Interactive effects of seawater carbonate chemistry, light intensity and nutrient availability on physiology and calcification of the coccolithophore Emiliania huxleyi

Abstract. Rising atmospheric carbonate dioxide (CO₂) levels lead to increasing CO₂ concentration and declining pH in seawater, as well as ocean warming. This enhances stratification and shoals the upper mixed layer (UML), hindering the transport of nutrients from deeper waters and exposing phytoplankton to increased light intensities. In the present study, we investigated combined impacts of CO₂ levels (410 μatm (LC) and 925 μatm (HC)), light intensities (80–480 μmol photons m⁻² s⁻¹) and nutrient concentrations [101 μmol L⁻¹ dissolved inorganic nitrogen (DIN) and 10.5 μmol L⁻¹ dissolved inorganic phosphate (DIP) (HNHP); 8.8 μmol L⁻¹ DIN and 10.5 μmol L⁻¹ DIP (LN); 101 μmol L⁻¹ DIN and 0.4 μmol L⁻¹ DIP (LP)] on growth, photosynthesis and calcification of the coccolithophore Emiliania huxleyi. HC and LN synergistically decreased growth rates of E. huxleyi at all light intensities. High light intensities compensated for inhibition of LP on growth rates at LC, but exacerbated inhibition of LP at HC. These results indicate that the ability of E. huxleyi to compete for nitrate and phosphate may be reduced in future oceans with high CO₂ and high light intensities. Low nutrient concentrations increased particulate inorganic carbon quotas and the sensitivity of maximum electron transport rates to light intensity. Light-use efficiencies for carbon fixation and calcification rates were significantly larger than that of growth. Our results suggest that interactive effects of multiple environmental factors on coccolithophores need to be considered when predicting their contributions to the biological carbon pump and feedbacks to climate change.