Acclimation of Red Sea macroalgae to solar radiation: Photosynthesis and thallus absorptance

Thallus absorptance, daily courses of maximal quantum yields of chlorophyll fluorescence of photosystem II (F_v/F_m) and electron transport rates (ETR) were determined in 3 macroalgae (Hypnea spinella, Rhodophyta; Sargassum vulgare, Phaeophyta; and Ulva lactuca, Chlorophyta) collected from a low nutrient supply (LNS) ecosystem (Red Sea, Gulf of Aqaba, Eilat, Israel). In addition, U. lactuca grown previously in high nutrient supply (HNS) fishpond effluents was used to evaluate the effect of nutrient enrichment on photosynthesis. Short-term (2 d) and mid-term (5 d) effects of both photosynthetically active radiation (PAR) and full solar irradiance (PAB: PAR+UVA+UVB) at 3 solar irradiance treatments (no neutral filters, no NF; 2 neutral filter layers, 2NF; and 4 neutral filter layers, 4NF simulating levels of irradiance at 5, 10 and 20 m depth respectively) were evaluated in algae incubated with running seawater. Significant effects of time (variation throughout the day) and irradiance (number of neutral filters) on both F_v/F_m and ETR were observed. Photoinhibition occurred at noon under the no NF (up to 40% decrease) and 2NF (up to 30% decrease) treatments; however, full recovery in all treatments was reached in the afternoon (dynamic photoinhibition) except in H. spinella. The highest maximum ETR was observed in the no NF treatment despite its strongest photoinhibitory effect. The decrease in F_v/F_m at noon was similar to or higher under the PAB than under the PAR treatment alone, depending on the species. Photoinhibition was lower and the maximum ETR was higher in HNS than in LNS U. lactuca, indicating positive effects of nutrient supply on both photosynthesis and photoprotection. The maximum ETRs exhibited by different algae may be related to their zonation in the field and the resulting energy supply.