Antioxidative Properties of Baltic Sea Keystone Macroalgae (, Phaeophyceae) under Ocean Warming and Acidification in a Seasonally Varying Environment.

The keystone macroalga (Phaeophyceae), dominating shallow hard bottom zones, encounters a strongly and rapidly changing environment due to anthropogenic change over the last decades in the Baltic Sea. Thus, in four successive benthic mesocosm experiments, the single and joint effects of increased temperature (Δ + 5 °C) and pCO (1100 ppm) under ambient irradiances were experimentally tested on the antioxidative properties of western Baltic in all seasons. The antioxidative properties (superoxide dismutase activity and lipid peroxidation) as well as the sensitivity of photosynthetic performance (i.e., effective quantum yield) to oxidative stress under these global change scenarios were seasonally examined. exhibited high and relatively constant photosynthetic performance under artificial hydrogen peroxide (HO) stress in all seasons. High activities of superoxide dismutase and a relatively low degree of the biomarker for lipid peroxidation (malondialdehyde concentration) were found in . Thus, Baltic is equipped with a high antioxidative potential to tolerate strong oxidative stress for at least short periods. Antioxidative properties of were more strongly affected by warming than by acidification, resulting in significantly increased malondialdehyde concentrations under elevated temperature levels in all seasons. Oxidative stress was enhanced in under warming but seem to be modulated by seasonally varying environmental conditions (e.g., high and low irradiances) and pCO levels. However, more frequent summer heatwaves reaching and surpassing lethal temperatures in shallow coastal waters may determine the population's overall persistence in the Baltic Sea.