Affinity for inorganic carbon of Gracilaria tenuistipitata cultured at lowand high irradiance

Regulation by irradiance level of the mechanism for dissolved inorganic car bon (DIC) acquisition was examined in the red macroalga Gracilaria tenuisti pitata Zhang et Xia. For this purpose, affinity for external DIC, carbonic anhydrase (CA; EC 4.2.1.1) activity and content of ribulose-1,5-bisphosphat e carboxylase/oxygenase (Rubisco; EC 4.1.1.39) were determined in thalli gr own at 45 and 500 mu mol photons m(-2) s(-1). Oxygen evolution rates declin ed by 50% when the medium pH was changed from 8.1 to 8.7, and the pH compen sation point attained was ca. 9.2. These characteristics were unaffected by the light treatments. In contrast, photosynthetic conductance for DIC at p H 8.7 was doubled in thalli grown at high irradiance compared with those gr own at low irradiance (to 0.74 x 10(-6) from 0.33 x 10(-6) m s(-1)). Photos ynthetic rates at saturating Die concentration were also higher by 60% in t halli grown at high irradiance. These differences could not be attributed t o changes in the use of external DIC, since external CA activity did not va ry. Although the irradiance level did not modify the pool size of Rubisco, Rubisco content expressed on a chlorophyll a basis was almost doubled at hi gh irradiance. These results likely indicate that the internal transport of DIC towards the active-site of Rubisco, rather than the external use of DI C, is enhanced in the thalli grown at high irradiance.