Toxin variability in natural populations of Alexandrium fundyense in Casco Bay, Maine—evidence of nitrogen limitation

The dinoflagellate Alexandrium fundyense is a common, recurring harmful algal bloom (HAB) species in the Gulf of Maine. To date, most physiological measurements of phytoplankton in the field provide data on the entire community, yet efforts to obtain species-specific data are particularly important for understanding the ecological and physiological dynamics of HAB species, such as, Alexandrium. Alexandrium spp., do not usually dominate the planktonic community in the Gulf of Maine, but are of great interest due to the potent toxins produced. In order to determine the nutritional status of Alexandrium spp. in natural populations, indicators of nutrient deprivation need to be identified that are specific to that one species. To date, the saxitoxin content of A. fundyense is known to vary under different environmental conditions such as nitrogen and phosphorous limitation. However, in batch culture the composition of the toxin (the relative amounts of each saxitoxin derivative per cell) appears to be a stable quantity and thus is sometimes viewed as a biochemical marker of individual strains. In more recent studies, toxin composition has been shown to vary during progressive N- and P- limitation, once the cells are given time to achieve steady state in semi-continuous, nutrient-limited cultures. Using both the absolute toxin concentrations and relative proportion (mole % total toxin) of each toxin derivative, N- and P-limitation can be distinguished based on the observed trends in the different saxitoxin derivatives. In this study, we examine the toxin content and composition in natural A. fundyense populations during a spring bloom in Casco Bay, ME from April–June of 1998. This allows us to examine whether A. fundyense populations in the western Gulf of Maine are sufficiently homogenous to permit the detection of toxin composition and toxin content differences through time and space, and if so, to determine whether those changes are indicative of a particular nutritional state (e.g., N-limitation). Using both toxin composition and toxin ratios determined from field samples during an A. fundyense bloom, the ratios generally correlated with N-limitation in the Casco Bay region.