Study On The Selective Enrichment And Metabolic Regulation Mechanism Of Electroactive Microorganisms By Electric Field

Bioelectrochemical systems(BESs)are a method that utilizes the unique extracellular electron transfer(EET)properties of electroactive microorganisms(EAMs)to provide controllable bioelectron transfer for pollutant degradation.Longterm studies have generally agreed that the electric field(EF)has a direct effect on the performance of BESs,but the mechanism of the effect is still unclear.In this paper,we systematically investigated the response mechanism of two important electroactive bacteria strains to local electric field by constructing a non-uniform strong electric field,and revealed the regulatory mechanism of electric field on the degradation of complex substrates propionate and phenol.Geobacter spp.are widely found to be dominant in biofilms in acetate-rich environments when an appropriate voltage is applied,but it is still largely unknown how these bacteria are selectively enriched.Herein,two key Geobacter spp.that have been demonstrated predominant in wastewater-enriched electroactive biofilm(EAB)after long-term operation,G.sulfurreducens and G.anodireducens,responded to EF differently,leading to a higher abundance of EF-sensitive G.anodireducens in the high EF(HEF)region after cocultivation with G.sulfurreducens.Transcriptome analysis indicated that two-component systems(TCSs)containing sensor histidine kinases and response regulators were the key for EF sensing in G.anodireducens rather than in G.sulfurreducens,which are closely connected to chemotaxis,c-di-GMP,fatty acid metabolism,pilus,oxidative phosphorylation and transcription,resulting in an increase in extracellular polymeric substance(EPS)secretion and rapid cell proliferation.We further investigated the regulatory mechanism of local EF strength on the degradation of complex substrates by using an electroactive biofilm that degrades propionate and found that HEF selectively enriched Geobacter(especially G.anodireducens).The selective effect of the EF was further enhanced in the propionatefed system compared with the acetate-fed system.In the HEF region,the content of biomass and extracellular proteins,and the relative percentage of Geobacter in EAB were 1.5-6.9 times higher than those in the low EF(LEF)region,while there was only1.2-1.3 times in the acetate-fed system.The strengthening effect of EF in propionate-fed system suggested that mechanism of EF regulation on EAMs may be different from that of the acetate-fed system.To further investigate the mechanism of action,the dominant microorganisms and their interactions during propionate degradation were first investigated.After five generations of acclimation,it was determined that Arcobacter butzleri(48% ～ 59%)and Comamonas testosteroni(10% ～ 19%)in suspension and G.anodireducens(56%～ 71%)in biofilm were dominant strain.The binary co-cultivation of A.butzleri and G.anodireducens showed that A.butzleri metabolized propionate to produce acetate for G.anodireducens to grow and produce electricity,while the trace acetate could act as an "activator" to activate the direct metabolism of propionate by G.anodireducens.HEF mainly regulated the significant up-regulation of genes related to cytochrome,NADH oxidation,ATP synthesis and propionate metabolism in G.anodireducens,which may accelerate the oxidation of propionate and extracellular electron transfer.The principle of EF regulation was further extended to the biodegradation of phenol,a typical toxic pollutant,and the effect of EF on the degradation of phenol was investigated by adjusting the EF intensity by setting different electrode spacing.Compared with the inter-electrode solution internal resistance difference(1.5 times),the local EF strength has a larger difference multiple(11 times).The sustained action of HEF was beneficial to improve the electrochemical performance and EPS secretion of BESs,and realize the selective enrichment of phenol-degrading bacteria Comamonas and exoelectrogens Geobacter.Compared with the LEF treatment system,the increase of the abundance of phenol-degrading bacteria(2.1 times)and the EPS content(1.5 times)in HEF jointly accelerated the degradation of phenol.In this paper,a series of studies showed that EF can be used as an easy-to-operate means to regulate EAB,and revealed the regulation mechanism of EF on EAMs.It was found that EF can indirectly selected enrichment of pollutant-degrading bacteria by regulating the metabolism of EAMs,which provides a new idea for the rapid degradation of pollutants and biomass energy conversion based on BESs.