Intraspecific variation in multiple trait responses of Alexandrium ostenfeldii towards elevated pCO₂

Dissolved oceanic CO₂ concentrations are rising as result of increasing atmospheric partial pressure of CO₂ (pCO₂), which has large consequences for phytoplankton. To test how higher CO₂ availability affects different traits of the toxic dinoflagellate Alexandrium ostenfeldii, we exposed three strains of the same population to 400 and 1,000 µatm CO₂, and measured traits including growth rate, cell volume, elemental composition, ¹³C fractionation, toxin content, and volatile organic compounds (VOCs). Strains largely increased their growth rates and particulate organic carbon and nitrogen production with higher pCO₂ and showed significant changes in their VOC profile. One strain showed a significant decrease in both PSP and cyclic imine content and thereby in cellular toxicity. Fractionation against ¹³C increased in response to elevated pCO₂, which may point towards enhanced CO₂ acquisition and/or a downscaling of the carbon concentrating mechanisms. Besides consistent responses in some traits, other traits showed large variation in both direction and strength of responses towards elevated pCO₂. The observed intraspecific variation in phenotypic plasticity of important functional traits within the same population may help A. ostenfeldii to negate the effects of immediate environmental fluctuations and allow populations to adapt more quickly to changing environments.