Photosynthetic Fluorescence Characteristics And Decomposition Of Macroalgae In Seaweed Bed Of Gouqi Island

Photosynthesis is the most important biochemical reaction on the earth.It is also the largest process for synthesizing CO2 and H2 O on organic matter and releasing oxygen.Photosynthetic organisms provide energy for all life activities on the earth through photosynthesis.On the ecological scale,photosynthesis of plants lays the foundation for material circulation and energy flow of ecosystem.It can be said that photosynthesis plays a key role in maintaining the normal operation of the earth's ecosystem.Macroalgae is a large plant in the ocean.In some areas,seaweed beds are widely covered all over the world's rocky reef coastline.Macroalgae such as Phylum Phaeophyta and Phylum Chlorophyta are important keystone species in kelp bed ecosystem,and they are important primary productivity source of off-shore ecosystem.Seaweed bed provides a good habitat for many small fish and economic fish juveniles,also gathered large quantities of macrobenthos like snails.Nitrogen and Phosphorus are fixed through macroalgae.At the same time,macroalgae are considered to be effective species for solving greenhouse gas problems(absorbing and fixing CO2).First,macroalgae is a primary producer with great biomass,which absorbs CO2 through photosynthesis and then removes CO2 by harvesting algae.Second,macroalgae gradually remove C through litter decomposition after fixation of CO2,like entering sediments or serving as food into the food web.The basis of the ecological functions of macroalgae is their photosynthesis as plant.Macroalgae play an extremely important role in kelp bed ecosystem and even island and reef sea area.It absorbs CO2 during its lifetime,and synthetic organic matter is stored in the macroalgae.It is often the case that only 10-15% of the production from kelp beds is consumed locally by herbivores,while 85-90% of the production is not directly consumed and becomes detritus,which is frequently exported from kelp beds to distant habitats where it is consumed.we selected seaweed bed of Gouqi Island as the study area and choose the common macroalgae as the target species.Photosynthetic characteristics of macroalgae was studied through chlorophyll fluorescence technology.At the same time,the natural decomposition process of macroalgae was analyzed by carbon and nitrogen stable isotope techniques.The results are as follows:1.The Fv/Fm(maximal quantum yield of PS?in the dark)of Ulva pertusa,Cladophora stimpsonii,Grateloupia livida,Polysiphonia urceolata,and Hizikia fusifarme are lower than those of the same study about same species,indicating that the potential maximum photosynthetic capacity of the five species macroalgae were inhibited.Among them,both U.pertusa and H.fusifarme were suffered from temperature stress.2.The induction curves shows that Y(effective quantum yield),q P(photochemical quenching),and r ETR(relative electronic transfer rate)finally keep steady state when macroalgae are irradiated with actinic light.The Y,the q P and the r ETR all shows brown algae> green algae> red algae.Although Fv/Fm of U.pertusa and C.stimpsonii are higher than Hizikia fusifarme and Sargassum thunbergii,the photosynthetic capacity of Hizikia fusifarme and Sargassum thunbergii is higher than U.pertusa and C.stimpsonii in nature environment.3.Rapid light curves shows H.fusifarme and S.thunbergii had higher r ETRm(maximal relative electron transport rate)and higher Ek(half-saturation light intensity),indicating that under same light environment,H.fusifarme and S.thunbergii had better carbon sequestration capacity and carbon sequestration efficiency are higher,followed by U.pertusa.4.During the decomposition period of macroalgae litter,the organic C and N contents of the litter showed an overall decreasing trend.There is an excellent linear correlation between C and N contents.The ?13C does not change much compared with fresh macroalgae,but ?15N had great changes.If ?15N of single species litter or ?15N of mixed species litter are neglected,it will cause deviations when we do trophic level calculations,and thus affect the entire food chain structure.