CO 2 ‐concentrating mechanisms in three southern hemisphere strains of E miliania huxleyi

Rising global CO 2 is changing the carbonate chemistry of seawater, which is expected to influence the way phytoplankton acquire inorganic carbon. All phytoplankton rely on ribulose‐bisphosphate carboxylase oxygenase ( RUBISCO ) for assimilation of inorganic carbon in photosynthesis, but this enzyme is inefficient at present day CO 2 levels. Many algae have developed a range of energy demanding mechanisms, referred to as carbon concentrating mechanisms ( CCM s), which increase the efficiency of carbon acquisition. We investigated CCM activity in three southern hemisphere strains of the coccolithophorid E miliania huxleyi W. W. Hay & H. P. Mohler. Both calcifying and non‐calcifying strains showed strong CCM activity, with HCO 3 − as a preferred source of photosynthetic carbon in the non‐calcifying strain, but a higher preference for CO 2 in the calcifying strains. All three strains were characterized by the presence of pyrenoids, external carbonic anhydrase ( CA ) and high affinity for CO 2 in photosynthesis, indicative of active CCM s. We postulate that under higher CO 2 levels cocco‐lithophorids will be able to down‐regulate their CCM s, and re‐direct some of the metabolic energy to processes such as calcification. Due to the expected rise in CO 2 levels, photosynthesis in calcifying strains is expected to benefit most, due to their use of CO 2 for carbon uptake. The non‐calcifying strain, on the other hand, will experience only a 10% increase in HCO 3 − , thus making it less responsive to changes in carbonate chemistry of water.