High plasticity in inorganic carbon uptake by Southern Ocean phytoplankton in response to ambient CO 2

Abstract

The fixation of dissolved inorganic carbon (DIC) by marine phytoplankton provides an important feedback mechanism on concentrations of CO 2 in the atmosphere. As a consequence it is important to determine whether oceanic primary productivity is susceptible to changing atmospheric CO 2 levels. Among numerous other factors, the acquisition of DIC by microalgae particularly in the polar seas is projected to have a significant effect on future phytoplanktonic production and hence atmospheric CO 2 concentrations. Using the isotopic disequilibrium technique the contribution of different carbon species (CO 2 and bicarbonate) to the overall DIC uptake and the extent to which external Carbonic Anhydrase (eCA) plays a role in facilitating DIC uptake was estimated. Simultaneous uptake of CO 2 and HCO 3 − was observed in all cases, but the proportions in which different DIC species contributed to carbon assimilation varied considerably between stations. Bicarbonate as well as CO 2 could be the major DIC source for local phytoplankton assemblages. There was a positive correlation between the contribution of CO 2 to total DIC uptake and ambient concentration of CO 2 in seawater suggesting that Southern Ocean microalgae could increase the proportion of CO 2 uptake under future high atmospheric CO 2 levels. Results will be discussed in view of metabolic costs related to DIC acquisition of Southern Ocean phytoplankton.