The Microbial Transformation Of Terrigenous Particulate Organic Matter And Its Ecological Effects In Seawater

Estuary,which connects the land and ocean,plays a fundamental role in the global biogeochemical element cycles.In the estuary,degradation of terrigenous particulate organic matter releases dissolved organic matter and nutrients to the surrounding environment,changing the elements cycle.A mass of field surveys show that the characteristics of terrigenous particles experience dramatic changes in estuary.Both the mixing of freshwater and seawater,and microorganisms can cause the changes in terrigenous particles.It is difficult to distinguish the effects of microorganisms from that of physical mixing on terrigenous particles in the field,so there is little knowledge on how microorganisms transform terrigenous particles.To study the transformation of terrigenous POM conducted by microorganisms,we set up an ultra-large-volume longterm incubation experiment utilizing the Aquatron tower tank located in Dalhousie University.(1)Firstly,ocean water from Halifax coastal ocean was poured into Aquatron Tower Tank,and the system was kept in dark for stabilization for 80 days.During the stabilization phase,it was observed that phototrophic groups decreased while chemoautotrophic nitrifiers increased over the course of time in the Aquatron incubation system.Functional prediction also supported that nitrification was an essential process.Coupling the enrichment of nitrifiers and the enhancement of nitrification with degradation of aromatic compounds in the incubation,we speculate that fresh organic carbon produced by the nitrifiers could promote the degradation of recalcitrant aromatic compounds.(2)After stabilization,river water was gently added to the seawater surface.The results indicated that the input of terrigenous particulate organic matter influenced the ocean microorganism community structure and function.Genes related to CAZymes and peptidases were mainly affiliated to Bacteroidetes,Planctomycetes and Proteobacteria,suggesting that these groups played important role in particulate organic matter degradation.Besides,although fungi and nitrifiers were not the most abundant groups,they were essential to terrigenous organic matter transformation.High molecular matter was degraded into low molecular matter by fungi and the dominant bacteria further transformed the low molecular matter.Ammonia released by terrigenous organic matter degradation further strengthened the nitrification(3)The results of particulate composition,POC:PN and content of stable carbon and nitrogen isotope,together with two-end member mixing model supplied information about how the microorganism transformed terrigenous particulate organic matter.Fungi and Bacteroidetes tended to degrade polysaccharide rather than nitrogenous compounds when a mass of terrigenous POM was present.Similarly,Planctomycetes preferred to utilize high C:N nitrogen-containing compounds.Besides,nitrification was strengthened by terrigenous organic matter degradation and compounds produced by nitrifiers have low C:N.Coupled with fungi,Bacteroidetes,Planctomycetes and nitrifiers,the value of C:N of residual particles was lower.Microorganism preferred to degrade compounds with low 815N.Bacterial biomass biosynthesis and aggregation,which preferentially utilizes 13C-enriched organic substances,may have resulted in the positive deviation of δ15N and δ13C.The accumulation of recalcitrant organic matter(aromatic compounds)and depletion of liable organic matter(nitrogenous compounds)also resulted from the selective degradation of microorganism.In summary,this research illustrated that the transformation of terrigenous organic matter in ocean was mainly dominated by microorganism metabolism.The cometabolism between Bacteroidetes and fungi may contribute to the degradation of terrigenous polysaccharide with high molecular matter in the marine environment.The C:N of residual particles decreased along with nitrification.Isotope fractionation caused by microorganism resulted in the deviation of particles.This study gave new insight on the impacts of terrigenous input to the carbon and nitrogen elements cycle in ocean.