Inter- and intra-specific responses of coccolithophores to CO₂-induced ocean acidification

Abstract. Oceanic uptake of anthropogenic carbon dioxide (CO₂) is altering the seawater chemistry of the world's oceans with consequences for marine bioregions, especially calcareous organisms such as corals, foraminifera and coccolithophores. The coccolithophores, one of the most abundant and widespread groups of calcifying plankton, are responsible for a large proportion of modern oceanic carbonate production. However, culture experiments examining the response of coccolithophores to elevated CO₂ partial pressure (pCO₂) have mostly been based on investigations of a single strain and have yielded contradictory results from different experiments between and even within species. Here, four strains of the coccolithophores Emiliania huxleyi (E. huxleyi) and Gephyrocapsa oceanica (G. oceanica), which contained separately naked and calcifying strains, were investigated simultaneously for the first time in a bubbling batch culture at four CO₂ grades ranging from approximately 380 to 2000 μatm. We synchronously determined multiple physiological parameters of four coccolithophore strains involving growth, photosynthesis, nitrogen uptake, elemental compositions and calcification efficiency in the process of cultivation. The results did not show a uniform response from different strains to elevated pCO₂ up to 2000 μatm, and the naked strain E. huxleyi (N-E) was seriously suppressed, in sharp contrast to the positive response of the different levels of the other three algae. In addition, we fitted nitrogen uptake rate response curves relative to changing pCO₂ for the four strains and applied kinetic constants from the response curves to further analyze the hypostatic difference among different strains, which reflected the same variational trend of the four stains above vs. increasing CO₂. We determined that the responses of coccolithophores to ocean acidification are inter- and intra-specific, and this variation may cause changes to biodiversity and other ecosystem processes in the future ocean.