The Different Responses Of Growth And Photosynthesis To NH₄⁺ Enrichments Between Gracilariopsis Lemaneiformis And Its Epiphytic Alga Ulva Lactuca Grown At Changed Atmospheric CO₂

The concentrations of inorganic carbon and NH₄⁺in seawater surrounding marine macroalgae are always change due to the human activities,climate change,and physiological metabolism of marine macroalgae.Such changes of carbon and nitrogen might affect the physiological characteristics of macroalgae.In this paper,the economic seaweed Gracilariopsis lemaneiformis,a species widely cultivated in South China Sea,and its epiphytic macroalga,Ulva lactuca,were used as the research materials.Different NH₄⁺concentrations were set to investigate the response of growth,photosynthesis and biochemical compositions of macroalgae to the change of NH₄⁺concentration gradient,as well as the effect of the elevated atmospheric CO₂ concentration on the response.In addition,the response of growth,photosynthesis,and biochemical compositions of macroalgae cultured under high NH₄⁺concentration to carbon restriction was discussed.The main findings are as follows:1.All algae were incubated under four nutritional conditions(control group?zero NH₄⁺addition?,eutrophication condition(100?mol·L^-1),high eutrophication condition(500?mol·L^-1),super eutrophication condition(2500?mol·L^-1))and two CO₂ levels?ambient air level?390 ppm?and elevated CO₂ level?1000 ppm??.The results showed that the eutrophication condition significantly promoted the relative growth rates?RGR?and the maximum photosynthetic rates?P_m?of both species,and increased the Chlorophyll a?Chl a?contents in G.lemaneiformis and the soluble protein?SP?contents in U.lactuca.When the concentration of NH₄⁺was not less than 100?mol·L^-1,the RGR of both species decreased with an increase in NH₄⁺concentrations,and the P_m,Chl a contents,and total antioxidant capacity?T-AOC?also declined with the increase in NH₄⁺concentrations.However,the SP contents in G.lemaneiformis rose first and then decreased,and the SP contents in U.lactuca increased with the increase in NH₄⁺concentrations.Under the super eutrophication condition,the RGR of G.lemaneiformis was negative,significantly lower than that of the control group,but there was no significant difference in RGR of U.lactuca compared with the control.Under the super eutrophication condition,elevated CO₂ level significantly enhanced the RGR and P_m of G.lemaneiformis.Even so,The RGR of G.lemaneiformis was still far lower than that of U.lactuca.This indicated that the eutrophication condition was favorable to both species.The toxicity of NH₄⁺to seaweed increased with the increase in NH₄⁺concentration,and the toxicity to G.lemaneiformis was larger than that to U.lactuca.Under the super eutrophication condition,the increase in atmospheric CO₂ concentration significantly reduced the NH₄⁺stress on the growth and photosynthesis of G.lemaneiformis.Nonetheless,in the environment with high concentration of NH₄⁺and atmospheric CO₂,G.lemaneiformis will still be at a disadvantage in the competition with U.lactuca.2.The two species were cultured under two NH₄⁺concentrations(ambient ammonium concentration?AA,zero NH₄⁺addition?and elevated ammonium concentration(EA,2000?mol·L^-1))and two CO₂ levels?ambient air level?AC?and decreased CO₂ level?DC,the air through a high concentration NaOH solution before incoming??.The results showed that the EA condition significantly suppressed the RGR of G.lemaneiformis,but insignificantly enhanced the RGR of U.lactuca.Under the EA condition,the photosynthesis and pigment contents in both species were inhibited,though the SP contents and T-AOC in U.lactuca were significantly increased.Under the AA condition,the DC condition inhibited the growth and photosynthesis in both species,but not significantly.However,under the EA condition,the SP contents and T-AOC in G.lemaneiformis showed stronger negative responses to the DC condition,and the RGR,P_n,and SP contents in U.lactuca showed similar responses.It indicated that U.lactuca had higher tolerance to the EA condition than G.lemaneiformis,and the tolerance might be related to the SP contents and T-AOC.Furthermore,there was no significant effect of the DC condition on both species,but the EA condition could enhance the effect of the DC condition.In conclusion,an increase in NH₄⁺concentration in relatively low ranges was beneficial to the growth of both kinds of algae,while the high NH₄⁺concentration inhibited the growth of both algae.Under high NH₄⁺level,adequate carbon supply could significantly alleviate the stress on growth and photosynthesis in G.lemaneiformis,while carbon restriction had no significant effect.On the other hand,under high NH₄⁺level,adequate carbon supply could enhance the photosynthesis in U.lactuca,but carbon limitation decreased photosynthetic rates.Under low NH₄⁺level,carbon supply had little effect on the growth of U.lactuca.However,under high NH₄⁺condition,both increase and decrease in carbon supply showed inhibitory tendency on the growth of U.lactuca.