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We studied the physiological response of phytoplankton to the interacting effects of 3 factors affected by global climate change: CO2, nutrient loading and irradiance. Treatments had a high and low level for each factor: CO2 was bubbled at 1000 ppm by volume versus present atmospheric values; high nutrient treatments had a combination of inorganic and organic nutrients; and light treatments were obtained by covering the tanks with a single or double layer of screen. We measured esterase activity, oxidative stress (ROS), cell death, DNA damage, photosynthetic efficiency and C-14 assimilation as particulate or dissolved organic material (POC and DOC respectively). Conditions simulating future global change scenarios showed similar chlorophyll-normalized primary productivity as present conditions. The main effect driving phytoplankton physiology was the downregulation of the photosynthetic apparatus by elevated CO2, which decreased esterase activity, ROS, cell death and DNA damage. Nutrient concentration and light acted as additional modulators, upregulating or contributing to downregulation. The percentage of (DOC)-C-14 extracellular release (PER) was low (0 to 27%), significantly lower under ultraviolet radiation (UVR) than under photosynthetically active radiation (PAR), and acted mainly to re-equilibrate the internal balance when cells grown under UVR were exposed to PAR. PER was almost 3 times lower under high CO2, confirming a higher resource use efficiency of phytoplankton under future CO2 concentrations.
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