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

Burkhardt, Steffen, Ingrid Zondervan, Ulf Riebesell

Limnol. Oceanogr., 44(3), 1999, 683-690 | DOI: 10.4319/lo.1999.44.3.0683

ABSTRACT: The effect of variable concentrations of dissolved molecular carbon dioxide, [CO2,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 [CO2,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 [CO2,aq] led to large changes in C: N: P ratios. Significant CO2-related changes in elemental ratios were observed at [CO2,aq] < 10 umol kg-1 and correlated with a CO2-dependent decrease in growth rate. At [CO2,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 [CO2,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 v 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.

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