Microbial Social Behavior And Regulation Mechanisms Mediated By AHLs In A UASB

Anaerobic digestion of organic matter is carried out by fermentative acidogens,syntrophic acetogens,methanogens.sitmulate the syntrophic acetogenesis and methanogensis rate is the key step to enhance the efficiency and operational stability of the anaerobic digestion.The granulation of anaerobic sludge not only improves the activity of syntrophic acetogens and methanogens,but also has the advantages in fast sedimentation rate,low sludge production rate and impact resistance.Hence,it is widely used in engineering practice.However,research on sludge granulation process and microbial mechanism remains unclear.The quorum sensing?QS?system is able to control various social behaviors such as microbial activity and extracellular polymeric Substances?EPS?secretion,which is a noval perspective to analyze the anaerobic sludge granular process and the coordination mechanism between functional floras,and it will significance enhance the development of anaerobic biological treatment technology.This study is carried out basing on the start-up and regulation of two up-flow anaerobic sludge blanket?UASB?reactors.Starting-up,organic loading rate?OLR?,sludge loading rate?SLR?,temperature reduction,and addition of C₆-HSL were performed to analyze the mechanism of the microbial QS system.The results showed that,during the methane fermentation process,most of the N-acyl-homoserine lactones?AHLs?are secreted in the starvation conditions.Among them,AHLs which are significantly positively correlated with the hydrolyzed fermentation bacteria include C₄-HSL and OXOC₆-HSL,while C₈-HSL,C₆-HSL,OXOC₁₀-HSL and C₁₄-HSL were positively related to homoacetogens,syntrophic propionate-oxidizing bacteria,syntrophic butyrate-oxidizing bacteria and aceticlastic methanogen,respectively.Hydrogenotrophic methanogen controlled their social behavior through a variety of AHLs(C₁₀-HSL,C₁₂-HSL,OXOC₆-HSL,OXOC₈-HSL,OXOC₁₂-HSL,and OXOC₁₄-HSL).By comparing the efficiency of UASB with different OLR lifting strategies,it was known that"fixed influent chemical oxygen demand?COD?,shortening hydraulic retention time?HRT?"is more suitable for OLR lifting than"fixed HRT,increasing the influent COD".However,"fixed HRT,increasing the influent COD"was more suitable for anaerobic activated sludge granlur process.Both OLR lifting strategies reduced the diversity of bacteria and archaea in UASB.The increased OLR enhanced the abundance of homoacetogenic acetogen Treponema,and the dominant methanogens was converted from hydrogenotrophic to aceticlastic.During the sludge domestication,the OLR was as low as 2 kgCOD/?m³?d?,and the activated sludge was starved.In order to achieve coexistence under starvation conditions,the functional floras regulated their social behavior by secreting AHLs to obtain slower reproduction with other populations?K-strategy?.Along with the increase in OLR,enough substrate stimulated the growth of each flora,and the functional flora again coordinated the metabolic balance by secreting AHLs.The results of SLR regulation and operation showed that the maximum SLR of the system daeling with carbohydrate organic wastewater was about 0.68kgCOD/?kgMLSS·d?.With the increase of SLR,the zeta potential gradually decreased to-15.80 mV,which resulted in increasing the electrostatic repulsion between microorganisms and reducing the self-flocculation ability,as well as the surface hydrophobicity.Hence,the average particle size of the granular sludge decreased from 714.41?m to 297.65?m.The increase of SLR also reduced the relative abundance of syntrophic acetogens such as Syntrophomonas,Syntrophobacter and Smithella,which led to the accumulation of propionate in UASB effluent.With the increment of SLR,the microbial starvation was alleviated,and the concentration of AHLs indicating the starvation of hydrolyzed fermentative bacteria,syntrophic propionate-oxidizing bacteria and aceticlastic methanogen drasticly decreased,while the concentration of AHLs indicating the starvation of hydrogenotrophic methanogen and homoacetogens gradually increased.Hence,they maintained a very low hydrogen partial pressure within the UASB.When the temperature decreased from 35°C to 20°C,the zeta potential,the self-flocculation ability and surface hydrophobicity of activated sludge decreased,while the protein?PN?and polysaccharide?PS?in EPS increased,resulting in the disintegration of anaerobic granular sludge.The phenomenon was exacerbated as the temperature decreased.With the decrease of temperature,the relative abundance of syntrophic acetogens also decreased.The hydrogenotrophic methanogens can tolerate low temperature and gradually replace the function of the aceticlastic methanogens.The growth of hydrophilic methanogens was stimulated by the secretion of OXOC₁₄-HSL.Both hydrophilic methanogens and homoacetogens maintained the low partial pressure of hydrogen in the UASB,which created favorable conditions for the growth and metabolism of the syntrophic propionate-oxidizing bacteria.When the OLR increased from 8 kgCOD/?m3?d?to 12 kgCOD/?m3?d?,the OLR shocking severely inhibited the activity of methanogens and syntrophic acetogens,dramatically reduced COD removal rate to about 60%and tended to disintegrate sludge particles.After added 1?g/L C6-HSL,the activity of methanogens and hydrogen-producing acetic acid bacteria was recovered and COD removal rate of the system increased to 76.54%.The additional C6-HSL did not significantly affect the abundance of hydrogen-producing acetic acid bacteria,but significantly sitmulated the growth of aceticlastic methanogens?Methanothrix?and the secretion of C14-HSL,which further enhanced the activity of aceticlastic methanogens.Exogenous QS signaling molecule C6-HSL could also induce the decrease of PS content in LB-EPS,increase Zeta potential of UASB system and hydrophobicity of sludge flocs,significantly enhance flocculation ability of microorganisms and promote sludge particle remodeling.