Assessing Allelopathic Effects of Alexandrium fundyense on Thalassiosira sp.

Dinoflagellates of the genus Alexandrium produce neurotoxins responsible for Paralytic Shellfish Poisoning. They bloom annually in the Gulf of Maine, raising concern for their impacts on human health and the shellfish industry. Attempts to understand the dynamics of such harmful proliferations have shown alternating higher abundances of A. fundyense and diatoms in the Gulf of Maine and it has been suggested that competitive interactions, in addition to cyst bed dynamics and abiotic factors, between these two groups are important in determining A. fundyense success. One way by which A. fundyense may be able to outcompete faster-growing diatoms is by releasing chemicals to inhibit diatom growth, a process known as allelopathy.

Despite well-documented allelopathic potential of Alexandrium spp., no studies have examined these interactions between A. fundyense and important Gulf of Maine diatoms. To assess the allelopathic potential of A. fundyense against a diatom competitor, I studied growth and nutrient acquisition by the chain-forming diatom Thalassiosira sp. in the presence and absence of filtrates obtained from Alexandrium cultures. Thalassiosira cells exposed to filtrate of A. fundyense exhibited “bleaching” and both growth and nutrient utilization ceased for up to 4 days compared to controls. Results from this study support the existence of chemically mediated interactions, though the relatively high A. fundyense concentrations required to elicit a response suggest a greater role of such interactions in the maintenance of already established populations of A. fundyense, rather than a role in the initial stages of population growth. The magnitude of the effect was dependent on filtrate concentration and Thalassiosira cell size. Thalassiosira cultures that had undergone cell enlargement via sexual reproduction were less sensitive to A. fundyense filtrate, recovering earlier and showing less “bleaching” which suggests that competitor cell size, independent from taxonomy, is likely to be important in shaping the outcome of allelopathic interactions. The findings presented here reveal a novel aspect of allelopathic interactions in the plankton but also signify a potential ecological impact of diatom cell size reduction and sexual reproduction that has not yet been described and that may be important in determining diatom survival and success.