Photosyntheitc Physiology And Ecological Effects Of Macroalgae In Gouqi Island

Macroalgae are the main supporting species in offshore reef habitats.The seaweed bed composed of macroalgae is the typical nearshore biological habitat with important ecological functions.It constructed the nearshore marine ecosystem together with coral reefs,mangroves,wetlands and other habitats.Macroalgae convert inorganic carbon into organic carbon and release oxygen through photosynthesis.At the same time absorb biogenic elements such as nitrogen and phosphorus in growth process,which not only improves ocean primary productivity,and provides nutritient for the herbivorous and detrital food chain,but also contributes to global biogeochemical cycle,effectively water eutrophication prevent and reduction greenhouse gases concentrations in the atmosphere.In addition,the biological community composited by macroalgae,namely seaweed bed,is of great significance for habitat construction,biodiversity improvement of coastal waters,the structure and function optimization of marine ecosystem and maintainance ecosystem stability.Coastal land-based pollution has gradually intensified alongside industry and agriculture development,as well as offshore aquaculture expansion and sewage disposal,which has seriously affected the marine environment and lead to the frequently occurrence of nearshore water eutrophication.On the other hand,with the wide application of macroalgae in many fields such as food,medicine,industry and agriculture in recent years,coastal fishermen driven by economic interests disorderly collect sargassum fusiformem,Undaria pinnatifida,Gelidium amansii Lamouroux and Sargassum thunbergii at low tide,resulting in a sharp decrease in the distribution and biomass of macroalgae in the offshore.With the increase of global energy consumption,atmospheric CO₂concentration and dissolved CO₂ and HCO₃^-in the sea water^-,the value of CO₃^2-and p H are decreasing,which contribute to the ocean acidification,thereby causing the degradation or even disappearance of algae and the bleaching of coral algae,reducing the primary productivity of seaweed bed ecosystem in nearshore reef area,and then affecting the related community structure of herbivorous benthos such as sea urchins and fish.In order to restore the function of offshore ecosystem and maintain the sustainable development of the ocean,China,Japan,South Korea and other Asian countries that mainly utilize macroalgae have successively launched researches on the reconstruction of seaweed bed and the utilization of ecosystem functions with the restoration of seaweed bed as the main means.However,at present the relevant domestic and international research mainly focuse on the cultured economic algae,such as Laminaria japonica and Gracilaria.The living habits of macroalgae in natural seaweed bed,the response of photosynthetic physiological activity to different environmental conditions,and the photosynthetic physiological activity characteristics of different algae communities in the same habitat is not reported specific and in-depth.To carry out the research on photosynthetic physiological activity of macroalgae could provide crucial support to the ecological restoration of natural seaweed bed,ecological function reconstruction of offshore seaweed bed,solving regional marine environmental problems,low-carbon economy development and the goal of achieving carbon neutralization.In this paper,the dominant species of typical macrobenthic algae in nature seaweed bed in Goiqi Island,in Shengsi Ma’an archipelago,Zhoushan,Zhejiang Province,China,are taken as the research objects.Sampling through SCUBA Diving,the photosynthetic pigment and chlorophyll fluorescence activities of benthic macroalgae are measured to explore the physiological response of different macroalgae to environmental factors.Taking the Sargassum horueri,the dominant species in seaweed bed,as an example,the photosynthetic activity,pigment composition and content in key growth stages are analyzed,trying to explore the physiological response mechanism of Sargassum horneri in different growth stages to environmental conditions.the ecological effects of seaweed bed is reported by analyzing the biomass of benthic macroalgae and raft epiphytic macroalgae and the content of total organic carbon（TOC）in the subtidal zone of seaweed bed of Gouqi Island.And on this basis,the carbon sequestration capacity of benthic macroalgae in the subtidal zone of coastal waters in China was preliminarily evaluated,with the results of the national coastal macroalgae survey.On the other hand,the adsorption and enrichment of heavy metals by macroalgae in the subtidal zone are detected and analyzed,and their ecological effects such as heavy metal purification are evaluated.The main results are as follows:（1）Through SCUBA Diving and Dving-PAM,the chlorophyll fluorescence parameters of 9 kinds of macroalgae of three categories:phaeophyta,chlorophyta and rhodophyta in the seaweed bed of Gouqi Island are on-site measured.There is no significant difference in Fv/Fm parameters of three kinds of large algae in green algae and brown algae under natural sea conditions,while three macroalgae of Rhodophyta have significant difference.The Fv/Fm of the nine species of macroalgae is as follows:Codium fragile>Ulva pertusa>Sargassum horueri>Chladophora>Hizikia fusifarme>Sargassum thunbergii>Hypnea boergesenii>Coralline algae>Grateloupia filicina.The relative electron transfer rate show a change trend of Phaeophyta>Chlorophyta>Rhodophyta,in which Sargassum thunbergii is the largest and the relative electron transfer rate is the highest.Among the three categories,Phaeophyta has stronger carbon sequestration ability,and Sargassum thunbergii,Chladophora and Hizikia fusifarme have higher carbon sequestration efficiency in the nine kinds of algae.Take ecosystem diversity into account,preference should be given to economic algae such as Sargassum Sargassum and sargassum fusiforme in the construction of artificial seaweed bed,which have high photosynthetic potential,high photosynthetic rate and strong carbon sequestration efficiency.（2）According to the analysis of the pigment content and photosynthetic activity parameters of Sargassum horneri at seedling stage and rapid growth stage,it is found that the height of Sargassum horneri increased and the density decreased with the decrease of temperature at its key growth stage.At different growth stages,pigment content has no significant difference,and Sargassum horneri grow well and is not suppressed under different temperature conditions.The relative electron transfer rate（r ETR）and non photochemical quenching（NPQ）is August>October>January.Sargassum horneri has higher maximum relative electron transfer rate（r ETRmax）and saturated light intensity（I_K）at seedling stage in August,and shows the strongest tolerance to strong light and photosynthesis.（3）A total of 49 species of epiphytic algae is collected in the aquaculture raft area throughout the year,belonging to 11 orders,20 families and 33 genera,the number of rhodophyta,chlorophyta and phaeophyta is 35,11 and 3,respectively.Macroalgae show the distinct replacement in different seasons,and the biomass is highest in summer,which is 5529.40g/m².Six kinds of algae distribute in the research area throughout the year,in which four kinds belong to rhodophyta,one in phaeophyta and one in chlorophyta.Macroalgae of phaeophyta has higher content of total organic carbon（TOC）,indicating that its carbon fixation ability is stronger than another two categories,and there are differences in the carbon fixation ability among different algae.The annual total carbon sequestration of raft epiphytic algae is 89.86t,which is equivalent to about 326.33 tons of CO₂ emission reduction.The micro habitat consisted of epiphytic algae on aquaculture raft has good ecological effects such as carbon sequestration.（4）Based on Biosonics MX echosounder and diving survey,19 species of macroalgae are found in the subtidal zone in summer,belong to 3 phyla and 16 genera,in which rhodophyta are dominant algae,followed by phaeophyta and chlorophyta.The distribution area of seaweed bed in Guoqi island is about 22.33 hectares,in which the biomass of macroalgae is about 650.71t（wet weight）in the subtidal zone.The estimated carbon sequestration of macroalgae in the subtidal zone of Gouqi Island is about 38.28t,the equivalent of about 129.07t CO₂ emission reduction.Combined with the national coastal survey,the ecological effects such as carbon sequestration of macroalgae in China’s coastal subtidal zone are preliminarily estimated.It is found that the annual carbon sequestration of macroalgae in China’s coastal subtidal zone is 10.2 million tons,and the nitrogen and phosphorus are 0.51 and 0.14 million tons respectively,which illustrate macroalgae has important ecological effects such as carbon sequestration and water purification.（5）By the exploratory data analysis,the enrichment characteristics of heavy metals in different algae are explored,which provide the basis for marine heavy metal pollution control and heavy metal monitoring indicator biological selection.The results show that the contents of heavy metals are different in different macroalgae.On the basis of cluster analysis,Phaeophyta has higher comprehensive enrichment ability for heavy metals,while Chlorophyta is the weakest.According to the content analysis of heavy metals by principal component analysis,the comprehensive ranking of the enrichment ability of 19 kinds of algae is as follows:Polysiphonia senticulosa>Hypnea boergesenii>Sargassum vachellianum>Pachydictyon coriaceum>Eucheuma sp>Sargassum thunbergii>Chaetomorpha aerea>Halymenia floresia>Hizikia fusiforme>Echinoptera constricta>Sargassum horneri>Grateloupia carnosa>Ulva lactuca>SP Corallina officinalis>Grateloupia turuturu>Grateloupia filicina>Chondria crassicaulis>Grateloupia livida>Jania Rubens.The toxic effects of heavy metals on Photosynthesis of macroalgae and the mechanism of macroalgae resisting the toxic effects of heavy metals remain to be studied.