Inorganic carbon acquisition in two green marine Stichococcus species

ABSTRACT The mechanism of inorganic carbon (Ci) uptake was examined in the marine green microalgae Stichococcus cylindricus and Stichococcus minor. External carbonic anhydrase (CA) activity was not detected in either species, by potentiometric assay or by mass spectrometry. Photosynthetic characteristics of Ci uptake indicate that both species have high apparent affinity for CO2 with a low K1/2(CO2) of about 10 µm. The O2 evolution rates in light exceeded the spontaneous CO2 formation rate by 2.5‐fold in both species, which thus have active bicarbonate uptake. Mass spectrometric monitoring of CO2 and O2 fluxes showed that rates of O2 evolution exceeded those of CO2 depletion by about three‐ and twofold in S. minor and S. cylindricus, respectively, and also showed, in cells photosynthesizing at pH 8.2, a rapid depletion of CO2 upon illumination to a CO2 compensation concentration of 15.42 and 12.03 µm in S. minor and S. cylindricus, respectively. Both species also exhibit active CO2 uptake: addition of bovine CA at CO2 compensation concentration caused a rapid rise in CO2 as the CO2–HCO3‐ equilibrium was restored. Accumulation of unfixed Ci by cells at pH 8.2 was calculated to be 84.33 mm in S. cylindricus, and 30.37 mm in S. minor to give internal accumulations of 23‐ and 8‐fold, respectively, compared to the external Ci concentration. This paper is the first to describe the occurrence of both active CO2 and active bicarbonate uptake in marine green algae and corrects previous published studies on Stichococcus species, which contended that no active transport of inorganic carbon occurred in these algae at seawater pH. The data also demonstrates that the rate of active CO2 uptake greatly exceeds that of bicarbonate uptake in a marine alga and also demonstrates the effectiveness of external carbonic anhydrase in stimulating inorganic carbon uptake in a marine alga.