Species Specific Responses to Grazer Cues and Acidification in Phytoplankton- Winners and Losers in a Changing World

Phytoplankton induce defensive traits in response to chemical alarm signals from grazing zooplankton. However, these signals are potentially vulnerable to changes in pH and it is not yet known how predator recognition may be affected by ocean acidification. We exposed four species of diatoms and one toxic dinoflagellate to future pCO(2) levels, projected by the turn of the century, in factorial combinations with predatory cues from copepods (copepodamides). We measured the change in growth, chain length, silica content, and toxin content. Effects of increased pCO(2) were highly species specific. The induction of defensive traits was accompanied by a significant reduction in growth rate in three out of five species. The reduction averaged 39% and we interpret this as an allocation cost associated with defensive traits. Copepodamides induced significant chain length reduction in three of the four diatom species. Under elevated pCO(2) Skeletonema marinoi reduced silica content by 30% and in Alexandrium minutum the toxin content was reduced by 30%. Using copepodamides to induce defensive traits in the absence of direct grazing provides a straightforward methodology to assess costs of defense in microplankton. We conclude that copepodamide signalling system is likely robust to ocean acidification. Moreover, the variable responses of different taxa to ocean acidification suggest that there will be winners and losers in a high pCO(2) world, and that ocean acidification may have structuring effects on phytoplankton communities.

Automated summary

Phytoplankton can respond to chemical alarm signals from grazing zooplankton by inducing defensive traits. However, these signals may be affected by changes in pH and it is not yet known how predator recognition will be affected by ocean acidification. In this study, we exposed diatoms and toxic dinoflagellates to future pCO(2) levels, along with predatory cues from copepods. The results showed that the effects of increased pCO(2) were species-specific, with a significant reduction in growth rate and changes in other traits. Our findings suggest that the copepodamide signalling system is likely to be robust to ocean acidification, but different taxa may have variable responses to it, indicating potential structuring effects on phytoplankton communities.