Study On The Coupling Effect Of Heavy Metal Cadmium, Nitrogen Nutrition And Ocean Acidification On The Macroalga Ulva Ulva

There are growing interests regarding the impacts of ocean acidification,caused by elevated atmospheric CO₂ concentrations,on marine ecosystems.However,ocean acidification did not happen in isolation,and it interplayed with other marine environmental changes which were caused by climate change or anthropogenic activities,such as heavy metal pollution and eutrophication,to influence on the survival and reproduction of marine organisms.Here,we investigated the interactions between ocean acidification and Cd²⁺,NO₃^-and NH₄⁺on the physiological performances?such as growth,photosynthesis,N assimilation?of a common green macroalgae Ulva lactuca by setting conditions for coupling different concentrations of Cd²⁺?NO₃^-and NH₄⁺with CO₂,respectively.Our study will provide the basis for predicting and evaluating the combined effects of ocean acidification and heavy metal cadmium and nitrogen availability on macroalgae.The main findings are listed as follows:1.The relative growth rate,net photosynthetic rate,the maximum and effective quantum yield of photosystem II of Ulva lactuca significantly decreased by Cd²⁺at ambient CO₂ level,and these negative effects were more obvious in high CO₂conditions,indicating an interaction between Cd²⁺and high CO₂.The contents of Chla,Chlb and Car also decreased with the increase of Cd levels,while the sensitivity to the interaction between Cd²⁺and CO₂ was Chla>Chlb>Car,respectively.While Cd²⁺significantly decreased the extracellular carbonic anhydrase activity at ambient CO₂ level,this inhibition was higher at high CO₂ level,also indicating an interaction between Cd²⁺and high CO₂.In conclusion,the high CO₂ concentration can affect the light usage efficiency of photosystem by regulating pigment contents and extracellular carbonic anhydrase activity,and then affected the photosynthesis and respiration rate to meet the higher energy demand for heavy metal detoxification.For the interactions,high CO₂ conditions exacerbated the negative effects of Cd²⁺on the physiological performances of U.lactuca,suggesting a synergistical interaction between these two factors.2.The high NO₃^-concentration?HN?increased the NO₃^-assimilate rate and nitrate reductase activity of U.lactuca at ambient CO₂ level,and these enhancements were more obvious at high CO₂ level,indicating an interaction between NO₃^-and CO₂.HN increased the relative growth rate,dark respiration rate,net photosynthetic rate,pigment contents,the maximum and effective quantum yield of photosystem II at ambient CO₂ level,however,these positive effects on net photosynthetic rate,pigment contents and the maximum and effective quantum yield of photosystem II were dampened by high CO₂,indicating an interaction between NO₃^-and CO₂.However,it did not affect the effect of NO₃^-on the relative growth rate,dark respiration rate,and glutamine synthetase activity of U.lactuca.In summary,our results indicated that more N entered the algae by nitrate reductase and glutamine synthase at high CO₂concentration.Furthermore,high CO₂ can affect the pigment contents and the efficiency of light energy utilization,resulting in the change of dark respiration rate and photosynthetic rate,but it did not effect the promoting effect of NO₃^-on growth.3.High NH₄⁺concentration increased its assimilation rates and glutamine synthase activity in U.lactuca at ambient CO₂ level,and higher CO₂ concentration did not cause the effect to increase.While the increase of CO₂ concentration inhibited the effect of NH₄⁺on relative growth rate,net photosynthetic rate and pigment contents.High NH₄⁺concentration inhibited the nitrate reductase activity,and this effect was alleviated by elevated CO₂.There were no significant changes in maximum and effective quantum yield of photosystem II at high NH₄⁺concentration,but high CO₂concentration led to a significant decrease.The effect of high NH₄⁺on dark respiration rate was only reflected in high CO₂ concentration.In summary,our results showed that higher activity of glutamine synthase increases N assimilation at high CO₂ level.At the same time,it affected the synthesis of pigments,further affected the efficiency of light energy utilization,and then inhibited the promoting effect of NH₄⁺on photosynthesis and growth of U.lactuca.