MEPS 483:47-66 (2013)  -  DOI: https://doi.org/10.3354/meps10279

ABSTRACT: Physiological characteristics of inorganic C uptake were examined in Southern Ocean ice algae and phytoplankton assemblages. Ice algal and phytoplankton assemblages were largely dominated 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. Extracellular carbonic anhydrase (eCA) was detectable in all samples, but with significantly lower activity in sea ice algae. Neither HCO₃^- transport nor eCA activity was related to the in situ partial pressure of CO₂ (pCO₂) or taxonomic composition of samples. The half-saturation constant (K_S) for inorganic C ranged from ~100 to 5000 µM, and showed significantly more variability among sea ice algae than phytoplankton assemblages. For the phytoplankton assemblages, there were significant positive correlations between in situ pCO₂ and K_S (higher C substrate affinity in low pCO₂ waters), and also between K_S and maximum C uptake rates (V_max). In contrast, K_S and V_max in sea-ice algal assemblages were not correlated to each other, or to any other measured variables. The C isotope composition of particulate organic carbon(δ¹³C-POC) in the phytoplankton assemblages showed modest variability (range -30 to -24.6‰) and was significantly correlated to the ratio of inferred growth rates (derived from V_max) and in situ CO₂ concentrations, but not to any measured C uptake parameters. δ¹³C-POC in sea ice algal samples (range -25.7 to -12.9‰) was significantly heavier than in the phytoplankton assemblages, and not correlated to any other variables. Our results provide evidence for the widespread occurrence of carbon-concentrating mechanisms in Southern Ocean sea ice algae and phytoplankton assemblages.

KEY WORDS: Phytoplankton · Sea ice algae · Inorganic carbon uptake · HCO₃^- · Carbonic anhydrase