Reduction in the toxicity of the dinoflagellate Gymnodinium catenatum when fed on by the heterotrophic dinoflagellate Polykrikos kofoidii

Abstract

To investigate the ability of the heterotrophic dinoflagellate Polykrikos kofoidii to reduce the toxicity produced by the dinoflagellate Gymnodinium catenatum (toxicity = 2.0 to 3.9 Mouse Units [MU]/105 G. catenatum cells), we used the mouse bioassay to measure the toxicity retained in a population of P. kofoidii, originally fed G. catenatum for 2 d (ingestion rate = 5.6 cells grazer-1 d-1) and then starved. As a control, we measured the toxicity retained in a population of P. kofoidii originally fed a non-toxic strain of Scrippsiella trochoidea. The toxicity retained in a population of P. kofoidii at Hour 0 (P. kofoidii starved for 0 to 48 h after being fed) was 17.3 MU/105 P. kofoidii cells. With increasing elapsed time after starvation the toxicity rapidly decreased to 3.6 MU/105 P. kofoidii cells at Hour 24, slowly decreased between Hours 36 and 81, and was not detectable at Hour 96. The decay constant in the exponential equation of the regression line for the toxicity in 105 P. kofoidii cells between Hours 0 and 96 was 0.059. No toxicity was detected in P. kofoidii fed S. trochoidea. This evidence suggests that the starvation of P. kofoidii fed G. catenatum may provide dissipation of the toxicity caused by this prey in marine food webs.