Study On The Responses Of Photosynthetic Function To Carbon Limitation In Marine Macroalgae

Marine macroalgae are considered as the main primary producer and play an indispensable role for ecosystem in coastal waters.Additionally,as the principal objects for cultivation,marine macroalgae cultivation brings huge economic and applied values for human.In recent years,the effects of increasing atmospheric CO₂ concentrations and the associated climate change on coastal ecosystem and marine macroalgae have been focused by many environmental researchers and ecologists.However,in realistic situations,a severe low dissolved inorganic carbon?DIC?concentration in seawater would be of frequent occurrence,especially under the conditions of slow water exchanges and high seaweeds standing stock.At present,it is poorly understood that how low DIC and other related environmental factors synergically affect the marine macroalgae.Therefore,the main cultivated macroalgae in Southern China Sea,Pyropia haitaneisis and Gracilaria lemaneiformis,and the common species of Grateloupia livida and Ulva lactuca,were used as the experimental materials in the present studies,in order to investigate the effects of lowered CO₂ interacting with other environmental conditions on algal growth,photosynthetic functions and related physiological processes.The main results were as follows:When P.haitaneisis and G.lemaneiformis thalli grown at decreased CO₂ supply condition?20 ppm?,the relative growth rates?RGR?,photosynthetic activity and NO₃^- uptake rates of the thalli were decreased,additionally,the RGR inhibition of G.lemaneiformis was greater than P.haitaneisis.The decreased sunlight-grown?sunlight intensity 20% of ambient sunlight?P.haitaneisis increased the RGR,photosynthetic activity and photosynthetic rates,and NO₃^- uptake rates,however,the RGR,photosynthetic activity,superoxide dismutase?SOD?and catalase?CAT?activities,and the NO₃^- uptake rates of G.lemaneiformis grown at decreased sunlight were decreased.Decreased CO₂ supply and lowered sunlight intensity in culture increased the photosynthetic pigments?chlorophyll a;Chl a,carotenoid;Car,phycoerythrin;PE,phycoerythrin;PC?contents,and elevated the capacity of HCO₃^- utilization and resistance to short-term temperature change in P.haitaneisis.Moreover,the recovery of the photosynthetic pigments,SOD,CAT and photosynthetic activity in G.lemaneiformis,which were treated with decreased CO₂ supply and reduced sunlight,were achieved in the laboratory.Both low CO₂ and high CO₂?1000 ppm?supplies in culture depressed the RGR and photosynthetic activity of G.livida.However,the PE and PC contents were increasd,when G.livida were cultured at decreased CO₂ supply.The stocking density increases with growth season during maricultivation.In the present study,the RGR,photosynthetic performance?photosynthetic activity and photosynthesis rate?and N metabolism?NO₃^-uptake rates and nitrate reductase activity?of P.haitaneisis grown at high stocking density(5 g L^-1)were decreased.High stocking density exhibited similar negative effects compared with reduction of carbon supply on the photosynthetic performance of P.haitanensis.Moreover,high stocking density aggravated effects resulting from decreased carbon supply on photosynthetic rates.In the natural sea area,the RGR and photosynthesis rates of G.lemaneiformis grown at low stocking density?4 g/cluster?were greater than the algae grown at high stocking density?12 g/cluster?.However,the photosynthetic pigments contents and photosynthesis rates were increased with G.lemaneiformis thalli sank to deeper depths which resulted from the increasing numbers of branches and mass.Therefore,appropriate measures are needed to trade off the stocking depth and density,in an effort to maintain a relative high photosynthesis rates during G.lemaneiformis maricultivation.The photosynthetic performances of algae varied greatly when it grown at different sites of intertidal zone with different biomass densities.For the U.lactuca grown at low tidal zone?emersed time 0 h?,the photosynthesis rates at high biomass density were lower than at low biomass density.However,for the algae grown at high?emersed time 4 h?and intermediate?emersed time 2 h?tidal zone,the photosynthesis rates at high biomass density were greater than at low biomass density.Under these different biomass density conditions,increased HCO₃^- concentration in seawater had different effects on the gross photosynthetic rates?Pg?of P.haitaneisis and U.lactuca.The Pg of P.haitaneisis was not obviously increased,however,the Pg of U.lactuca was significantly elevated with increased HCO₃^- concentration in seawater.These indicated that the thalli in P.haitaneisis and U.lactuca mats were in carbon limitation,with the limitation degree being greater in U.lactuca than in P.haitaneisis.Moreover,the Pg was not influenced by elevated flow motion in the tested range(?3.5 cm s^-1).In conclusion,lowered CO₂ supply in seawater decreased growth and photosynthetic functions and other related physiological processes of P.haitaneisis,G.lemaneiformis and G.livida.High stocking density displayed similar negative effects compared with reduction of carbon supply on the photosynthetic performance of P.haitanensis.The photosynthesis behaviors of G.lemaneiformis,P.haitaneisis,and U.lactuca were inhibited under high biomass density.The photosynthesis rates of the thalli in P.haitaneisis and U.lactuca mats were inhibited by carbon limitation,and the inhibition for U.lactuca was greater than for P.haitaneisis.These studies would contribute to more comprehensive understanding of the relationship between the macroalgal physiological and ecological functions and the inorganic carbon supply.Meanwhile,as the present studies are combined closely with realistic environmental conditions,the results would provide important theoretical basises and technical support for high yield and high efficiency with macroalgae cultivation.