Inorganic C utilization and C isotope fractionation by pelagic and sea ice algal assemblages along the Antarctic continental shelf

Abstract

Physiological characteristics of inorganic C uptake were examined in Southern Oceanice algae and phytoplankton assemblages. Ice algal and phytoplankton assemblages were largelydominated by diatoms and Phaeocystis antarctica, and showed a high capacity for HCO3−utilization, with direct HCO3− transport accounting for ~60% of total inorganic C uptake. Extracellularcarbonic anhydrase (eCA) was detectable in all samples, but with significantly lower activityin sea ice algae. Neither HCO3− transport nor eCA activity was related to the in situ partial pressureof CO2 (pCO2) or taxonomic composition of samples. The half-saturation constant (KS) for inorganicC ranged from ~100 to 5000 μM, and showed significantly more variability among sea ice algaethan phytoplankton assemblages. For the phytoplankton assemblages, there were significant positivecorrelations between in situ pCO2 and KS (higher C substrate affinity in low pCO2 waters), andalso between KS and maximum C uptake rates (Vmax). In contrast, KS and Vmax in sea-ice algal assemblageswere not correlated to each other, or to any other measured variables. The C isotopecomposition of particulate organic carbon (δ13C-POC) in the phytoplankton assemblages showedmodest variability (range −30 to −24.6‰) and was significantly correlated to the ratio of inferredgrowth rates (derived from Vmax) and in situ CO2 concentrations, but not to any measured C uptakeparameters. δ13C-POC in sea ice algal samples (range −25.7 to −12.9‰) was significantly heavierthan in the phytoplankton assemblages, and not correlated to any other variables. Our results provideevidence for the widespread occurrence of carbon-concentrating mechanisms in SouthernOcean sea ice algae and phytoplankton assemblages.