Bicarbonate uptake by Southern Ocean phytoplankton

Abstract

Marine phytoplankton have the potential to significantly buffer future increases in atmospheric carbon dioxide levels. However, in order for CO2 fertilization to have an effect on carbon sequestration to the deep ocean, the increase in dissolved CO2 must stimulate primary productivity; that is, marine phototrophs must be CO2 limited [Riebesell et al., 1993]. Estimation of the extent of bicarbonate (HCO3−) uptake in the oceans is therefore required to determine whether the anthropogenic carbon sources will enhance carbon flux to the deep ocean. Using short‐term 14CO2‐disequilibrium experiments during the Southern Ocean Iron Experiment (SOFeX), we show that HCO3− uptake by Southern Ocean phytoplankton is significant. Since the majority of dissolved inorganic carbon (DIC) in the ocean is in the form of bicarbonate, the biological pump may therefore be insensitive to anthropogenic CO2. Approximately half of the DIC uptake observed was attributable to direct HCO3− uptake, the other half being direct CO2 uptake mediated either by passive diffusion or active uptake mechanisms. The increase in growth rates and decrease in CO2 concentration associated with the iron fertilization did not trigger any noticeable changes in the mode of DIC acquisition, indicating that under most environmental conditions the carbon concentrating mechanism (CCM) is constitutive. A low‐CO2 treatment induced an increase in uptake of CO2, which we attributed to increased extracellular carbonic anhydrase activity, at the expense of direct HCO3− transport across the plasmalemma. Isotopic disequilibrium experimental results are consistent with Southern Ocean carbon stable isotope fractionation data from this and other studies. Although iron fertilization has been shown to significantly enhance phytoplankton growth and may potentially increase carbon flux to the deep ocean, an important source of the inorganic carbon taken up by phytoplankton in this study was HCO3−, whose concentration is negligibly affected by the anthropogenic rise in CO2. We conclude that biological productivity in this region of the world's ocean is unlikely to be directly regulated by natural or anthropogenic variations in atmospheric CO2 concentrations because of the presence of a constitutive CCM.