TEMPERATURE EFFECTS ON MICROALGAL PHOTOSYNTHESIS‐LIGHT RESPONSES MEASURED BY O2 PRODUCTION,PULSE‐AMPLITUDE‐MODULATED FLUORESCENCE,AND 14C ASSIMILATION1

Short‐term temperature effects on photosynthesis were investigated by measuring O₂ production, PSII‐fluorescence kinetics, and ¹⁴C‐incorporation rates in monocultures of the marine phytoplankton species Prorocentrum minimum (Pavill.) J. Schiller (Dinophyceae), Prymnesium parvum f. patelliferum (J. C. Green, D. J. Hibberd et Pienaar) A. Larsen (Coccolithophyceae), and Phaeodactylum tricornutum Bohlin (Bacillariophyceae), grown at 15°C and 80 μmol photons · m^?2 · s^?1. Photosynthesis versus irradiance curves were measured at seven temperatures (0°C–30°C) by all three approaches. The maximum photosynthetic rate (P^C_max) was strongly stimulated by temperature, reached an optimum for Pro. minimum only (20°C–25°C), and showed a similar relative temperature response for the three applied methods, with Q₁₀ ranging from 1.7 to 3.5. The maximum light utilization coefficient (α^C) was insensitive or decreased slightly with increasing temperature. Absolute rates of O₂ production were calculated from pulse‐amplitude‐modulated (PAM) fluorometry measurements in combination with biooptical determination of absorbed quanta in PSII. The relationship between PAM‐based O₂ production and measured O₂ production and ¹⁴C assimilation showed a species‐specific correlation, with 1.2–3.3 times higher absolute values of P^C_max and α^C when calculated from PAM data for Pry. parvum and Ph. tricornutum but equivalent for Pro. minimum. The offset seemed to be temperature insensitive and could be explained by a lower quantum yield for O₂ production than the theoretical maximum (due to Mehler‐type reactions). Conclusively, the PAM technique can be used to study temperature responses of photosynthesis in microalgae when paying attention to the absorption properties in PSII.