Evidence for K+-Dependent HCO3− Utilization in the Marine Diatom Phaeodactylum tricornutum

Abstract

Abstract Photosynthetic utilization of inorganic carbon in the marine diatom Phaeodactylum tricornutum was investigated by the pH drift experiment, measurement of K1/2 values of dissolved inorganic carbon (DIC) with pH change, and comparison of the rate of photosynthesis with the rate of the theoretical CO2 formation from uncatalyzed HCO3− conversion in the medium. The higher pH compensation point (10.3) and insensitivity of the photosynthetic rate to acetazolamide indicate that the alga has good capacity for direct HCO3− utilization. The photosynthetic rate reached 150 times the theoretical CO2 supply rate at 100 μmol L−1 DIC (pH 9.0) in the presence of 10 mmol L−1 K+ and 46 times that in the absence of K+, indicating that for pH 9.4-grown P. tricornutum, HCO3− in the medium is taken up through K+-dependent and -independent HCO3− transporters. The K1/2 (CO2) values at pH 8.2 were about 4 times higher than those at pH 9.0, whereas the K1/2 (HCO3−) values at pH 8.2 were slightly lower than those at pH 9.0 whether without or with K+, providing further evidence for the presence of the two HCO3− transport patterns in this alga. Photosynthetic rate and affinity for HCO3− in the presence of K+, respectively, were about 2- and 7-fold higher than those in the absence of K+, indicating that K+-dependent HCO3− transport is a predominant pattern of HCO3− cellular uptake in low DIC concentration. However, as P. tricornutum was cultured at pH 7.2 or 8.0, photosynthetic affinities to HCO3− were not affected by K+, implying that K+-dependent HCO3− transport is induced when P. tricornutum is cultured at high alkaline pH.