Effects Of Iron Addition On Seagrass Growth And Sulfur Speciation Transformation And Its Application

Seagrass bed,known as“blue carbon”ecosystem with high primary productivity and biodiversity,is declining due to sulfide toxicity caused by increased organic loading.The addition of exogenous iron（Fe）may effectively improve the state of seagrass in such sediments.At present,there are few reports on the mitigation of seagrass sulfide stress by different amounts and forms of Fe addition.In this study,we combined field investigation,indoor simulation and field control experiment to explore the effects of Fe addition amount and forms on seagrass growth,pore water and sulfur cycle.Through the investigation,growth characteristics and the content of nutrient elements in seagrass bed,we found that the pivotal factors of its decline were not N or P but sulfide.Furthermore,we found aboveground productivity had a very significant positive correlation with Fe in pore water,which indicated Fe may limit the growth of seagrass in this area.In order to explore the optimal amount of iron addition,starch(5 g·kg^-1)was added to two different（calcareous and siliceous）sediments in greenhouse to simulate the high organic load of seagrass bed.Different amounts of Fe（II）(0,0.05,0.15,0.5 g·kg^-1)were added at the same time to explore the response characteristics of photosynthetic pigment content,chlorophyll fluorescence parameters and antioxidant enzyme activity of seagrass leaves to Fe addition.The results showed that during the experiment（30 d）,the addition of exogenous Fe in the two types of sediments could effectively improve the content of photosynthetic pigment,the level of chlorophyll fluorescence parameters and the activity of antioxidant enzymes,and the effect was best at 0.5 g·kg^-1.Fe addition had a very significant effect on pore water Fe²⁺,S^2-,photosynthetic pigment content,chlorophyll fluorescence parameters and antioxidant enzyme activity,and had a significant（P<0.05）or very significant（P<0.01）effect on the contents of EXC-Fe,OM-Fe,CRS and S⁰.While sediment type had a very significant（P<0.01）effect on pore water Fe²⁺,leaf antioxidant enzyme activity,sediment CRS,TS and S⁰.The interaction between sediment type and Fe addition had a significant（P<0.05）or very significant（P<0.01）effect on photosynthetic pigment content,antioxidant enzyme activity,EXC-Fe,RED-Fe,RES-Fe and AVS.In general,the effect of Fe addition on photosynthesis and antioxidant enzyme activity is higher than that of sediment type.The addition of exogenous Fe can significantly affect the cycle of iron and sulfur in sediment,effectively improve the physiological and ecological indexes of T.hemprichii in calcareous and siliceous sediments.According to the indooor experiment,two different field seagrass beds,Xiaodong Bay（T.hemprichii）and Xincun Bay（Enhalus acodoides）,were selected for further research.The effects of different forms of iron[Fe（0）,Fe（II）,Fe（III）]on seagrass growth,physiology and sediment were explored.The results showed that the addition of different forms of exogenous iron in sediments increased the content of leave photosynthetic pigment and pore water Fe²⁺.While the iron addition had no significant effect on pore water S^2-.The addition of different forms of exogenous iron has the trend of increasing RED-Fe and OM-Fe in sediments,and increase the AVS and AVS+CRS in sediments of E.acodoides.Iron addition also affected bacterial community composition of seagrass sediments and change the abundance of sulfur cycle related bacteria,and same form has different effects on different seagrass.Divalent iron can reduce the abundance of Proteobacteria and Firmicutes,and significantly reduce the abundance of sulfur oxidizing bacteria（Sulfurovum）,while increase the abundance of Proteobacteria and Firmicutes in E.acodoides,but the inner mechanism needs to be further studied.In general,Fe addition could significantly increase the content of photosynthetic pigment,increase pore water Fe²⁺and affect S cycle in seagrass sediments,and the effect varied with the amount,form of iron added and the species of seagrass.