Effect of CO2 concentration on C:N:P ratio in marine phytoplankton: A species comparison

Abstract

The effect of variable concentrations of dissolved molecular carbon dioxide, [CO(2,aq)], on C:N:P ratios in marine phytoplankton was studied in batch cultures under high light, nutrient-replete conditions at different irradiance cycles. The elemental composition in six out of seven species tested was affected by variation in [CO(2,aq)]. Among these species, the magnitude of change in C:N:P was similar over the experimental CO2 range. Differences in both cell size and day length-dependent growth rate had little effect on the critical CO2 concentration below which a further decrease in [CO(2,aq)] led to large changes in C:N:P ratios. Significant CO2-related changes in elemental ratios were observed at [CO(2,aq)] < 10 μmol kg-1 and correlated with a CO2-dependent decrease in growth rate. At [CO(2,aq)] typical for ocean surface waters, variation in C:N:P was relatively small under our experimental conditions. No general pattern for CO2-related changes in the elemental composition could be found with regard to the direction of trends. Either an increase or a decrease in C:N and C:P with increasing [CO(2,aq)] was observed, depending on the species tested. Diurnal variation in C:N and C:P, tested in Skeletonema costatum, was of a similar magnitude as CO2-related variation. In this species, the CO2 effect was superimposed on diurnal variation, indicating that differences in elemental ratios at the end of the photoperiod were not caused by a transient buildup of carbon-rich storage compounds due to a more rapid accumulation of carbohydrates at high CO2 concentrations. If our results obtained under high light, nutrient-replete conditions are representative for natural phytoplankton populations, CO2-related changes in plankton stoichiometry are unlikely to have a significant effect on the oceanic carbon cycle.