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 0.3d⁻¹ and 0.1d⁻¹. CO₂ and temperature conditions were 300, 550 and 900μatm pCO₂ at 14°C and 900μatm 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 900μatm pCO₂ compared to 300 and 550μatm pCO₂. At 900μatm pCO₂, elevation of temperature as expected for a greenhouse ocean, further increased cell densities and POC concentrations. In contrast to POC concentration, C-quotas (pmolCcell⁻¹) were similar at D=0.3d⁻¹ in all cultures. At D=0.1d⁻¹, a reduction of C-quotas by up to 15% was observed in the 900μatm 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=0.3d⁻¹, and from 0.11 to 0.17 at D=0.1d⁻¹, with variations primarily induced by the changes in POC. At D=0.1d⁻¹, cell volume was reduced by up to 22% in cultures grown at 900μatm 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.