Effects Of Ocean Acidification,Temperature And Phosphorus On Photosynthesis And Agar Yield In Gracilaria Lemaneiformis

With the development of industrialization and the extensive use of fossil fuels,the concentration of CO₂ is increasing,which aggravates global changes?such as seawater warming and ocean acidification?.The eutrophication of seawater is becoming more and more serious,the increase of phosphorus concentration in seawater has an important impact on the material metabolism and energy conversion of marine plants.The change of marine ecological environment will also have an impact on marine primary productivity,and then affect the whole marine ecosystem.How marine macroalgae adapt to these environmental changes has attracted more and more attention.In these studies,the economical macroalgae Gracilaria lemaneiformis was employed to investigate the changes of environmental conditions such as ocean acidification and temperature rise were simulated in the laboratory to study the photosynthetic physiological and ecological effects of phosphorus concentration,atmospheric CO₂ concentration and seawater temperature.It provides a theoretical basis for predicting and evaluating the future global marine environmental changes on the cultivation and comprehensive utilization of marine macroalgae.The main findings are as follows:The growth and photosynthesis of Gracilaria lemaneiformis significantly increased,while the dark respiration rate decreased when the concentration of phosphorus increased from 0.5?mol·L^-1 to 30?mol·L^-1 under ambient CO₂ conditions?400 ppmv?.However,this effect was muted by the increasing in CO₂ concentration with no significant effect at the CO₂concentration of 1000 ppmv.At both CO₂ levels,the increaing in phosphorus did not show any effect on the contents of chlorophyll a and carotenoid,and the effective photochemical efficiency.Our results also showed that the increasing in phosphorus signicantly increased the ATPase activity regardless of the CO₂ level.Taken together,the findings indicated that the rising CO₂ concentration decreased the sensitivity of Gracilaria lemaneiformis to the increased phosphorus.When looking at the interactive effects of temperature and phosphorus concentration on Gracilaria lemaneiformis,we found that the optimum growth temperature for Gracilaria lemaneiformis was 20?at both phosphorus levels,while no effect of phosphorus on growth was observed regardless of the CO₂ levels.The results showed that there were no significant changes in the biochemical contents of Gracilaria lemaneiformis,such as soluble carbohydrate,chlorophyll a,carotenoid and soluble protein within the changes of temperature.Different from that of temperature,the inreasing in phosphorus concentration significantly decreased the contents of soluble carbohydrate,agar and sulfate by 19.48%,6.30%-19.79%,and 8.59%-29.29%,respectively.The phycoerythrin showed the highest content at the temperature of 20?,which showed similar trend with that of growth.The phycoerythrin content increased by 63.36%,38.71%and 45.27%at 15?,20?and 25?.Our results also showed that the high phosphorus enhanced the respiration of Gracilaria lemaneiformis by96.6%at high temperature of 25?,but it did not show any effect on the photosynthesis.However,at the low incubation temperature of 15?,the increasing in phosphorus significantly increased the maximum photosynthetic oxygenation evolution rate?68.43%?and ATPase activity?98%?,suggesting an accelerated metabolic rate under high phosphorus conditions.The contents of soluble carbohydrate of Gracilaria lemaneiformis was enhanced by50.26%when the concentration of phosphorus increased from 0.5?mol·L^-1 to 30?mol·L^-1 at ambient CO₂ level.However,the increasing in phosphorus significantly decreased the contents of soluble carbohydrate at high CO₂ level,suggesting that the rising CO₂ inhibited the positive effect of phosphorus on soluble carbohydrate.At both CO₂ levels,the gum yield and sulfate content decereased by 42.45%-45.16%and 47.17%-57.46%,respectively,at the phosphorus concentration of 30?mol·L^-1.However,no significant effect of CO₂ on the gum yield was detected in the present study.The findings indicated that the concentration of phosphorus had more pronounced effect than that of CO₂ on the gum yield and agar quality,that is,the high concentration of phosphorus significantly decreased the gum yield,but improved the agar quality.Taken together,our results showed that?1?Both the increasing in phosphorus and CO₂enhanced the growth of Gracilaria lemaneiformis.In the high CO₂ ocean acidification conditions,the increased phosphorus promoted the growth of Gracilaria lemaneiformis by modulating its photosynthetic efficiency,dark respiration and ATPase activity and hence the energy usage efficiency.?2?The optimum temperature for the growth and photosynthesis of Gracilaria lemaneiformis is 20?.The deceasing in ATPase activity with the increasing of temperature from 15?to 20?resulted an enhanced accumulation of organic compounds.However,when the temperature increased from 20?to 25?,the increased ATPase activity with the accelerated metabolic rates resulted a decrease in growth,phycoerythrin and organic compounds.?3?The nutrient level of phosphorus is a key factor that regulate the gum yield and agar quality of Gracilaria lemaneiformis,that is,the high phosphorus decreased the gum yield but improved the agar quality.However,no significant effect of temperature and CO₂ on the gum yield and agar quality was observed in the present study.