultures

The effects of a mixture of penicillin G and streptomycin on the growth and C2 toxin production of a marine dinoflagellate, Alexandrium tamarense CI01, were investigated to determine if antibiotic treatment would increase the toxin yield of the cultured algae in batch cultures. Algal growth and toxin production were both enhanced markedly when the culture was supplemented with the antibiotics, each at an initial concentration of 100 unit ml⁻¹ in medium,² but were severely inhibited when the concentration was 500 unit ml⁻¹ or higher. Short-term pretreatment of algal inocula with the antibiotics at 100, 500, and 1000 unit ml⁻¹ all produced the enhancing effects on the algal cultures in an autoclaved medium. A prolonged antibiotic pretreatment of the algal culture followed by repeated sterile cultivation resulted in an algal culture free of cultivable bacteria. This “drug-treated” culture became more resistant to the toxicity and more responsive to the enhancing effects of the antibiotics. Our results indicated that the antibiotics can enhance growth and C2 toxin productivity not only through their inhibition of the growth of bacteria that compete for nutrients with the coexisting algae, but also through their direct effects on the physiology of the algae. Supplementation of the two antibiotics therefore is an efficient way to increase the yield of C2 toxin in the production cultures of A. tamarense CI01.