Study On The Physiological Effects And Mechanism Of Important Environmental Factors On Golden Tide Causative Species Sargassum Horneri

The occurrence frequency and distribution area of golden tide caused by the floating Sargassum horneri have risen sharply in the coasts of the Yellow Sea and the East China Sea since 2015,which indicates that after the green tide,the golden tide will become another ecological disaster in coastal regions of China.To reveal the basic characteristics of inorganic carbon utilization and ecophysiological mechanism of the rapid biomass accumulation of S.horneri during the floating process and eventually develops into the golden tide,the basic characteristics of inorganic carbon utilization in S.horneri,and the effects of environmental factors such as light,temperature and nutrient on its growth and photosynthetic characteristics were studied.The main research results are as follows:1.TRIS buffer had no significant?P>0.05?effect on the photosynth-etic carbon fixation rates of S.horneri,but the carbonic-anhydrase inhibi-tors acetazolamide?AZ?and ethoxzolamide?EZ?,the anion exchange in-hibitor 4,4'-Diisothiocyano-2,2'-stilbenedisulfonic acid?DIDS?significa-ntly?P<0.05?suppressed them,with the suppression efficiency in the or-der of DIDS>EZ>AZ.The photosynthetic carbon fixation rates of S.horneri increased with enhanced exogenous inorganic carbon concentration until arrived at the carbon dioxide saturation point.Moreover,the maximum photosynthetic carbon fixation rates(V_max)of algae decreased while the half-saturation constant(K_0.5)increased with increased pH.The pH compensation point of S.horneri was about 9.0.It suggested that the existing of various inorganic carbon utilization strategies in the photosynthetic carbon sequestration of S.horneri might enable it to rapidly absorb exogenous inorganic carbon in seawater and grow rapidly during the floating on sea surface.2.Under the same nitrate?NO₃^--N?level,the growth rate,soluble sugar content,soluble protein content,nitrogen absorption rate,maximum photosynthetic rate(P_max)of S.horneri increased with increased light intensities.Under the same phosphorus level,the growth rate,content of soluble protein,phosphorus absorption rate and P_maxax increased with the increased light intensities.In addition,under the same light intensity,the pigment and soluble protein content,nitrogen absorption rate and P_maxax of S.horneri increased with increased of nitrogen levels.Compared with natural seawater,the enrichment of nitrate promoted the growth,photosynthetic carbon sequestration,pigment and nitrogen absorption rate of S.horneri.Furthermore,the soluble protein content,phosphorus absorption rate and P_maxax increased with increased phosphorus levels.The results indicated that thegrowth rate and photosynthetic potential of S.horneri were stimulated by higher light,especially during floating on the surface of eutrophic seawater.3.In addition,under the same nitrate level,the growth rate,soluble protein content and P_maxax of S.horneri increased firstly and then decreased with increased temperatures.Under the same phosphorus level,the growth rate,soluble protein content and phosphorus absorption rate of S.horneri increased with increased temperatures.On the other hand,The growth rate,pigment,soluble protein content and phosphorus absorption rate of S.horneri increased firstly with increased temperatures but then decreased with further increased temperatures.Furthermore,under the same temperature,the growth rate,pigment and soluble sugar content of S.horneri increased with the increased of nitrate levels,but the P_maxax increased firstly and then decreased with further increased nitrate levels.The soluble protein content and phosphorus absorption rate increased with the increased phosphorus levels.The results indicated that the optimum temperature combined with eutrophic seawater could stimulate the growth and photosynthesis of S.horneri.These above results indicated that S.horneri could utilize both CO₂and HCO₃^-in seawater for the photosynthesis.In addition,S.horneri could absorb HCO₃^-directly by transporting of the anion exchange protein and utilize HCO₃^-indirectly by converting the HCO₃^-to CO₂through carbonic anhydrase.The efficient absorption and utilization of HCO₃^-in S.horneri may be one of the important factors to ensure its high photosynthetic rate,rapid biomass accumulation,which eventually evolves into a gold tide.Meanwhile,it is easy to obtain high light intensity and suitable temperatures for the rapid growth when S.horneri floating on the seawater surface.Furthermore,the high nitrogen and high phosphorus levels in the coastal eutrophic seawater will further stimulate its growth and photosynthetic potential.All the suitable conditions will intensify the outbreak of golden tide caused by S.horneri.