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The aim of this study was to investigate whether increased phytoplankton growth rates and biomass caused by higher nutrient availability will result in lower oxidative stress in microalgae by reducing effective-light exposure, a process called self-shading. This hypothesis was tested by a 6 d mesocosm experiment carried out at the Zingst Marine Station, Germany (southern Baltic Sea, 8 to 9 psu) in June 2001. Out-door mesocosms filled with 1400 l of natural seawater sieved through a 100 µm plankton net were manipulated by daily additions of nitrate and phos- phate (NP treatment), and nitrate, phosphate and silicate (NPSi treatment). Oxidative stress para- meters monitored daily were H2O2 concentrations in the seawater and concentrations of superoxide dismutase (SOD) activity, catalase activity and lipoperoxidation products (thiobarbituric acid-reac- tive substances, TBARS) in the phytoplankton. Chlorophyll a (chl a) concentration, pH and photo- synthesis (O2 evolution) were measured throughout the experiment to describe the physiological status of the phytoplankton. Our data suggest that fast growth of nutrient-saturated microalgae strongly limits oxidative stress by self-shading. This was shown by significantly lower SOD and catalase activities in the nutrient treatments than in the control treatment without nutrient addi- tions. No differences were observed between the 2 nutrient treatments (with or without Si), despite higher photosynthetic rates in the diatom-rich mesocosms (NPSi). Our findings may contribute to a better understanding of the close relationships between eutrophication, the occurrence of algal blooms and antioxidant activity in microalgae.

Self-shading protects phytoplankton communities against H2O2-induced oxidative damage