ENVIRONMENTAL MODULATION OF KARLOTOXIN LEVELS IN STRAINS OF THE COSMOPOLITAN DINOFLAGELLATE,KARLODINIUM VENEFICUM (DINOPHYCEAE)1

We examined the influence of N or P depletion, alternate N‐ or P‐sources, salinity, and temperature on karlotoxin (KmTx) production in strains of Karlodinium veneficum (D. Ballant.) J. Larsen, an ichthyotoxic dinoflagellate that shows a high degree of variability of toxicity in situ. The six strains examined represented KmTx 1 (CCMP 1974, MD 2) and KmTx 2 (CCMP 2064, CCMP 2283, MBM1) producers, and one strain that did not produce detectable karlotoxin under nutrient‐replete growth conditions (MD 5). We hypothesized that growth‐limiting conditions would result in higher cell quotas of karlotoxin. KmTx was present in toxic strains during all growth phases and increased in stationary and senescent phase cultures under low N or P, generally 2‐ to 5‐fold but with some observations in the 10‐ to 15‐fold range. No karlotoxin was observed under low‐N or low‐P conditions in the nontoxic strain MD 5. Nutrient‐quality (NO₃, NH₄, urea, and glycerophosphate) did not affect growth rate, but growth on NH₄ produced 2‐ to 3‐fold higher cellular toxicity and a 50% higher ratio of KmTx 1‐1:KmTx 1‐3 in CCMP 1974. CCMP 1974 showed higher cellular toxicity at low salinity (≤5 ppt) and high temperature (25°C). Our results suggested that given the presence of a toxic strain of K. veneficum in situ, the existence of environmental conditions that favor cellular accumulation of karlotoxin is likely a significant factor underlying K. veneficum–related fish kills that require both high cell densities (10⁴ · mL^?1) and high cellular toxin quotas relative to those generally observed in nutrient‐replete cultures.