Alternative methods of photosynthetic carbon assimilation in marine macroalgae.

Two green macroalgae, Codium decorticatum and Udotea flabellum, differ photosynthetically. Codium had high O(2)-sensitive, and Udotea low O(2)-insensitive, CO(2) compensation points; Codium showed a Warburg effect at seawater dissolved inorganic carbon levels and had photorespiratory CO(2) release, whereas Udotea did not. Seawater dissolved inorganic carbon levels did not saturate photosynthesis. For Codium, but not Udotea, the Warburg effect was increased by ethoxyzolamide, a carbonic anhydrase inhibitor, at high but not low pH. Isolated chloroplasts from both macroalgae showed a Warburg effect that was ethoxyzolamide-insensitive. In both macroalgae, chloroplastic and extrachloroplastic carbonic anhydrase activity was present. P-enolpyruvate carboxykinase (PEPCK) carboxylating activity in Udotea extracts was equivalent to that of ribulose bisphosphate carboxylase, and enzyme activities for C(4) acid metabolism and P-enolpyruvate regeneration were sufficient to operate a limited C(4)-like system. In Udotea, malate and aspartate were early-labeled photosynthetic products that turned over within 60 seconds. Photorespiratory compounds were much less labeled in Udotea. Low dark fixation rates ruled out Crassulacean acid metabolism. A limited C(4)-like system, based on PEPCK, is hypothesized to be the mechanism reducing photorespiration in Udotea. Codium showed no evidence of photosynthetic C(4) acid metabolism. Marine macroalgae, like terrestrial angiosperms, seem to have diverse photosynthetic modes.