Effects Of Salinity Stress On Synechococcus Growth And Iron Isotope Fractionation

In the special estuary environment,material exchange is frequent and the source of energy is complicated,which leads to the large biomass and biodiversity of the estuary environment.In addition,the salinity of seawater changes with spatial changes,and phytoplankton has different responses to different salinities,which in turn leads to the diversity and structural complexity of phytoplankton communities.As the salinity increases,microorganisms will face the salinity stress,and the osmotic pressures inside and outside the cell will change.In addition,high-salinity seawater can also accumulate ROS(reactive oxygen species)in cells.ROS can cause changes in cell structure and peroxidation of cell membranes.These factors can inhibit cell growth.Although some scholars have detected the presence of freshwater phytoplankton at the estuary of rivers through molecular biology techniques,whether freshwater phytoplankton can survive in the estuary of rivers is still an uncertain scientific question.The response mechanism of freshwater phytoplankton to the salinity gradient research is still very limited.Iron is not only an essential nutrient element for the growth and development of phytoplankton,but also an important component of antioxidant enzymes.Existing research results indicate that higher plants can use iron to resist salinity stress.Does phytoplankton have the same mechanism?Does it reduce the resistance of phytoplankton to salinity in an iron-deficient environment?In addition,the iron isotope value can provide a meaningful understanding of the mechanism of iron absorption by phytoplankton.Generally speaking,phytoplankton cells have priority over the enrichment of heavy iron isotopes.The smaller the degree of enrichment from the original iron pool,the lower the iron concentration of the cells,the higher the isotope value.In this paper,the freshwater cyanobacteria Synechococcus sp.PCC 7942cultivated in varying salinities.The media with different salinities adjusted by seawater were used to simulate the environment of the salinity gradient of the river estuary,and the synthesized ferrihydrite was supplied as iron source.The experimental results show that:1)As the concentration of seawater increased,the biomass of Synechococcus decreased.However,when the proportion of seawater rose to 50%,the biomass of Synechococcus increases and was similar as that growing in 0%seawater.2)As the proportion of seawater changed,the iron isotope composition associated with cells also changed.When the proportion increased from 0 to 25%,the?⁵⁶Fe of Synechococcus cells increased from0.436‰to 1.847‰,but when the proportion of seawater increased to 50%,the?⁵⁶Fe of cells decreased to 0.879‰.3)In the process of increasing the proportion of seawater,the secretion of siderophores(representing the ability of cells to obtain iron)is roughly the same as the change in the biomass of Synechococcus.4)The biomass of Synechococcus has a negative correlation with?⁵⁶Fe.In general,this study proved that iron plays a key role in the survival of phytoplankton under salinity stress,and showed that siderophores introduced by seawater can accelerate the dissolution of iron,increase the bioavailability of iron,and make planktonic plant cells overcome the adverse effects of high salinity.In this paper,we measured the biomass of Synechococcus,the secretion of siderophores and the value of?⁵⁶Fe of cells to reveal the growth of Synechococcus in different salinities and the absorption of iron by Synechococcus cells.These results provide new ideas and perspectives for phytoplankton physiology and its role in iron biogeochemistry in estuary environment.