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

Photosynthetic utilization of inorganic carbon in the marine diatom Phaeodactylum tricornutum was investigated by the pH drift experiment, measurement of K(1/2) values of dissolved inorganic carbon (DIC) with pH change, and comparison of the rate of photosynthesis with the rate of the theoretical CO(2) formation from uncatalyzed HCO(3)(-) 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 HCO(3)(-) utilization. The photosynthetic rate reached 150 times the theoretical CO(2) supply rate at 100 micromol 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, HCO(3)(-) in the medium is taken up through K(+)-dependent and -independent HCO(3)(-) transporters. The K(1/2) (CO(2)) values at pH 8.2 were about 4 times higher than those at pH 9.0, whereas the K(1/2) (HCO(3)(-)) 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 HCO(3)(-) transport patterns in this alga. Photosynthetic rate and affinity for HCO(3)(-) in the presence of K(+), respectively, were about 2- and 7-fold higher than those in the absence of K(+), indicating that K(+)-dependent HCO(3)(-) transport is a predominant pattern of HCO(3)(-) cellular uptake in low DIC concentration. However, as P. tricornutum was cultured at pH 7.2 or 8.0, photosynthetic affinities to HCO(3)(-) were not affected by K(+), implying that K(+)-dependent HCO(3)(-) transport is induced when P. tricornutum is cultured at high alkaline pH.