Short-term effects of increased CO2, nitrate and temperature on photosynthetic activity in Ulva rigida (Chlorophyta) estimated by different pulse amplitude modulated fluorometers and oxygen evolution

Short-term effects of pCO(2) (700-380 ppm; High carbon (HC) and Low carbon (LC), respectively) and nitrate content (50-5 mu M; High nitrogen (HN) and Low nitrogen (LN), respectively on photosynthesis were investigated in Ulva rigida (Chlorophyta) under solar radiation (in-situ) and in the laboratory under artificial light (ex-situ). After six days of incubation at ambient temperature (AT), algae were subjected to a 4 degrees C temperature increase (AT+4 degrees C) for 3 d. Both in-situ and ex-situ maximal electron transport rate (ETRmax) and in situ gross photosynthesis (GP), measured by O-2 evolution, presented highest values under HCHN, and lowest under HCLN, across all measuring systems. Maximal quantum yield (Fv/Fm), and ETRmax of photosystem (PS) II [ETR(II)(max)] and PSI [ETR(I)(ma)(x)], decreased under HCLN at AT+4 degrees C. Ex situ ETR was higher than in situ ETR. At noon, Fv/Fm decreased (indicating photoinhibition), whereas ETR(II)(max) and maximal non-photochemical quenching (NPQ(max)) increased. ETR(II)(max) decreased under AT+ 4 degrees C in contrast to Fv/Fm, photosynthetic efficiency (alpha(ETR)) and saturated irradiance (E-K). Thus, U. rigida exhibited a decrease in photosynthesis under acidification, changing LN, and AT+4 degrees C. These results emphasize the importance of studying the interaction between environmental parameters using in-situ versus ex-situ conditions, when aiming to evaluate the impact of global change on marine macroalgae.