TOXIN VARIABILITY DURING THE CELL-CYCLE OF THE DINOFLAGELLATE ALEXANDRIUM FUNDYENSE

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 conte nt, toxin composition, and several other physiological parameters were monitored every 2 h for 94 h. Toxin production was discontinuous, ind uced 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 similar to 8-10 h in early G(1) and dropped to zero for the remainder of the interphase and mitosis. Analysis of t oxin composition showed that cellular concentrations of all the saxito xin derivatives followed a similar pattern of increase, stabilization, and decrease throughout one generation time. A putative sequence of i nterconversions between the derivatives could be established, with C2 as the first compound to appear. Division of a subset of the populatio n during the first 24 h of the experiment and the ensuing total synchr ony of the culture suggest the existence of two transition points in t he cell cycle of this dinoflagellate. The first transition point, at t he 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-depend ent and arrests the cells in G(1). We propose a model of the cell cycl e of A. fundyense in which progression through the cell cycle can be a rrested at two different transition points located in G(1) and toxin p roduction is induced by light during G(1). The restriction of toxin pr oduction to a relatively short segment of the cell cycle provides a to ol for comparing cells that are and are not synthesizing toxin.