Photosynthesis of Ulva sp: III. O(2) Effects, Carboxylase Activities, and the CO(2) Incorporation Pattern.

Ulva, a common green seaweed, performs at the biochemical level as a typical C(3) plant. Over 90% of label was found in glycerate 3-phosphate following a 3 second (14)C pulse in the light, and the label was subsequently transferred to sugars. Also, the level of ribulose-1,5-bisphosphate carboxylase activity in crude extracts was about 10 times higher than that of phosphoenolpyruvate carboxylase. Concerning gas exchange, photosynthetic rates of Ulva showed no O(2) sensitivity, indicating that photorespiratory CO(2) losses are repressed as in C(4) plants. This apparent anomaly could be explained by the efficient HCO(3) (-) uptake system of Ulva which might concentrate CO(2) to the chloroplasts, thus suppressing the oxygenase activity of ribulose-1,5-bisphosphate carboxylase.