Toxin variability during the cell cycle of the dinoflagellate <i>Alexandrium fundyense</i>

Abstract

Cultures of the toxic dinoflagellate Alexandrium fundyense Balech were subjected to conditions that induced two synchronized divisions over a period of 48 h. Before, during, and after this interval, toxin content, toxin composition, and several other physiological parameters were monitored every 2 h for 94 h. Toxin production was discontinuous, induced by light, and always occurred during a defined time frame within the G 1 phase of the cell cycle. Specific toxin production rates were positive for a period of ∼8–10 h in early G 1 and dropped to zero for the remainder of the interphase and mitosis. Analysis of toxin composition showed that cellular concentrations of all the saxitoxin derivatives followed a similar pattern of increase, stabilization, and decrease throughout one generation time. A putative sequence of interconversions between the derivatives could be established, with C2 as the first compound to appear. Division of a subset of the population during the first 24 h of the experiment and the ensuing total synchrony of the culture suggest the existence of two transition points in the cell cycle of this dinoflagellate. The first transition point, at the beginning of G 1 , is light‐dependent and holds the cells in a G 0 ‐like period. The second block point at the end of G 1 is size‐dependent and arrests the cells in G 1 . We propose a model of the cell cycle of A. fundyense in which progression through the cell cycle can be arrested at two different transition points located in G 1 and toxin production is induced by light during G 1 . The restriction off toxin production to a relatively short segment of the cell cycle provides a tool for comparing cells that are and are not synthesizing toxin.