A Comparative Study Of Photosynthetic Capacity In Three Marine Green Algae

“Green tide”is an ecological disaster in the coastal areas globally.Green macroalgae in intertidal zone proliferates and accumulates and caused the high biomass locally or harmful algal bloom by disengagement from attachment and floating into the sea area.Its outbreak is not only driven by environmental factors such as temperature,light intensity and nutrients,but also closely related to its own physiological characteristics.Previous studies demonstrated that the green-tide species show higher carbon fixation capacity and environmental tolerance.In this study,three green-tide species including Ulva prolifera,U.intestinalis and U.expansa,were selected for the study of the photosynthetic pathway and antioxidant capacity differences of these species under the conditions of high temperature and high light intensity in summer based on the field culture experiment.The correlation of enzyme activities of key photosynthetic enzymes and tissue carbon stable isotope(?¹³C)values of three macroalgal speices were also studied.This study is hope to provide scientific basis for the biological mechanisms study of the outbreak of green tide.The main results are as follows:1.The field culture experiment were conducted under the conditions of high temperature and high light intensity.The key enzyme activities of three photosynthetic enzymes(ribulose-1,5-bisphosphatecarboxylase/oxygenase(Rubisco),phosphoenolpyruvate carboxylase(PEPCase)and phosphoenolpyruvate carboxylated kinase(PEPCKase))and antioxidant enzymes(superoxide dismutase(SOD),peroxidase(POD))of the three species were detected,and their corresponding relationship with tissue?¹³C were analyzed.The results showed that significant differences were found in photosynthetic pathways and antioxidant capacities among U.prolifera,U.intestinalis and U.expansa.The?¹³C values in tissue of U.prolifera and U.intestinalis ranged from-19.4‰to-17.4‰and from-17.1‰to-15.7‰separately in the process of photosynthesis,and the key enzyme activity of Hatch-Slack cycle(C₄)was highly expressed.While the key enzyme activity of calvin cycle(C₃)was inhibited under high light intensity,which indicated that the photosynthesis of the two Ulva species was significantly correlated with light intensity and temperature and both C₃and C₄ pathway should be involved in.Besides,the two Ulva species indicated strong antioxidant capacity in the culture experiment.In contrast,the activity expression of C₄ photosynthetic enzymes of U.expansa was relatively lower and there was no significant correlation between light intensity and temperature.And?¹³C in tissue ranged from-23.5‰to-21.9‰,indicated that the photosynthesis was mainly C₃pathway.By contrast,the activities of photosynthetic enzymes and antioxidant enzymes of U.prolifera collected from the Yellow Sea were significantly higher than U.intestinalis and U.expansa,indicating that its photosynthetic capacity and antioxidant capacity were significantly higher than U.intestinalis and U.expansa.2.For further explanation of the characteristics of photosynthetic pathways of U.prolifera,the uptake ability of U.prolifera to CO₂ and HCO₃^-under the conditions of high temperature and high light intensity was studied and the activities of C₃ and C₄key photosynthetic enzymes(Rubisco,PEPCase,PEPCKase)and carbonic anhydrase(CA),and their corrections with tissue?¹³C values were analyzed through the field culture experiment.The results showed that the photosynthesis of U.prolifera involved C₃,C₄ pathways and CA mechanism.The photosynthesis mainly depends on C₃pathway to absorb CO₂,with HCO₃^-being used through CA mechanism under low CO₂,and C₄photosynthetic pathway being activated under high temperature and high light.The?¹³C values in tissue of U.prolifera(-19.4‰to-17.4‰)are between the ranges of the?¹³C values in tissue of C₃ and C₄ plants.The joint contribution of C₄ pathway and CA mechanism exceeds 50%in carbon fixation under high temperature and high light.The efficient and varied photosynthetic pathways of U.prolifera explained its physiological mechanism of rapidly forming massive floating algae mats during the outbreaks.Most marine algae assimilate CO₂ through C₃ cycle,and have developed CO₂-concentrating mechanisms(CCMs)by catalyzing HCO₃^-to gather CO₂at low CO₂level via CA to increase the efficiency of carbon fixation.Our results showed that the photosynthesis of U.prolifera should involve C₃,C₄ pathways and CA mechanism.And the photosynthesis of U.intestinalis also might involve C₃ and C₄ pathways.The results explained the capacity of these species in efficient utilization of HCO₃^-and quick accumulation of massive biomass during the green tide.In adddition,the results also indicated that some green-tide species would not perform the photosynthetic characteristics.For U.expansa,the carbon fixation and antioxidant capacity were weak during the experiment,without expression of the C₄ pathway and CCM.Therefore,the comparison study on the photosynthetic mechanism in different green-tide species needs further exploration to clarify the biological principles of their interspecific differences.