ELEVATED ATMOSPHERIC CARBON DIOXIDE INCREASES ORGANIC CARBON FIXATION BY <i>EMILIANIA HUXLEYI</i> (HAPTOPHYTA), UNDER NUTRIENT‐LIMITED HIGH‐LIGHT CONDITIONS¹

Abstract

Phytoplankton play a key role in determining the partitioning of CO 2 between the atmosphere and the ocean on seasonal, interannual, and millennial time scales. The magnitude of biological draw‐down of atmospheric CO 2 and C storage in the oceans is affected by concurrent changes in other environmental factors, like nutrient supply. Furthermore, variations in carbon‐to‐nitrogen (C:N) and carbon‐to‐phosphorus (C:P) assimilation ratios modify the oceanic CO 2 storage capacity. Here we show that increased atmospheric CO 2 concentration enhances CO 2 fixation into organic matter by a noncalcifying strain of Emiliania huxleyi (Lohmann) Hay &amp; Mohler only under certain conditions, namely high light and nutrient limitation. Enhanced organic matter production was accompanied by marked deviations of the C:N:P ratio from the canonical stoichiometry of marine particulate matter of 106:16:1 (C:N:P) known as the Redfield ratio. Increased cell organic carbon content, C:N, and C:P were observed at high light when growth was either nitrogen or phosphorus limited. Elevated CO 2 led to further increases in the particulate C:N and C:P ratios. Enhanced CO 2 uptake by phytoplankton such as E. huxleyi , in response to elevated atmospheric CO 2 , could increase carbon storage in the nitrogen‐limited regions of the oceans and thus act as a negative feedback on rising atmospheric CO 2 levels.