Photosynthesis, carbon uptake and antioxidant defence in two coexisting filamentous green algae under different stress conditions

The physiological basis for ecological processes is in many cases little understood. The purpose of this work was to link 3 important physiological processes in algae—photosynthesis, carbon uptake and antioxidant defence—to life form. The organisms used were the morphologically similar filamentous green algal speciesCladophora glomerataandUlva procera, which seemingly occupy the same niche when they co-occur in dense belts in the upper littoral zone of the brackish Baltic Sea in summer. Their life strategies are different: the annualC. glomeratausually stays attached throughout summer, while the ephemeralU. proceracan appear and disappear from the same site from week to week. The algae were growing in the field under exactly the same conditions (mixed stands) and were immediately used in experiments at a field station. Fundamental ecophysiological differences were found between the 2 species. (1) Higher photosynthetic activity was detected inU. procera. (2) More shade-adaptations were found inU. procera, and more sun-adaptations inC. glomerata. (3)C. glomeratauses a proton pump for HCO₃⁻ transport, and carbon uptake does not depend on periplasmic carbonic anhydrase. This is an advantage in dense algal belts with longer periods of carbon limitation. (4)C. glomeratainvests more in carotenoid protection against oxidative stress, including high carotenoid/chlorophyll ratios and a functional violaxanthin xanthophyll cycle. (5)U. procerawas more sensitive to oxidative stress created by UV-B radiation thanC. glomerata, which correlates with a more effective intracellular (carotenoid and enzymatic) defence against oxidative stress inC. glomerata. (6) H₂O₂ in the seawater medium had a negative effect on photosynthesis inC. glomerata, but not inU. procera. This suggests that a high release of H₂O₂ byU. proceraunder oxidative stress may damageC. glomerata. While the ecophysiological traits ofC. glomerataseem to be directed toward persistence, those ofU. proceraseem to be more engaged with large but short-term gains. This is in accordance with their different life strategies.