Biodegradation Of Steroid Estrogens Mediated By Biogenic Organic Matter In Shallow Freshwater Lakes

Due to the global climate change and increasing anthropogenic activities,the co-occurrence of eutrophication and emerging contaminants has become one of the most serious problems in shallow freshwater waters.In eutrophic lakes,the occurrence of cyanobacterial blooms releases a large amount of biogenic organic matter（OM）,the biogeochemical processes of which can regulate the fate of other contaminants.Steroid estrogens（SEs）,a common endocrine disruptor,have raised serious concerns due to their ecological risks and toxic effects in freshwater lakes.The natural biological attenuation of SEs in freshwater lakes is believed to be associated with the quantity and quality of OM.However,previous studies mostly investigated the effect of standardized OM on SEs degradation,and few studies have focused on the influence mechanism of biogenic OM,especially the evolution of microbial community function mediated by biogenic OM.In this study,Lake Taihu,the third largest shallow freshwater lake in China,is of interest due to the notorious cyanobacterial blooms in recent years.The fresh cyanobacterial-and macrophyte-derived OM（COM and MOM）were used as the representatives of biogenic OM.The natural estrogen E2and synthetic estrogen EE2 were used as the target compounds.The aim of this study was to reveal the role of biogenic OM on the microbial degradation of SEs in water column and sediments in freshwater lakes.The main results are listed as follows:（1）Relationships between COM molecular bioavailability and E2 degradation in water column was firstly investigated.The COM molecules with decreasing bioavailabilty were separated and characterized through the application of multiple stage biofilm reactor.The labile components in COM mainly included proteins,polysaccharides,and unsaturated hydrocarbons,while the semilabile and recalcitrant components are composed of lignins,tannins,lipids,and condensed aromatics.The abundant labile compounds in COM did not exhibite substrate competition and metabolic repression in E2 degradation,but maintained the growth of bacterial consortium and significantly increased E2 degradation rate from 0.017 h^-1 to 0.097 h^-1.However,the semilabile component did not serve as the availabile carbon source for bacteria and lead to the lower degradation rate.Unexpected,the recalcitrant components also accelerated the E2 degradation without increasing the growth of biomass,which may be due to that the aromatic molecules stimulated the expression of E2-degrading enzymes in bacterial consortium.As the main products of E2 degradation,the accumulation of E1 was also affected by the bioavailability of COM.Microbial community analysis showed that the existence of COM regulated the microbial composition and structure.The relative abundance of Betaproteobacteria increased during E2 degradation process,whereas the recalcitrant OM was favorable to the survival of Stenotrophomonas and Brevundimonas.Furthermore,the role of OM bioavailability in E2degradation was also demonstrated through assessing the E2 degradation potential at 10 sites in the north of Lake Taihu.It was found that the E2 degradation potential was positively correlated with the sum of active and recalcitrant fractions in OM（P<0.001）.These results suggested that the release of numerous labile and recalcitrant OM molecules during cyanobacterial blooms played a significant role in controlling the natural attenuation of SEs in water column.（2）Effects of the quantity and quality of biogenic OM on the microbial degradation of EE2 and the bacterial acitivity in sediments were studied.Analysis of EEM–PARAFAC indicated that the fluorescent composition of COM and MOM was dominated by protein-like components（tryptophan-and tyrosin-like substances）,and the contents of humic-like components,including microbially humic-,humic acid-,and fulvic acid-like substances,were relatively low.In the two months of microcosms,the leachates of COM and MOM both promoted the aerobic EE2 degradation in Lake Taihu sediments.Moreover,the EE2 degradation efficiency was positively correlated with the decrease in organic carbon concentrations in sediments（P<0.01）,suggested that the biogenic OM mainly enhance the degradation of EE2 via co-metabolism.However,the biogenic humics may also accelerate EE2 degradation through the stimulated cellular enzymes.Analysis of enzymetic acitivity showed that the decomposition of COM and MOM significantly increase the activity of cellcular metabolic enzymes,including fluorescein diacetate hydrolase,dehydrogenase,urease,and neutral protease.Meanwhile,the production of proteins and polysaccharides in microbial colloidal and bound extracellular polymeric substances（EPS）was also positively related to the input of protein-like OM.The increased concentrations of EPS encapsulated more EE2 in bacterial communities.These results suggested that the enhanced degradation of EE2 was attributed to the enhanced metabolic activity of indigenous microorganisms by co-metabolism of biogenic OM,as well as to the intensed access to extracellular degrading enzymes.（3）The influence of biogenic OM on composition and function of bacterial community in sediments during EE2 degradation was further analyzed.The input of COM and MOM both facilitated the bacterial growth and increased the microbial diversity in the incubation.The co-occurrence network analysis showed that the compositon of bacterial community was closedly linked to the quantity and quality of biogenic OM in EE2 degradation.The strong correlations between OTUs and protein-like substances in OM suggested that the labile components were the primary factor affecting the niche space of microbial groups.Meanwhile,the input of biogenic OM also enhanced the interspecies interactions among microorganisms,which may coordinate the development of microbial communities and facilitate the degradation of EE2 in sediments.Metabolic pathway analysis based on KEGG indicated that the existence of COM and MOM both promoted the evolution of carbohydrates,amino acids,and xenobiotic metabolism functional genes in bacterial community.These results suggested that the input of biogenic OM not only enhanced the metabolism genes of organic carbon in sediments,but also increased the relative abundance of contaminants metabolism genes.Therefore,the production of biogenic OM during cyanobacterial blooms may accelerate the natural attenuation of SEs in water column and sediments,and more attenation should be paied to the fate and ecotoxicity of SEs degradation products in eutrophic freshwater lakes.