Effects of high irradiance and temperature on photosynthesis and photoinhibition in Nannochloropsis gaditana Lubian (Eustigmatophyceae)

Photosynthesis and photoinhibition of the marine microalga Nannochloropsis gaditana Lubian (Eustigmatophyceae) have been determined under different irradiance and temperature regimes by using O-2 evolution and pulse amplitude modulated fluorescence techniques. The photochemical efficiency (F-v/F-m) decreased to a similar extent (65 % of the initial) at both 25 and 35 degrees C at high irradiance of white light (1500 mu mol m(-2) s(-1)) as well as at low irradiance (150 mu mol m(-2) s(-1)) (75 % of the initial); however, photoinhibition was less pronounced at low irradiance after 4h of exposure. Recovery of photochemical efficiency at 25 and 35 degrees C treatments occurred during the first 2h when transferred to darkness at 25 degrees C. O-2 evolution showed a similar variation at 25 and 35 degrees C. Recovery of photosynthesis at 25 and 35 degrees C treatments was slower than that of photochemical efficiency, taking 24h to return to the initial values. At 40 degrees C, fluorescence parameters as well as O-2 evolution dropped after 15 min of exposure at both quanta fluence rates and did not return during the recovery period in darkness. At this temperature, damage to the photosynthetic apparatus was found even at low light, possibly through an effect of temperature on the water-splitting complex. Violaxanthin decreased and antheraxanthin and zeaxanthin increased under high irradiance at all temperatures. This result indicates the activity of the xanthophyll cycle acting as a photoprotection system. According to the data, three processes are proposed to be involved in the acclimation of the photosynthetic apparatus to the irradiance-temperature stress: 1) PSII reaction center turnover accompanied by changes in open PSII reaction center efficiency; these pathways have been found to fluctuate complementary to each other, 2) zeaxanthin-associated dissipation of excess absorbed energy and 3) reduction of the effective cross-section.