Production Of Organic Matter By Cultivated Macroalgae And Effects On Phytoplankton

Marine macroalgae are photosynthetic autotroph and important primary producers in coastal ecosystems.Macroalgae can convert dissolved inorganic carbon into organic carbon through photosynthesis,as well as absorb dissolved nitrogen and phosphorus during the growth process.Portions of the photosynthetically fixed inorganic carbon are released to the seawater as particulate organic matter(POM)and dissolved organic matter(DOM).The remainder becomes the organic carbon of macroalgal tissue.In addition to the well-known fixation of carbon dioxide and release oxygen,macroalgae are also widely involved in the coastal biogeochemical cycles of nitrogen,phosphorus,silicon and many metal.However,we know little about the ecological and environmental effects of macroalgae on coastal ocean.By using physiological technology and 18S rDNA sequencing,through laboratory incubation experiments and ecological survey of cultivated macroalgae(Saccharina japonica,Pyropia haitanensis and Gracilaria lemaneiformis)in Nanri Island,Fujian Province,this study analyzed the production of organic matter released by cultivated macroalgae and their ecological effects on phytoplankton.The main results are as follow:(1)The production of DOM and POM by macroalgae were analyzed to reveal the effect of macroalgae on coastal C,N,and P element cycles.In this study,we carried out 17 five-day laboratory incubation experiments with 11 species of typical intertidal macroalgae.During incubation,the average proportions of released dissolved organic carbon(DOC),nitrogen(DON),and phosphorous(DOP)as compared to total carbon,nitrogen,and phosphorus were 14.39%,37.38%,and 31.32%,respectively.The average released proportions of particulate organic carbon(POC),nitrogen(PON),and phosphorous(POP)as compared to total carbon,nitrogen,and phosphorus were 9.02%,12.67%,and 13.89%,respectively.This study showed that,during the vegetative growth stage,the intertidal macroalgae released both DOM and POM,and that more DOM than POM was released into the seawater.This research also found that the growth rate of the macroalgae had a linear correlation with algal tissue C,N,and P accumulation rates and the release rates of POM and DOM.The carbon:nitrogen(C:N)and carbon:phosphorus(C:P)ratio of both DOM and POM were substantially lower than that of algal tissue,which suggested that macroalgae tend to release organic nitrogen and phosphorus and enrich carbon.(2)In order to study the ecological and environmental effects of kelp aquaculture,this study investigated the growth rate,the tissue elemental contents(C,N,and P)and ratios of cultured commercial kelp(Saccharina japonica),in Nanri Island,Fujian Province,and the seasonal variations of the POM and DOM in the cultured area.The result showed that the wet weight of kelp varied as a power function of the length(W=0.02 L7.57,R2=0.84).Besides,the wet weight(W=5.45 t-30.50,R2=0.96),the length(L=1.72t+41.18,R2=0.97),and the width(W=0.30 t+5.28,R2=0.93)were linearly increased with the culture days.The tissue C/N ratio varied from 9.13 to 18.66,N/P ratio ranged from 11.32 to 18.48,and C/P ratio varied from 153.18 to 267.99.The tissue C/N ratio varied as an exponential function of N content(Y=748.30 e-4.10X,R2=0.88),and the C/P ratio varied as an exponential function of tissue P content(Y=50.21 e-0.060X,R2=0.92).The seawater POC,PON,POP,DOC,DON,and DOP in the culture zone were varied from 0.33 to 0.86 mg/L,from 0.07 to 0.1lmg/L,from 0.01 to 0.02 mg/L,from 1.98 to 17.06 mg/L,from 0.20 to 0.55 mg/L,and from 0.01 to 0.04 mg/L,respectively.The average tissue C,N,and P contents of at harvest period were 26.17%,1.76%,and 0.29%,respectively.The total production of cultured kelp in Fujian Province was about 768300 tons,which indicates that the removable C,N,and P contents were about 201 300,13 500,and 2 200 t,respectively.Our result demonstrated that the kelp aquaculture is an important source of seawater organic matter in the culture area,and is one of the important components of coastal carbon cycle.(3)In order to study the ecological and environmental effects of two cultivated macroalgae(P.haitanensis and G lemaneiformis),this study investigated the tissue elemental contents(C,N,and P),spatial distribution of the POM and DOM in Nanri Island,Fujian Province.The result showed that the tissue C,N,and P contents of P.haitanensis and G lemaneiformis were 37.19±2.41%and 29.86±1.63%,5.07 ±0.42%and 29.86±1.63%,0.24 ± 0.02%and 0.48±0.04%,respectively.The tissue C/N,N/P,and C/P ratios of P.haitanensis and G lemaneiformis were 8.58 ± 0.37 and 10.77 ± 0.94,22.88 ± 1.91 and 29.19 ± 1.43,196.4 ± 18.5 and 314.9 ± 37.1,respectively.The seawater POM,POC,PON,POP and BSi in the cultivated macroalgae zone were varied from 28.09 to 114.56 mg/L,from 0.39 to 0.97 mg/L,and from 0.06 to 0.13 mg/L,from 0.01 to 0.03 mg/L,from 1.40 to 19.38 μg/L,respectively.The DOC,DON,and DOP in seawater of the cultivation zone were varied from 4.07 to 17.45 mg/L,from 0.09 to 0.83 mg/L,and from 0.01 to 0.04 mg/L,respectively.18S rDNA sequencing of surface seawater samples from the cultivation zone and non-cultivation zone were performed on high throughput sequencing technology.Diversity analysis revealed that there were no significant difference in the phytoplankton diversity betweend the G.lemaneiformis cultivation zone and the non-cultivation zone,but the phytoplankton diversity in the P.haitanensis cultivation zone was significantly higher than the other two.However,there were no significant differences in the phytoplankton richness of the three sample zones.Through NMDS analysis,it was found that there were no significant differences in the seawater samples betweend the G.lemaneiformis cultivation zone and non-cultivation area,but the seawater samples was significant different between the P.haitanensis cultivation zone and the other two zones.The heatmap of species abundance found that the abundance of the diatoms(Thalassiosiraceae and Skeletonemataceae)and Mamiellaceae in seawater samples from P.haitanensis cultivation zone was significantly lower than that of the non-cultivation zone,but the abundance of Chlorophyta Ulvacea in seawater samples from the P.haitanensis cultivation zone was significantly higher than that of the non-cultivation zone.The results indicated that the cultivated P.haitanensis can effectively increased the phytoplankton diversity.There were a large amount of Ulva attached to the P.haitanensis cultivation rafts may release spores and detries into the seawater,resulting in-the higher abundance of Chlorophyta Ulvacea.This study helps to comprehensively evaluate the ecological and environmental effects of the cultivated P.haitanensis and G.lemaneiformis.In this study,the ecological and environmental effects of the primary commercial macroalgaes in Fujian Province were comprehensively analyzed.The results show that the C:N:P ratios of the algal tissue,particulate organic matter,and dissolved organic matter from cultivated macroalgae zone were different,and the underlying mechanism and potential biogeochemical effects need further study.In particular,it is worthy to study the mechanism of increase of C:N:P ratios after dissolved organic matters were released into seawater.The cultivated P.haitanensis can effectively increased the phytoplankton diversity,but increase abundance of Chlorophyta Ulvacea.