Carbon Fixation And Associated Influencing Factors In Sediments Of The Yangtze Estuary And Adjacent Coastal Areas

Dark carbon fixation process driven by microorganisms has become a research hotspot of carbon cycle as an important part of biological carbon fixation.As a transitional zone between land and ocean,the estuary and adjacent sea areas are significantly influenced by various biogeopsychemical factors,including environmental geomorphology and nutrient input.Abundant nutrients provide sufficient substrate support for microbial communities,thereby regulating the biogeochemical cycling processes including dark carbon fixation of regional critical biogenic elements to a certain extent.While the dark carbon fixation process in the open ocean water column and deep-sea sediments has been extensively studied,the dark carbon fixation process and influencing factors in sediment of the estuary and adjacent sea areas are not yet clear.Therefore,this study employed ¹⁴C isotope labeling combined with molecular biology techniques to investigate the dark carbon fixation process and carbon-fixation microorganism diversity in sediments of the Yangtze River Estuary and adjacent sea areas,which serves as a typical region,the aim is to reveal the spatiotemporal characteristics and influencing factors of the dark carbon fixation rate and carbon-fixing microorganism distribution in sediments,and clarify the ecological environmental effects of dark carbon fixation in sediments of the estuary and adjacent sea areas.The research result will help deepen the understanding of the carbon cycling process and its influencing mechanism in the estuarine and nearshore ecosystems.The main findings and understanding of this study include the following:（1）The average dark carbon fixation rate of sediment in Chongming Dongtan reached 1.17 mmol m^-2 d^-1,indicating greater potential of dark carbon fixation in the Yangtze Estuary.The dark carbon fixation rate of sediments shows significant seasonal and spatial differences,and increasing with seasonal warming but inhibited by extreme high temperatures.Relative to mudflat,the Spartina alterniflora vegetation promotes dark carbon fixation of sediments,while the Phragmites australis vegetation inhibits it.Different vegetation regulates the dark carbon fixation in sediment by adjusting key factors NH₄⁺,Fe³⁺,S^2-,and functional gene（cbb L,cbb M）abundance.（2）The dark carbon fixation rate of sediment in the adjacent sea area of the Yangtze River Estuary ranges from 0.18-1.15 mmol C m^-2 d^-1,showing significant seasonal differences and spatial heterogeneity in both seasons.Temperature,dissolved oxygen,S^2-,Fe²⁺,and cbb M gene abundance are the main environmental factors affecting the heterogeneity of dark carbon fixation rate,aerobic sulfur oxidation may be the main driving force of sedimentary dark carbon fixation.The direct contribution of ammoxidation to dark carbon fixation of sediments may be small,but there is a significant positive and negative correlation between dark carbon fixation rate and ammoxidation rate in winter and summer,respectively.It is estimated that the total amount of dark carbon fixation in sediments in the adjacent sea area of the Yangtze River Estuary is expected to reach 2.0×10⁶ t C y^-1.（3）In sediments of adjacent sea areas of the Yangtze River Estuary,There is no significant seasonal and spatial heterogeneity in the distribution of cbb M encoding microbial communities,while a significant spatial heterogeneity in the distribution of cbb M encoding microbial communities.The composition of the chemolithoautotrophy communities in sediments is relatively stable,and cbb M encoding communities may possess superior adaptability.The structure of cbb L encoding chemolithoautotrophic communities is significantly influenced by total organic carbon（TOC）and total nitrogen（TN）,while the structure of cbb M encoding chemolithoautotrophic communities is significantly influenced by salinity,S^2-,and sulfate.In addition,the diversity of chemolithoautotrophy communities is significantly correlated with NH₄⁺and total carbon（TC）,whereas the abundance of chemolithoautotrophy is significantly influenced by dissolved oxygen（DO）of overlying water,p H,and NO₃^-.