The Role And Mechanisms Of Dissolved Organic Matter In Promoting Degradation Of Pyrene By Fungus

Polycyclic aromatic hydrocarbons(PAHs)contain two or more fused benzene rings.They are hydrophobic,bio-accumulative and resistant to degradation.In addition,trapped and stayed in soil micropores,they age and persist in soil.Therefore,PAH-contaminated soils are very difficult to remediate.As partitioning plays a major role in the adsorption of PAHs by microbes and the adsorption-desorption process is reversible,the PAHs adsorbed on microbes can undergo desorption and then degradation,or undergo degradation directly.Interactions between dissolved organic matter(DOM)and PAH-degrading microbes can increase the bio-accessibility of PAHs.In this study,pyrene was used as a model PAH,and Phanerochaete chrysosporium(PC)was a typical PAH-degrading fungusamong white rot fungi.The effect of DOM on the processes of adsorption and desorption of pyrene to and from soil was investigated.The mechanisms of how DOM influences the adsorption of pyrene by mycelia were explored.The relationship between DOM and the production process of PAH-degrading enzymes was also investigated.Other Pyrene-degrading fungi were isolated,indentified and compared to PC in the degradation of PAHs.The following major conclusions were drawn:(1)The influence of DOM on the adsorption of PAHs on soil: Endogenous DOM inhibited the adsorption of pyrene by soil.Therefore,removal of endogenous DOM enhanced the adsorption of pyrene by soil.The effect of exogenous DOM on the adsorption of pyrene on soil was decided by the concentration of exogenous DOM: when lower than a certain concentration,co-adsorption and cumulative adsorption played a major role and adsorption of pyrene on soil was enhanced;when higher than a certain concentration,DOM had major influence on the solubilization of organic matters,which was favorable for the desorption and mobility but unfavorable for the adsorption of pyrene.DOM of various molecular weights had different influences on the adsorption of pyrene on soil.When DOM concentration was higher than a certain value,DOM of high molecular weights stimulated adsorption,otherwise inhibited adsorption.The influence of DOM of different sources on the adsorption of pyrene on soil was mainly controlled by the proportion of high molecular weight DOM(HMW-DOM).Pyrene was dissolved more by DOM derived from pig manure than that from the wormcast,the more the HMW-DOM,the stronger the influence on the solubilization of pyrene.(2)The influence of DOM on the adsorption of pyrene on mycelia: DOM enhanced the adsorption of pyrene by mycelia.Dead mycelia had a stronger pyrene adsorption ability than live mycelia.(3)Stimulation effects of DOM on the adsorption of pyrene on mycelia and the degradation of pyrene: Results of adsorption of pyrene on dead and live mycelia and adsorption-degradation of pyrene on live mycelia showed that dead mycelia had a stronger pyrene adsorption ability than live mycelia at the earlier stage,but the adsorbed pyrene was easy to undergo desorption.Pyrene was completely removed by live mycelia with PAH-degrading enzymes.Addition of DOM stimulated enzyme production by mycelia,and the more DOM was added,the earlier enzyme activities were detected.When the amount of pyrene desorbed was slightly higher than the amount degraded by enzymes,pyrene concentration stayed at a low value;when the amount of pyrene desorbed was approximately equal to the amount degraded by enzymes,pyrene concentration was very low and close to zero.HMW-DOM had solubilization effects,which was favorable for PAH desorption and improved the bio-accessibility of PAHs;low molecular weight DOM(LMW-DOM)could be subjected to aerobic degradation as the organic C source of electron donors and provided C source and energy for PC growth.Therefore,LMW-DOM was a relatively ideal co-metabolic substrate which led to the increase of mycelium biomass and correspondingly increased biosorption of pyrene on mycelia.LMW-DOM induced the production of PAH-degrading enzymes at high C/N and when PAHs co-existed,shortened the distance between PAHs and PAH-degrading enzymes via biosorption,improved bio-accessibility and enhanced bio-degradation.(4)Stimulation effects of DOM on the adsorption/degradation of pyrene on soil and mycelia: HMW-DOM increased the desorption of PAHs from soil and mycelia,increased dissolved PAHs,and thus improved the bio-accessibility of PAHs.DOM of moderate molecular weights stimulated the adsorption of PAHs on soil particles,shortened the distance between the PAH-degrading enzymes and PAHs,and improved the bio-accessibility of PAHs.LMW-DOM was easily utilized by fungi as energy to increase the biomass of mycelium.As a result,biosorption was increased,which was favorable for bio-degradation.(5)C/N and PAH-degrading enzymes: C/N monitoring during the incubation showed that increase in mycelium dry weight was related to enzyme activities and nitrogen concentration;in addition,the higher the enzyme activities,the earlier the mycelium dry weight increased to the maximum value;the higher the nitrogen concentration,the earlier the mycelium pellets aged and turned yellow.During the incubation,the excretion of enzymes was related to nitrogen concentration and C/N;extremely low nitrogen concentration and specific C/N were indispensable conditions for the excretion of lignin peroxides(Li P)and manganese peroxidase(Mn P);polysaccharides influenced enzyme activities,nitrogen and oxygen limitation led to the production of polysaccharides,and excessively produced polysaccharides inhibited enzyme activities.(6)Influence of Pyrene-degrading fungistrains: Three pyrene-degrading fungal strains,H1,H2,and H3,were isolated after enrichment incubation of earthworm cast originated from pig manure using PAHs as C source.Observation with microscope and molecular identification showed that the cultural and morphological characteristics of H1,H2,and H3 were most close to those of Asperqillus fumigates,Trichoderma asperellum,and Mucor circinelloides,respectively.When the initial concentration of pyrene was 5 mg/L,the maximum degradation rate of pyrene by H1,H2,and H3 was 75.63%,81.82%,and 95.98%,respectively.After 30 days of incubation,pyrene-degradation rate achieved by H2 and H3 was 8.49% and 22.65%,respectively,higher than that by PC.