Costs and Benefits of Putatively Anti-Grazing Defenses in the Marine Dinoflagellate Alexandrium catenella

The typical framework for studies and models of bloom dynamics of toxigenic algae is based on environmental determinants (e.g., light, nutrients) of cell growth rate and toxin production rate. There is mounting evidence, however, of grazer control of toxin production, which then may impact cell growth. This thesis examines the costs and benefits of grazer-induced toxin production in the toxigenic dinoflagellate Alexandrium catenella. Paralytic Shellfish Toxin (PST) production was significantly increased regardless of growth phase when cells were contemporaneously exposed directly or indirectly to grazers, relative to cells not exposed to grazers. The same effect was observed for cells that had been previously exposed to grazers regardless of nutrient regime. In contrast, the pattern for cell growth rate was the opposite – cells directly or indirectly exposed to grazers showed lower growth rates than unexposed cells, with little effect of growth phase or nutrient regime. The results suggest a trade-off between toxin production and growth rate. A subsequent experiment confirmed a trade-off using toxin production rate and the relative expression of a cell growth gene, which is correlated with cell division rate. A fitness tax equivalent to a reduction in cell division rate of 56% was evident when grazers induced cells to increase toxin production. This grazer-mediated fitness tax should be incorporated in population dynamic models of toxigenic prey. Costs and benefits of toxin production were simultaneously tested in laboratory studies that measured net cell growth rates of three Alexandrium catenella strains that differed in PST production (low, moderate and high PST production, respectively) versus a gradient of grazing pressure. Both the moderate toxigenic and the high toxigenic strain showed a fitness benefit relative to the low toxigenic strain; however, the highest benefit was derived in the moderate toxigenic strain, which also reduced cell size and increased cell division rate in response to grazing. This suggests that in addition to toxin production other mechanisms of defense are simultaneously expressed, adding complexity to the study of defense mechanisms