Effects Of Salinity Stress On Photosynthetic Characteristics And Carbon Metabolic Pathways Of Gracilaria Chouae

1.Effects of short period high-salinity stress on photosynthetic characteristic of Gracilaria chouaeTo verify the feasibility of the pretreatment method:high-salinity seawater immersion,and ascertain the optimum reaction conditions in production practice,the effect of high-salinity stress on the photosynthesis of G.chouae was studied using methods from experimental ecology.In this study,branches of G.chouae were incubated at five up-regulated salinity levels?40,45,50,55 and 60 psu?for 30minutes,while the natural seawater?33 psu?was taken as control.Afterwards,these incubated branches of algae were transferred to the natural seawater for recovery for12 and 24 hours,respectively.The values of pH compensation point,chlorophyll fluorescence parameters,oxygenic photosynthesis and the contents of photosynthetic pigments of incubated algae were measured at different salinity levels and recovery time.The pH compensation point increased slightly,while RO and Fv/Fm decreased significantly after algae was incubated at 40 psu for 30 minutes.The pH compensation point and Y?II?decreased,while RO fluctuated downward with increasing salinity level once the incubating salinity was higher than 40 psu.The Chla/Car value was significantly different among different levels of salinity treatments?P<0.01?,Between different incubation time within the same level of salinity,no significant difference was found.Significant change in PE content?P<0.01?was observed,especially in 45 and 55 psu groups.After 12 h recovery,the overall Fv/Fm value of stressed algae was still significantly lower than that in control?P<0.05?,however,the value of Y?II?began to increase.After 24 h recovery,the photosynthetic indexes of stressed algae included Fv/Fm,Y?II?,Chla,Car,PE,PC and Chla/Car were close to fully recovery.This study indicated that salinitythe algae can adjust its photosynthetic characteristics under high-salinity stress,as a result,its photosynthetic parameters was able to return to normal level after 24 h recovery.It was considered that the pretreatment method is feasible,and the salinity of 50-55 and soaking for 0.5 h is a suitable treatment condition.2.Effects of short period high-salinity stress on antioxidant enzyme activities and photosynthesis enzyme activities of Gracilaria chouaeUsing methods from experimental ecology,the effects of short period high-salinity stress on the antioxidant enzyme activities and photosynthesis enzyme activities of G.chouae were studied following the exposure to different salinities ranging from 35?control?,40,45,50,55 to 60 in laboratory conditions.The incubated branches of algae were then transferred to the natural seawater and recovered for 12and 24 hours,respectively.With salinity increasing,the activity of antioxidant enzyme SOD gradually increased?P<0.05?,the activity of POD was fluctuated?P<0.05?,the activity of CAT gradually decreased without significant difference?P>0.05?but the content of MDA was significantly increased?P<0.01?.The activity of each antioxidant enzyme has an extreme value in 50-55 psu groups;The activity of Rubisco,a key enzyme in C 3 pathway,gradually decreased with the increase of salinity?P<0.01?.The activity of NADP-MDH,a key enzyme in C 4 pathway,showed a reverse trend?P<0.01?,and the content of CA slightly increased with the increase of salinity?P<0.05?.With the increase of recovery time,SOD,POD and CAT activities gradually increased?P<0.05?,but MDA content considerably decreased?P<0.05?.Rubisco activity significantly higher than that in 0.5 h?P<0.05?,NADP-MDH activity increased overall?P<0.05?,and CA content fluctuated with salinity.Short period high-salinity stress significantly affected the activities of antioxidant enzymes and photosynthetic enzymes in G.chouae.Algae responded to high-salinity stress by advancing the activities of antioxidant enzymes,enhancing the absorption and utilization of inorganic carbon,and enhancing the key enzyme in C4 pathway.The alga gradually returned to the normal level after the stress was removed.3.Effects of salinity stress on the pathway of photosynthetic carbon metabolism of Gracilaria chouaeThe effect of salinity stress on the pH compensation point,stable carbon isotope ratio?¹³C?‰?,activities of key enzymes in photosynthetic carbon assimilation pathways of C3 and C4 and tnec ontents of key metabolite of G.chouae was studied using methods.Branches of G.chouae were incubated at three up-regulated salinity levels?10,32 and 55 psu?for 1d,3d and 6d.Under different stress time,the pH compensation point of algae increased firstly and then decreased with the increase of salinity.The salinity of 32 groups was above 9,while that of 55 groups was below 9,and decreased significantly with the increase of stress time?P<0.05?.The value of?¹³C increased firstly and then decreased with the increase of salinity on the 1st and6th day of salt treatment,and decreased significantly with the increase of salinity on the 6th day of salt treatment?P<0.05?.The value of?¹³C in salinity 10 treatment decreased significantly with the increase of treatment time,i n salinity 55 treatment,the value of?¹³C was significantly higher than that in salinity 10 and 32 groups,and both were less than-28‰.The activity of Rubisco did not change significantly with the increase of salinity?P>0.05?at the early stage of stress?1d?and the activity of Rubisco in salinity 55 treatment was significantly higher than that in salinity 10treatment?P<0.05?.And the activity of RAC showed the same trend.With the increase of stress time,the activity of PEPC had no significant difference in salinity10 treatment group?P>0.05?,but increased significantly in salinity 55 group?P<0.05?,and was significantly higher than that in salinity 10 and 32 groups.The activities of PPDK and NADP-MDH increased significantly with the increase of stress time and salinity?P<0.05?.Compared with the natural seawater group?salinity32?,the contents of GAP,PG,F6P,G6P and DHAP,the key metabolites of C3pathway,and PEP,the key metabolites of C4 pathway,decreased significantly in salinity 10 and 55 treatments after 1 day of salt treatment.After 6 days of salt treatment,the contents of F6P,G6P and Pu5P,the key metabolites of C3 pathway,gradually increased with the increase of salinity.The contents of PEP and Asp,the key metabolites of C4 pathway,were significantly higher in salinity 55 group than those in other salinity treatments.