Effects of copepod chemical cues on intra- and extracellular toxins in two species ofDinophysis

Abstract

Copepods may contribute to harmful algal bloom formation by selectively rejecting harmful cells. Additionally, copepods and the chemical cues they exude, copepodamides, have been shown to induce increased toxin production in paralytic and amnesic toxin producing microalgae. However, it is unknown if diarrhetic shellfish toxin (DST) producers such asDinophysisrespond to copepods or copepodamides in a similar fashion. Here we expose laboratory cultures ofDinophysis sacculusandD. acuminatato direct grazing byAcartiasp. copepods or copepodamides and measure their toxins after three days. Total DSTs increased by 8 - 45% inD. sacculusbut was significantly different from controls only in the highest (10 nM) copepodamide treatment whereas toxin content was not affected inD. acuminata. Growth rate was low across all groups and explained up to 91% of the variation in toxin content. DSTs were redistributed from internal compartments to the extracellular medium in the highest copepodamide treatments (5 - 10 nM), which were two to three times higher than controls and indicates an active release or passive leakage of toxins. Untargeted analysis of endometabolomes indicated significant changes in metabolite profiles for both species in response to the highest copepodamide treatments, independent of known toxins. However, it is not clear whether these are stress responses or caused by more complex mechanisms. The relatively small grazer-induced effect inDinophysisobserved here, compared to several species ofAlexandriumandPseudo-nitzschiareported previously, suggests that DST production inDinophysisis likely not induced by copepods, except perhaps in patches with high copepod densities. DSTs may, however, represent a constitutive chemical defence forDinophysis, or serve an altogether different purpose.