Mechanism Of Sea Level Rise Affecting Spartina Alterniflora Tolerance To Cd Stress Mediated By Sulfur Fluctuations

Coastal wetland ecosystems are transitional zones between the ocean and land,playing a crucial ecological role.However,coastal wetlands are currently facing challenges such as rising sea levels,heavy metal pollution,and plant invasion threats.With the global sea level rising rapidly at a rate of approximately 3.6 mm yr^-1,there is a risk of degradation and even loss of coastal wetland ecosystem functionality.In China,coastal wetlands are severely threatened by the invasive plant Spartina alterniflora,which alters the biogeochemical processes of native plant populations and essential ecological elements.Additionally,cadmium（Cd）poses significant risks and ecological toxicity in coastal wetlands.Sulfur（S）is a fundamental element involved in amino acid biosynthesis and protein structure formation within the coastal wetland habitat.The interaction between Cd and S influences the speciation,migration,transformation,and biological toxicity of Cd driven by the occurrence and transformation of S forms in sediments.Moreover,S regulates plant growth and mediates the plant’s response to Cd stress.Therefore,the increased duration of tidal inundation caused by rising sea levels affects the speciation,migration,transformation,and distribution of S and Cd in the coastal wetland ecosystem.To explore the differences in sulfur cycling mediated by the invasive plant Spartina alterniflora and the native plant Phragmites australis in tolerating Cd stress under the backdrop of rising sea levels,a controlled pot experiment was conducted.The study primarily focused on two aspects:（1）The impact of rising sea levels on the differential tolerance of sulfur cycling mediated by Spartina alterniflora and Phragmites australis to Cd stress;（2）The influence of rising sea levels and Spartina alterniflora invasion on the biogeochemical behavior of Cd and S in sediment.The main findings and conclusions of the study are as follows:（1）For the invasive species Spartina alterniflora,Cd stress induced intracellular lipid peroxidation.This led to a decrease in the content of photosynthetic pigments,a reduction in the maximum quantum yield of photosynthesis（QY）,and an increase in the non-photochemical quenching coefficient（NPQ）.Ultimately,these factors inhibited the growth of plants.However,Spartina alterniflora possesses its own regulatory mechanisms to cope with Cd stress.Glutathione（GSH）had higher content in the leaves(2660.79 nmol g^-1 DW)and roots(485.70nmol g^-1 DW),facilitating the removal of reactive oxygen species in leaf cells.In addition,Spartina alterniflora had a higher content of plant chelating peptides（PCs）in the roots(46872.15nmol g^-1 DW)compared to the leaves(18215.53 nmol g^-1 DW),promoting chelation and immobilization of Cd,thereby inhibiting the transport of Cd from roots to leaves and protecting the photosystem from oxidative damage caused by Cd stress.Although sea level rise did not significantly inhibit the GSH content in Spartina alterniflora leaves or the PCs content in the roots,indicating a higher efficiency of sulfate assimilation and utilization,the decrease in the extractable Cd content in sediment（Ex-Cd）influenced by rising sea levels facilitated the enhancement of Cd tolerance in Spartina alterniflora.Moreover,the increase in sediment sulfur pool promoted the synthesis of chelating and antioxidative compounds within Spartina alterniflora,improving plant metabolism and mitigating the adverse effects of Cd stress and rising sea levels on plant growth.（2）There were differences in the physiological responses of Phragmites australis and Spartina alterniflora to rising sea levels.Although the content of water-soluble sulfur（WS-S）in the rhizosphere sediment was significantly reduced in both species,the key substances for Cd tolerance,namely GSH and PCs,did not show a significant decrease in Spartina alterniflora but were significantly inhibited in Phragmites australis.This discrepancy reflects the difference in assimilating sulfate for enhancing Cd tolerance between the two plants.Furthermore,the increased sulfur pool highlighted Spartina alterniflora higher ability to absorb and assimilate sulfate,which contributes to its competitive advantage over Phragmites australis under the combined stress of rising sea levels,Cd pollution,and increased sulfur storage.The Ex-Cd content in sediment was 11.78%higher in the Phragmites australis group than in the Spartina alterniflora group.However,the rhizosphere sediment of Phragmites australis had a higher content of WS-S（mean of 180.29±56.31μg g-1 DW）,which may provide sufficient sulfate for Spartina alterniflora invasion to support its physiological and biochemical processes in resisting Cd stress,thereby counteracting the negative effects of higher Ex-Cd levels in the rhizosphere sediment.The increase in sediment sulfur pool effectively reduced the bioavailability of Cd and enhanced Spartina alterniflora ability to assimilate sulfate and resist Cd stress,thereby strengthening its competitive advantage over Phragmites australis under Cd stress.Moreover,Spartina alterniflora exhibited a higher ability to assimilate sulfate compared to Phragmites australis.Considering these factors,Spartina alterniflora has an advantage in the competition with Phragmites australis in Cd-contaminated coastal wetlands affected by rising sea levels and fluctuating sulfur environments,which may further contribute to its expansion in Cd-polluted coastal wetlands.This study elucidates the mechanism of sea level rise affecting Spartina alterniflora tolerance to Cd stress mediated by S fluctuations.In the future,with the backdrop of rising sea levels,this research can provide scientific evidence and theoretical support for the prevention and control of Spartina alterniflora invasion in coastal wetlands contaminated by Cd.