Photosynthetic responses and anatomical features of two marine macroalgae with different CO 2 compensation points

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Codium decorticatum (Woodward) Howe and Udotea flabellum (Ellis and Solander) Lamouroux are marine, siphonaceous macroalgae in the Chlorophyta. The C. decorticatum thallus consisted of chloroplastic and achloroplastic filaments of 132 μm diameter, while the filaments of the U. flabellum thallus were small (24 μm diameter) and chloroplastic. At 2 mM dissolved inorganic carbon (DIC), photosynthesis in C. decorticatum, but not in U. flabellum, was inhibited by O 2; the CO 2 compensation points in these algae were high and low (54 and 7 μl CO 2 l −1), respectively. Growth under different temperature, photoperiod and CO 2 regimes failed to alter the CO 2 compensation points, indicating stability in the high and low photorespiratory states. Both macroalgae were shade species, with light compensation and saturation points for photosynthesis of 5–10 and 250 μmol quanta m −2 s −1, respectively. Although U. flabellum had a greater photosynthetic affinity for CO 2 and HCO 3 − than C. decorticatum, neither was saturating at seawater DIC levels (2.0 mM at pH 8). Apparent K m values indicated free CO 2 was preferred, but the high HCO 3 − in seawater was probably the major photosynthetic DIC source. The photosynthetic Q 10 of U. flabellum was almost twice that of C. decorticatum; the former was able to photosynthesize and grow at temperatures above 30°C, whereas the latter could not. For U. flabellum, a C 4-like mechanism is probably a factor minimizing photorespiration and influencing its tropical distribution, whereas C 3-like photosynthesis in C. decorticatum may account for its high photorespiration and more temperate range.