The Production Pathway Of Reduced Sulfur In Urban River Surges

Urban rivers and streams are of vital importance to the coordinated development of urban society,economy and environment.However the imbalance of water ecosystem caused by the phenomenon of black odorization of river wells,which is affected by industrial,domestic and agricultural wastewater has once become a prominent hot issue in China,especially in the Guangdong-Hong Kong-Macao Greater Bay Area.Even though the reduction of organic matter in river water/sediment by means of endogenous removal or exogenous control is considered to be effective in alleviating the black odor problem,some of the treated river surges are still subject to re-blackening and re-stinking.In order to achieve the long term treatment of river wells,we need to fundamentally understand the evolutionary process and factors influencing the black odor of river wells.In this study,we used a combination of in situ investigation and indoor simulation experiments to reveal the main sources of reduced sulfur(∑S^2-)and its microbial mechanisms in urban river sediments by means of enzyme activity inhibition and high-throughput sequencing.The results of this study can help to provide a theoretical basis for the treatment of black smelly water bodies in urban rivers and the optimization of the regulatory system.The main conclusions of this thesis are as follows:（1）The apparent degree of black odor in urban river wells is related to the∑S^2-content of sediment.The sediment∑S^2-content of H1 and H2,which had relatively high black odor,was0.71±0.46 mg/g and 0.87±0.30 mg/g,respectively,which was significantly higher than that of H3（0.25±0.19 mg/g）and H4（0.31±0.16 mg/g）,which had low black odor（P<0.05）.Overall,the∑S^2-content in the sediment was significantly higher in summer（0.68±0.46 mg/g）than in winter（0.31±0.22 mg/g）.Dissolved oxygen（DO）concentrations varied in the opposite direction,being significantly lower in the summer（3.15±1.81 mg/L）than in the winter（5.50±1.42 mg/L）.The sediments of urban rivers have high nutrient contents,with total nitrogen（TN）,total phosphorus（TP）and total organic matter（TOM）contents of 2.9±0.7 mg/g,4.4±1.5 mg/g and 70±30 mg/g,respectively.In the sediment interstitial water,especially in H1 and H2,inorganic nitrogen is mainly in the form of ammonium nitrogen（NH₄⁺）,which shows an overall higher concentration in summer than in winter.In contrast,the overlying water of the river surges is mainly in the form of nitrate（NO₃^-）,and shows the opposite seasonal difference with NH₄⁺.Pearson correlation analysis shows that the DO,NO₃^-,Moisture,TN and NH₄⁺content of the overlying water are environmental factors that significantly affect the accumulation of∑S^2-in the river sediment.（2）The∑S^2-production rate（0-3.04μg/g/h）in the sediments of urban wells with different degrees of black odor is significantly different,and the average rate in H1 and H2（0.71μg/g/h）is significantly higher than that in H3 and H4（0.05μg/g/h）.The TOM,Moisture and NH₄⁺content of sediment were important environmental factors influencing the activity of sulfate-reducing bacteria（SRB）and methanogenic bacteria（MPB）-mediated∑S^2-production in riverine sediments.Macro-transcriptome analysis showed that the pathways of sulfide production in river sediments include three main pathways:singlet sulfur reduction process（reversible）,sulfite reduction process mediated by SRB,and methyl mercaptan methyl transfer process mediated by MPB.Therefore,SRB and MPB are the main microorganisms mediating∑S^2-production in river sediments,in which the pathway mediated by SRB dominates,but we speculate from the experimental results that MPB may have a higher potential for∑S^2-production compared to SRB.（3）The SRB community in the river sediment is mainly composed of five orders of Syntrophales,Desulfobacterales,Desulfovibrionales,Syntrophobacterales and Eubacteriales,Desulfosarcina and Desulfobulbus which belong to the Desulfobacterales are the dominant genera in all river wells,but their abundance varies considerably in river sediments with different degrees of black odor,which may be an important SRB taxon leading to the formation of black odor.The MPB community of river surge sediments is mainly composed of Methanobacteriales,Methanomassiliicoccales,Methanomicrobiales,Methanosarcinale and Norank Candidatus＿Methanofastidiosa total of five orders.The dominant genus were Methanobacterium,Methanothrix and Methanospirillum,accounting for 53%of the total number of sequences.Notably,the relative abundance of the Candidatus＿Methanofastidiosum（methylotrophic methanogenic bacteria）was significantly higher in H1 and H2 than in H3 and H4,which may have an important contribution to the production of∑S^2-in the river surge sediments.Both MPB and SRB communities in the river sediments were significantly influenced by the content of SO₄^2-,DO,TN,and Moisture.In addition,ecological network analysis showed that there were complex symbiotic and competitive relationships between MPB and SRB in the river sediments,and most of the keystone species that maintained the structural stability of MPB and SRB communities belonged to unclassified SRB,and their relative abundance was significantly correlated with sediment NH₄⁺,TN and SO₄^2-.（4）When MPB and SRB were inhibited separately or no inhibition was applied,the∑S^2-production rate of the stream sediments increased significantly with temperature（15-35℃）,which was more obvious in the sediments with higher in situ∑S^2-production rate,and the MPB-mediated∑S^2-production process is more sensitive to temperature.Unlike temperature,the effect of p H on the different∑S^2-production pathways in surge sediments differed,with∑S^2-production rates in sediments under neutral conditions（p H=7）with and without MPB inhibition being lower than those under acidic（p H=6）and alkaline（p H=8,9）conditions.Howerer,the MPB-mediated∑S^2-production rate was significantly enhanced at p H=6.In the low oxygen range of 0-3 mg/L,the increase in DO concentration showed a significant inhibition of the rate of∑S^2-production in the river sediment,but the response of different pathways to DO differed,in which MPB-mediated∑S^2-production had a relatively higher oxygen tolerance than SRB to DO.