Domoic acid production by the diatom Pseudo-nitzschia multiseries as a function of division rate in silicate-limited chemostat culture

In batch culture, the pennate diatom Pseudo-nitzschia multiseries (=Pseudo-nitzschia pungens f. multiseries) produces the neurotoxin domoic acid (DA) primarily during the stationary phase when cell division ceases due to limitation by silicate or phosphate. Here we show that P. multiseries is also capable of producing DA while growing in silicate-limited chemostat cultures. Two experiments were carried out to explore the apparent paradox of DA production by dividing cells in a continuous culture system. Pseudo-nitzschia multiseries was grown in non-axenic silicate-limited chemostats at division rates of ca. 0.2, 0.4, 0.6, and 0.8 d super(-1). There was an inverse linear relationship (r super(2) = 0.998 and 0.949 for the two experiments) between division rate and DA per cell. Cellular DA values were 8.5 and 5.4 pg cell super(-1) for the slowest dividing cultures, and 0.5 and 0.6 pg cell super(-1) for the fastest dividing cultures for Experiments 1 and 2, respectively. In Experiment 2, we reciprocally switched the dilution rates of the chemostats after 40 d. This resulted in the expected corresponding increase or decrease in cellular DA content, depending on the initial dilution rate of the culture. The results support the conclusion that DA is a secondary metabolite produced during conditions of stress due, e.g., to silicate limitation.