Karlodinium veneficum feeding responses and effects on larvae of the eastern oyster Crassostrea virginica under variable nitrogen:phosphorus stoichiometry

Mixotrophic feeding can be promoted by nutrient-enriched prey, a nutritional strategy which can provide benefits to some toxic microalgae under nutrient-imbalanced conditions. However, it is unclear how the nutritional condition of the predator or the prey affects the mixotrophy and toxicity of toxin-producing mixotrophs. Laboratory experiments were conducted to measure growth and feeding rates of Karlodinium veneficum with addition of Rhodomonas salina as prey under varied nitrogen (N):phosphorus (P) stoichiometry (molar N:P of 4, 16 and 32) of both predator and prey and with K. veneficum initially in different growth phases (exponential and stationary). Growth rates of initially exponential- and stationary-phase K. veneficum were enhanced in the presence of prey with reciprocal nutrient conditions. Feeding rates (measured as prey death rates) were highest for low-NP K. veneficum initially growing exponentially and mixed with N-rich prey. Maximum feeding rates of low-NP K. veneficum on N-rich prey during exponential growth were ~4-fold higher than the rates of high-NP K. veneficum on N-rich prey. The nutritionally different K. veneficum were tested with larvae of the eastern oyster Crassostrea virginica to compare putative toxicity. Larval mortality was significantly increased in 2 d exposures to high-NP K. veneficum monocultures in both growth phases. When mixed with N-rich prey, the presence of K. veneficum resulted in significantly enhanced larval mortality, but this was not the case for low-NP K. veneficum in exponential phase. Enhanced growth of K. veneficum and increased negative effects of K. veneficum on larval survival appeared to be highest when fed prey with higher N:P content.