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: Marine dinoflagellates in the genus Alexandrium synthesize Saxitoxin (STX) and several of its derivatives. The aim of this thesis was to analyze novel aspects of toxigenesis and the cell cycle in Alexandrium and to apply molecular techniques to gain new insights on the regulation of STX biosynthesis. Synchronized cultures of A. fundyense were studied to determine the dynamics of toxin production throughout the cell cycle. STX biosynthesis was shown to be discontinuous and constrained to approximately 10 hours at the beginning of G1. Two cell cycle transition points were identified: a light-dependent one and a size-dependent one. Semi-continuous cultures of A. fundyense were used to determine the effects of temperature and phosphate limitation on the coupling of STX biosynthesis to O%. Inferred toxin production rates were constant and correlated to G1 over a range of growth rates. The effects of phosphate on STX biosynthesis were analyzed and a scheme of interconversions among STX derivatives is proposed. Differential Display (DD) analysis was applied to the identification of up- or downregulated genes during toxigenesis in A. fundyense. The expression patterns of S-adenosylhomocysteine hydrolase, methionine aminopeptidase and a histone-like protein were related to periods of STX biosynthesis.

Cell cycle dynamics and the physiology of saxitoxin biosynthesis in Alexandrium fundyense (Dinophyceae)