Study On The Photosynthetic Functions Of Marine Macroalgae In Response To Elevated Atmospheric CO₂

In recent years,the effects of increasing atmospheric CO₂ concentration and the associated climate change have been focused on by many environmental researchers and ecologists.As the largest carbon sink of the earth,the ocean has absorbed large quantities of CO₂ caused by anthropogenic activities,which led to changes of seawater carbonate systems and the decrease of p H,termed ocean acidification?OA?.The global warming and eutrophication in coastal areas had emerged correspondingly.Those changes will have a profound impact on primary producers,marine macroalgae,which play an indispensable role along the coast.Many of the macroalga species are used for the marine cultivation.How they respond to the elevation of CO₂ has become a hotspot of study.At present,it is poorly understood how increasing CO₂ and temperature,and other related environmental factors synergically affect the marine macroalgae.Therefore,the main cultivated macroalgal species in southern area of C hina,Gracilaria lemaneiformis and Pyropia haitaneisis,and the common specie,Ulva lactuca,were used as experimental materials in this study,in order to investigate the effects of elevated CO₂ interacting with different environmental factors on the growth,the biochemical components,the assimilation of nitrogen,photosynthetic characteristics,respiration and antioxidant enzymes.The main results were followed:Firstly,the results exhibited that the elevation of CO 2 and temperature had no obvious effect on relative growth rates?RGR?of U.lactuca,while elevated CO₂ and temperature coinstantaneously increased the RGR of G.lemaneiformis.As temperature rose,elevated CO₂ markedly promoted chlorophyll a?Chla?and carotenoid?Car?of U.lactuca and phycobiliprotein?PB?of G.lemaneiformis,while it did not affect soluble carbohydrates?SC?and soluble proteins?SP?of two algae mentioned above.Total fatty acids?TFAs?of U.lactuca reduced by 8% with elevated CO₂ but that of G.lemaneiformis increased by 3-5%,whereas the former rose by 18% with increased temperature and the latter declined within 14%.Secondly,according to the interaction analysis of variance among elevated CO 2 and temperature and short-time overshadowing,temperature was considered as the main factor to influence photosynthesis and respiration in U.lactuca and G.lemaneiformis,both of which showed a low-temperature acclimation when exposure to thermal gradient.With the interaction of elevated CO₂ and temperature,photosynthesis and the nitrate reductase?N R?activity of U.lactuca were dramatically improved.Responses of G.lemaneiformis to elevated CO₂ and temperature were different with that of U.lactuca.Regardless of CO₂ levels,increased temperature enhanced the C hl a/PB ratios but decreased the maximum electron transfer rates?ETRm?,photosynthetic efficiency???and activities of SOD,POD and CAT of G.lemaneiformis.Likewise,high-temperature grown algae expressed lower values of the photochemical quenching?qP?and the effective light quantum yield(Y_?)than low-temperature grown algae,together with increased the non-photochemical quenching?NPQ?and quantum yield of light-induced non-photochemical quenching(Y_NPQ)and fixed quantum yield of non-light-induced non-photochemical quenching(Y_NO).Thirdly,greenhouse performed different effects on the growth,photosynthesis and respiration of P.haitaneisis.Increased CO₂ or temperature had no effect on RGR of the algae.Elevated CO₂ restrained its photosynthetic rates?Pn or Pg?,the inhibition of which could remit when temperature increased.Dark respiratory rates?Rd?were not promoted by elevated CO₂ but rather increased temperature.At 14-30 oC,ratios of Rd to Pg of algae maintained constant with the similar Q10 values between Pg and Rd,while the Rd/Pg ratios rapidly increased when temperature exceeded 34 oC.Additionally,the interaction of elevated CO₂ and eutrophication significantly increased the RGR,ETRm and NR activity of P.haitaneisis.According to curves of NR activities versus p H and temperatures,NR activities of algae were reduced by lower?pH<7.6?or higher pH?pH>9.1?,and the range of temperature optima for NR became broader and lower with a reduction in elevated CO 2 or eutrophication.Finally,temperature responses of photosynthesis in U.lactuca and G.lemaneiformis were closely related to light intensity.Photosynthesis of the two algae were more sensitive to temperature when light intensity exceeded 320 ?mol photons m-2 s-1,while they were unchanged with temperature when light intensity was under 160 ?mol photons m-2 s-1.At low temperature,for U.lactuca,high light reduced the apparent carboxylation efficiency?ACE?and Y_NOwith an enhancement of NPQ,Y_NPQand pH compensation point.However,high light declined the K_m and increased ACE and Y_?of low-temperature grown G.lemaneiformis,along with no change of NPQ,Y_NPQ,Y_NOand pH compensation point under the same condition.Consequently,elevated CO₂,together with increased temperature and eutrophication synergically affected the growth,biochemical components,photosynthesis,respiration,nitrogen metabolism,and regulations of antioxidant processes in macroalgae.Through the regulation of their photosynthetic function,they could acclimate the climate change physiologically.