Differential effects of ocean acidification on carbon acquisition in two bloom‐forming dinoflagellate species
Eberlein, Tim | Van de Waal, Dedmer B. | Rost, Björn
Dinoflagellates represent a cosmopolitan group of phytoplankton with the ability to form harmful algal blooms. Featuring a Ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RubisCO) with very low CO₂ affinities, photosynthesis of this group may be particularly prone to carbon limitation and thus benefit from rising atmospheric CO₂ partial pressure (pCO₂) under ocean acidification (OA). Here, we investigated the consequences of OA on two bloom‐forming dinoflagellate species, the calcareous Scrippsiella trochoidea and the toxic Alexandrium tamarense. Using dilute batch incubations, we assessed growth characteristics over a range of pCO₂ (i.e. 180–1200 µatm). To understand the underlying physiology, several aspects of inorganic carbon acquisition were investigated by membrane‐inlet mass spectrometry. Our results show that both species kept growth rates constant over the tested pCO₂ range, but we observed a number of species‐specific responses. For instance, biomass production and cell size decreased in S. trochoidea, while A. tamarense was not responsive to OA in these measures. In terms of oxygen fluxes, rates of photosynthesis and respiration remained unaltered in S. trochoidea whereas respiration increased in A. tamarense under OA. Both species featured efficient carbon concentrating mechanisms (CCMs) with a CO₂‐dependent contribution of HCO₃ ⁻ uptake. In S. trochoidea, the CCM was further facilitated by exceptionally high and CO₂‐independent carbonic anhydrase activity. Comparing both species, a general trade‐off between maximum rates of photosynthesis and respective affinities is indicated. In conclusion, our results demonstrate effective CCMs in both species, yet very different strategies to adjust their carbon acquisition. This regulation in CCMs enables both species to maintain growth over a wide range of ecologically relevant pCO₂.
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