Two green macroalgae, Codium decorticatum and Udotea flabellum, differ photosynthetically. Codium had high O2-sensitive, and Udotea low 02-insensitive, CO2 compensation points; Codium showed a Warburg effect at seawater dissolved inorganic carbon levels and had photorespiratory C02 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 ethoxyzolamideinsensitive. 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 C4 acid metabolism and P-enolpyruvate regeneration were sufficient to operate a limited C4-like system. In Udotea, malate and aspartate were earlylabeled 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 C4-like system, based on PEPCK, is hypothesized to be the mechanism reducing photorespiration in Udotea. Codium showed no evidence of photosynthetic C4 acid metabolism. Marine macroalgae, like terrestrial angiosperms, seem to have diverse photosynthetic modes.In the three divisions of marine macroalgae, Rhodophyta (reds), Phaeophyta (browns), and Chlorophyta (greens), Ru-BPCO2 has been reported to be a major carboxylase, and labeling studies point to a functional PCR cycle (6,22,24). Photorespiration measurements and labeling of glycine and serine suggest that the PCO cycle also operates, and, furthermore, all three divisions contain species which exhibit 02 inhibition of photosynthesis or the Warburg effect and high r values (5-7, 22, 24, 28). This has led to the general conclusion that marine macroalgae are C3 (5) There is evidence that many, but not all, marine macroalgae use HCO3 ions for photosynthesis (4,6,10,22,24) and that CA is required (4, 10, 22). Thus, the possibility that they concentrate C02, in a manner comparable to unicellular organisms, has been broached (4, 6, 10); though there are no direct measurements of internal DIC levels to substantiate this hypothesis.Enzymes of C4 acid metabolism and C4 acid labeling during photosynthesis occur in some macroalgae (6,(22)(23)(24). In the case of brown macroalgae and marine diatoms, substantial ,8-carboxylation of PEP in the light and dark has been reported (20,24). The enzyme responsible is PEPCK acting in a carboxylating mode, not PEPC which occurs in terrestrial C4 plants (6,20,22,24). Dark fixation and diel titratable acidity changes associated with the PEPCK activity (22,24) are small, and a CAM-like system in brown macroalgae has been discounted (21). The operation of C4-like photosynthesis in marine macroalgae (23) is ...

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