Ocean acidification effects on haploid and diploid Emiliania huxleyi strains: Why changes in cell size matter

Abstract

The effects of ocean acidification (OA) on Emiliania huxleyi non-calcifying haploid strain CCMP 374 (s374) and calcifying diploid strain CCMP 2668 (s2668) were investigated in this study. Both E. huxleyi strains were cultured semi-continuously for ~20 generations under three pCO2 levels, one representing equilibrium with contemporary atmospheric pCO2 and two representing different OA scenarios projected for the end the century. During the course of the experiments, measurements were made for cell size in s374 and coccosphere size in strain s2668, cellular intrinsic growth rates, and the per cell (s374) and per coccosphere (s2668) concentrations of particulate organic carbon (POC) and nitrogen (PON), particulate inorganic carbon (PIC), chlorophyll a (chl a) and particulate dimethylsulfoniopropionate (DMSPp). In agreement with other studies, elevated pCO2 led to a small increase in POC and PON in the diploid strain, but not in the haploid strain. Although the PIC was static across pCO2 treatments in the diploid calcifier, PIC:POC was reduced under OA due to the disproportionate increase in POC. The concentration of E. huxleyi DMSPp cell−1 increased (s2668) or responded non-linearly (s374) to OA. Exposure to OA caused cell and coccosphere volumes to increase by 18% and 37% from the ambient to high pCO2 treatment in the haploid and diploid strain, respectively, despite equivalent cross-treatment growth rates. Therefore, when normalized to cell and coccosphere volume, small to negligible increases in cellular constituents under OA translated into decreased cellular and coccosphere densities (e.g. mass vol−1) of those same constituents. This produces seemingly contradictory cellular responses to OA depending on data normalization, and highlights the need to cautiously interpret OA experimental data when cell and coccosphere size is not reported. A consistent OA-induced increase in cell and coccosphere size has implications for E. huxleyi cellular physiology, ecology and trophic interactions, as cell size is a master characteristic that governs cellular rate processes, predator encounter rates, and predator ingestion efficiencies. Further, because POC and PON did not scale with volume increases, there is a dilution in the carbon and nitrogen concentration in E. huxleyi cells under OA. This result could have cascading effects on planktonic food webs as predator ingestion rates and growth efficiencies adjust.