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

Abstract

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 pCO2 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 pCO2 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 pCO2Skeletonema 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 pCO2 world, and that ocean acidification may have structuring effects on phytoplankton communities.

Quantitative Analysis of the Trade-Offs of Colony Formation for Trichodesmium

“…Colony formation in marine phytoplankton can be subject to various environmental and ecological drivers, such as temperature constraints, turbulence, predator cues, or nutrient availability ( 27 – 29 ). Colony adaptations can often be species-specific, with additive effects when phytoplankton are exposed to more than one environmental change ( 30 ). We added a parameter to our model to test the influence of external drivers on colony performance in Trichodesmium .…”

Mass spectroscopy reveals compositional differences in copepodamides from limnic and marine copepods

“…Bioluminescent taxa such as Lingulodinium polyedrum and Alexandrium tamarense increase light production 12 . Chain forming diatoms split up colonies into smaller units 13 , 14 . Other diatoms have been shown to increase silica content and stickiness, which results in cell aggregation 15 .…”

“…Other diatoms have been shown to increase silica content and stickiness, which results in cell aggregation 15 . Induced toxin production and morphological changes are accompanied by increased resistance to grazers 14 , 16 , 17 . Well defended taxa may subsequently benefit from a competitive edge which can contribute to the formation of HABs.…”

Copepodamides in marine and freshwater copepods-similar but different

“…Other diatoms have been shown to increase in silica content and stickiness, which results in cell aggregation [13]. Induced toxin production and morphological changes are accompanied by increased resistance to grazers [10,12,14,15]. Well defended taxa may subsequently benefit from a competitive edge which can contribute to the formation of HABs.…”

“…Bioluminescent taxa such as Lingulodinium polyedrum and Alexandrium tamarense increase light production [10]. Chain forming diatoms split up colonies into smaller units [11,12]. Other diatoms have been shown to increase in silica content and stickiness, which results in cell aggregation [13].…”