Characterization Of Natural Dissolved Organic Matter From South China Sea And Contribution Of Bacterial Exopolysaccharides Into Marine Carbon Pool

Marine dissolved organic matter(DOM)is a large carbon reservoir(700 Pg C)similar in magnitude to atmospheric CO2.More than 95%of marine DOM is refractory dissolved organic matter(RDOM)that resists microbial utilization and can be stored for several millennia in the oceans;changes in marine RDOM may mediate the atmospheric C02 pool.Hence,understanding the mechanisms of oceanic carbon sequestration is crucial as it impacts global climate change.The sources and sequestration of RDOM are thought to be intrinsically linked to microbial processes.However,the chemical composition and cycling processes of marine RDOM are not fully understood.Exopolysaccharides(EPS)represent an important source of DOM in marine ecosystems.Bacteria are known to be key participants in the turnover of EPS,but studies on bacterial communities associated with EPS degradation remain fragmentary.Understanding the chemical composition of natural DOM in the ocean and addition of known EPS to long-term incubation allow observing chemical structure of DOM to resolve the formation pathways of RDOM.To achieve these goals,14 natural DOM samples were solid phase extracted from different stations and different depths of South China Sea,and were measured in ultra-high resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry(FT-ICR-MS)to uncover the molecular composition of which.Besides,two marine bacterial EPS samples were purified,and microbial EPS degradation was investigated through incubation experiments in seawater from estuary and open ocean sites(surface and deep water samples were tested).We found that(i)DOM composition significantly varies among the surface and deep seawater samples.The weighted average of molecular weight,O/C ratios,DBE,degradation index,and IOS-RDOM were found relatively high in deep seawater DOM samples,while weighted average of H/C ratios were found higher in surface DOM samples;compound group analysis indicates that surface DOM samples have relatively high percentage of labile organics,whereas deep sea samples have higher contribution of recalcitrant compounds.(ii)bacteria-derived EPS samples were efficiently reutilized within the first 30 d.The major consumer of EPS during the early incubation period was Gammaproteobacteria,an affiliation that was likely shaped by nutrients at the family level.In detail,Oceanospirillaceae dominated the bacterial community in the estuarine and deep seawater incubations enriched in nutrients,while Alteromonadaceae dominated in surface open seawater incubations which were oligonutrient.After 360 d,residue DOM in experimental samples were found to have relatively high proportions of refractory compounds compared to the controls.These results suggest that the bacterial communities from different oceanic biogeographic regions efficiently reutilized bacterial EPS and produced refractory DOM that was similar to naturally occurring refractory DOM from the deep ocean,which represents an important carbon reservoir.