Effects Of Interspecific Competition On Physiological Responses In Economic Seaweed Sargassum Fusiforme Under Ocean Acidification

Due to increased anthropogenic CO₂ emissions,ocean acidification becomes the third major environmental problem following global warming and ozone depletion.Changes in seawater carbonate systems caused by ocean acidification directly affected the physiological characteristics and chemical compositions of algal primary producers in marine ecosystems.Artificial cultivation of economic seaweed Sargassum.fusiforme in large scale played an important role in the stability of offshore ecosystem.During cultivation practice,S.fusiforme was affected by ocean acidification and the competition of epiphytic macrophytes simultaneously.Thus,S.fusiforme and its common epiphytes Ulva lactuca were employed in these studies.After reasonable exploration of culture density,the interspecific competition between S.fusiforme and U.lactuca under ocean acidification was studied under laboratory conditions.Characteristics of photosynthesis and nitrogen absorption and assimilation of these algae were evaluated.This is aiming to provide data reference for the future seaweed cultivation at the background of climate change,and for the practice of epiphytes removal.Studies on culture densities of two seaweeds showed that,the photosynthesis and photosynthetic pigments of both algae were reduced by the shading effects under high density conditions.Under low-density,excessive light inhibited photosynthetic activities in S.fusiforme and U.lactuca.There were higher growth rates under moderate culture densities for both S.fusiforme and U.lactuca.Therefore,moderate culture densities were chosen to carry out experiments of ocean acidification and interspecific competition.This was also indicated a reasonable density for S.fusiforme marine cultivation practice.Studies on the two seaweeds affected by ocean acidification showed that,ocean acidification had different effects on the activity of their photosynthesis-related enzymes,while enhanced the photosynthesis of the two algae.Ocean acidification significantly increased activities of nitrate reductase and glutamate synthase in S.fusiforme,while decreased them in U.lactuca.Ocean acidification significantly reduced the activities of glutamine synthetase both in S.fusiforme and U.lactuca.Although the promotion of ocean acidification on U.lactuca growth was not significant,it's beneficial to the growth of the two seaweeds.Studies on the competitions between S.fusiforme and U.lactuca showed that,under ambient air,interspecific competition significantly reduced the activity of sucrose phosphate synthase in S.fusiforme,and inhibited its photosynthesis.but had different effects on different N-metabolism-related enzyme activities.Competition significantly increase dark respiration in S.fusiforme.Under interspecific competition,S.fusiforme needed more energy to regulate its metabolic process physiologically to counter the competition with U.lactuca.Moreover,under condition of ocean acidification,competitions of photosynthetic light and CO₂ utilization between S.fusiforme and U.lactuca were aggravated,and the photosynthesis of S.fusiforme was also significantly inhibited.However,photosynthetic efficiency of U.lactuca was significantly increased.Interspecific competition enhanced the activities of nitrogen metabolism related enzyme and increased the C/N ratio in S.fusiforme,this alleviated the competition for nutrition with U.lactuca.In addition,under ocean acidification,interspecific competition significantly reduced the total amino acid content of S.fusiforme,which lowered the quality of the commercial product.