Photosynthetic response of laboratory-cultured Halophila ovalis to thermal stress

Chlorophyll a fluorescence was able to rapidly detect responses of laboratory-cultured Halophila ovalis to acute changes in temperature. Six heating (27.5, 30.0, 32.5, 35.0, 37.5 and 40.0°C and 6 chilling (10.0, 12.5, 15.0, 17.5, 20.0 and 22,5"C) stress levels were used over a 96 h exposure period, followed by a 5 d recovery period, to provide a comprehensive measure of the overall thermal stress effects and responses. The acute (5 h) response of H. ovalis to thermal shock was characterised by a change in photochemical quenching, whilst after 96 h the effective and maximum quantum yields were able to detect temperature changes of ±2.5OC Maximum fluorescence declined for both heating and chilling, whilst minimum fluorescence was stable for chilling and increased with moderate heating. H. ovalis was susceptible to thermal stress outside the optimum photosynthetic range of 25 to 3OoC, where extreme temperatures (10.0, 12.5, 37.5 and 40°C caused a complete collapse of the PSI1 electron transport system. When thermal stress was applied in darkness, chlorophyll a fluorescence was not able to detect the onset of thermal stress (except at 40.0°C H. ovalis tolerated thermal shock from 15 to 30° for up to 96 h , and was able to completely recover on return to standard growth conditions.