Biogeochemical response of Emiliania huxleyi (PML B92/11) to elevated CO2 and temperature under phosphorous limitation: A chemostat study

The present study investigates the combined effect of phosphorous limitation, elevated partial pressure of CO2 (pCO2) and temperature on a calcifying strain of Emiliania huxleyi (PML B92/11) by means of a fully controlled continuous culture facility. Two levels of phosphorous limitation were consecutively applied by renewal of culture media (N:P = 26) at dilution rates (D) of 0.3 d− 1 and 0.1 d− 1. CO2 and temperature conditions were 300, 550 and 900 μatm pCO2 at 14 °C and 900 μatm pCO2 at 18 °C. In general, the steady state cell density and particulate organic carbon (POC) production increased with pCO2, yielding significantly higher concentrations in cultures grown at 900 μatm pCO2 compared to 300 and 550 μatm pCO2. At 900 μatm pCO2, elevation of temperature as expected for a greenhouse ocean, further increased cell densities and POC concentrations. In contrast to POC concentration, C-quotas (pmol C cell− 1) were similar at D = 0.3 d− 1 in all cultures. At D = 0.1 d− 1, a reduction of C-quotas by up to 15% was observed in the 900 μatm pCO2 at 18 °C culture. As a result of growth rate reduction, POC:PON:POP ratios deviated strongly from the Redfield ratio, primarily due to an increase in POC. Ratios of particulate inorganic and organic carbon (PIC:POC) ranged from 0.14 to 0.18 at D = 0.3 d− 1, and from 0.11 to 0.17 at D = 0.1 d− 1, with variations primarily induced by the changes in POC. At D = 0.1 d− 1, cell volume was reduced by up to 22% in cultures grown at 900 μatm pCO2. Our results indicate that changes in pCO2, temperature and phosphorus supply affect cell density, POC concentration and size of E. huxleyi (PML B92/11) to varying degrees, and will likely impact bloom development as well as biogeochemical cycling in a greenhouse ocean.

► E. huxleyi cell density and POC concentration were higher at elevated CO2 and T.
► Carbon-cell-quotas were similar under P-limitation for all CO2 and T conditions.
► E. huxleyi shows a large plasticity of the P-cell-quota while grown at high CO2.
► Cell sizes were significantly decreased at high CO2 under severe P-stress.
► High C:N:P ratios suggest an enhanced carbon export in the greenhouse ocean.