Photosynthesis and cell composition of Porphyra leucosticta discs grown at low (< 0.0001% in air), current (control) and high (1% CO2 in air) inorganic carbon (C-i) concentrations were analyzed. Carbohydrate content in discs grown at high C-i increased (15.1 mg g(-1) FW) with respect to the control (6.4 mg g FW-1), whereas soluble protein content decreased to one-third (5.6 to 2.1 mg g(-1) FW). Carbohydrate content was unaffected and soluble protein slightly increased in discs grown at low C-i. As a consequence of these changes, a lower C/N molar ratio (8.6) was found in the discs grown at low compared to high C-i (12.4). Nitrate reductase activity increased at high C-i from 0.3 +/- 0.2 to 1.7 +/- 0.4 mu mol NO2- g(-1) FW h(-1) indicating that reduction and assimilation of nitrate were uncoupled. The response of photosynthesis to increasing irradiance, estimated from O-2 evolution vs. irradiance curves, was affected by the treatments. Maximum quantum yield (Phi (O2)degrees) and effective quantum yield (Phi (O)2) at 150 mu mol photon m(-2) s(-1) decreased by 20% and 50%, respectively, at low C-i. These differences could be due to changes in photosynthetic electron flow between PSII and PSI. Treatments also produced changes in maximal (F-v/F-m) and effective (Delta F/F-m') quantum yield for photosystem II charge separation.
