Photosynthetic utilisation of inorganic carbon and its regulation in the marine diatom Skeletonema costatum.

Photosynthetic uptake of inorganic carbon and regulation of photosynthetic CO affinity were investigated in Skeletonema costatum (Grev.) Cleve. The pH independence of K(CO) values indicated that algae grown at either ambient (12 μmol L) or low (3 μmol L) CO predominantly took up CO from the medium. The lower pH compensation point (9.12) and insensitivity of photosynthetic rate to di-isothiocyanatostilbene disulfonic acid (DIDS) indicated that the alga had poor capacity for direct HCO utilisation. Photosynthetic CO affinity is regulated by the concentration of CO rather than HCO, CO or total dissolved inorganic carbon (DIC) in the medium. The response of photosynthetic CO affinity to changes in CO concentration was most sensitive within the range 3-48 μmol L CO. Light was required for the induction of photosynthetic CO affinity, but not for its repression, when cells were shifted between high (126 μmol L) and ambient (12 μmol L) CO. The time needed for cells grown at high CO (126 μmol L) to fully develop photosynthetic CO affinity at ambient CO was approximately 2 h, but acclimation to low or very low CO levels (3 and 1.3 μmol L, respectively) took more than 10 h. Cells grown at low CO (3 μmol L) required approximately 10 h for repression of all photosynthetic CO affinity when transferred to ambient or high CO (12 or 126 μmol L, respectively), and more than 10 h at very high CO (392 μmol L).