Influence Of Silver Nanoparticles On Bioremediation Of White-rot Fungus In Wastewater And The Toxicology

Silver nanoparticles?AgNPs?have become the most widely used metal nanomaterials in the current market due to their excellent superconducting,optical,catalytic,and antimicrobial properties.Extensive production,processing,and wide application of AgNPs result in their unavoidable release into the environment,and their environmental and health risks are increasingly attracting widespread attention.The efficiency of microorganisms in wastewater treatment for toxic pollutants will be affected,and unknown risks to environmental microorganisms and human beings will also be increased.Thus,only by clarifying the interaction manner and internal mechanism of between AgNPs and environmental microorganisms,can we enhance the removal performance of environmental microorganisms more specifically,find a more beneficial method for wastewater treatment,and understand the cytotoxicity of AgNPs fundamentally.In this work,AgNPs and Phanerochaete chrysosporium?P.chrysosporium?with unique removal ability to organic pollutants and heavy metals were applied to wastewater treatment.The influences of AgNPs on biodegradation of2,4-dichlorophenol?2,4-DCP?by P.chrysosporium and toxicity modes of AgNPs were investigated.The effects of exogenous silver ions?Ag⁺?on AgNP cytotoxicity were explored.Systematic studies focused on the production of extracellular secretions and antioxidative responses of under stresses of Ag⁺and AgNPs were conducted.The toxicity of cysteine to Ag⁺and AgNPs and the bioremediation ability of P.chrysosporium were also investigated.Finally,a way to enhance the physiological state of P.chrysosporium was sought,further improving its efficiency of wastewater treatment.The specific research works and results of this paper can be summarized in the following five aspects.The first section describes the influences of AgNPs on 2,4-DCP degradation by P.chrysosporium and the related mechanism.Firstly,the inoculated P.chrysosporium and as-synthesized AgNPs were applied to degrade 2,4-DCP from wastewater.The effects of AgNPs,AgNO₃,and 2,4-DCP on the degradation efficiency were investigated.Variations of dissolved Ag⁺,total Ag?including Ag⁺and AgNPs?,solution pH,and extracellular proteins in the system were monitored.The results can be concluded as follows:?1?AgNPs at low doses?0–60?M?have greatly enhanced the degradation ability of P.chrysosporium to 2,4-DCP with the maximum degradation rates of more than 94%,and 2,4-DCP was broken into linear chain organics and eventually turned into CO₂ and H₂O through reductive dechlorination and reaction with hydroxyl radicals.Meanwhile,excellent synergies between AgNPs and P.chrysosporium in 2,4-DCP degradation were exhibited,and total Ag removal was also at high levels and highly pH-dependent.?2?Significant inhibition was highlighted on 2,4-DCP biodegradation and Ag removal upon treatment with AgNPs at high doses and AgNO₃ at low-level exposure.?3?AgNPs-induced cytotoxicity could arise from the?Trojan-horse?mechanism executing particle effects,ion effects,or both,ruling out extracellularly released Ag⁺.?4?FTIR analysis showed that amino,carboxyl,carbonyl,and sulfurcontaining functional groups played crucial roles in Ag transportation and the reduction of Ag⁺to Ag⁰.The second section focuses on the exogenous Ag⁺effects on AgNP toxicity by determining the variation tendency of extracellular secretions of P.chrysosporium under Ag⁺and AgNP stress.The results showed that?1?oxalate production was elicited with moderate concentrations of 2,4-DCP and AgNPs reaching a plateau at 10mg/L and 10?M,respectively.Increased oxalate accumulation was accompanied by higher activities of manganese peroxidase?MnP?and lignin peroxidase?LiP?.However,the secretion of oxalate,MnP and LiP was significantly inhibited owing to Ag⁺incorporation into AgNP solution.?2?Production of extracellular polymeric substances?EPS?significantly elevated with an increase in 2,4-DCP concentrations;however,after 24 h of exposure to 100 mg/L 2,4-DCP,an obvious decrease in EPS occurred,indicating that part of EPS could be consumed as carbon and energy sources to ameliorate biological tolerance to toxic stress.?3?AgNP-induced?Particle-specific?cytotoxicity was substantially enhanced with additional Ag⁺as evidenced by its significant negative impact on cellular growth,plasma membrane integrity,and morphological preservation.The third section describes the antioxidative responses of P.chrysosporium against AgNPs under stress of exogenous Ag⁺.The results indicated that?1?malondialdehyde content was elevated by 2,4-DCP,AgNPs,and/or Ag⁺in concentration-and time-dependent manners within 24 h,indicating an increase in lipid peroxidation.However,beyond 48 h of exposure,lipid peroxidation was alleviated by upregulation of intracellular protein production and enhancement in the activities of superoxide dismutase?SOD?,catalase?CAT?,and peroxidase?POD?.Comparatively,POD played more major roles in cell protection against oxidative damage.?2?The dynamic change in reactive oxygen species?ROS?level was parallel to that of oxidized glutathione?GSSG?,and ROS levels correlated well with GSSG contents?R²=0.953?after exposure to AgNPs for 24 h.This finding suggested that elimination of oxidative stress resulted in depletion of reduced glutathione.?3?AgNP-induced cytotoxicity could originate from the original AgNPs,rather than dissolved Ag⁺or the biosynthesized AgNPs.The fourth part examines cysteine?Cys?on toxicity of AgNPs and Ag⁺and bioremediation capability of P.chrysosporium.The results showed that?1?Total Ag removal,2,4-DCP degradation,extracellular protein secretion,and cellular viability were enhanced to some extent after supplement of various concentrations of cysteine under stress of AgNPs and Ag⁺.?2?An obvious decrease in total Ag uptake was observed after 5-50?M cysteine addition in the groups treated with 10?M AgNPs and 1?M Ag⁺,especially at a Cys:Ag molar ratio of 5.More stabilization in uptake pattern at this ratio was detected under Ag⁺exposure than that under AgNP exposure.?3?In the absence of cysteine,all Ag⁺treatments stimulated the generation of ROS more significantly than high-dose AgNPs did.However,cysteine supply under AgNP/Ag⁺stress aggravated ROS levels,albeit alleviated at 100?M Ag⁺,indicating that the toxicity profiles of AgNPs and Ag⁺to P.chrysosporium could be exacerbated or marginally mitigated by cysteine.The results obtained were possibly associated with the lability and bioavailability of AgNP/Ag⁺-cysteine complexes.In the fifth part,the way to mitigate oxidative stress in P.chrysosporium caused by pollutants was pursued,thus ameliorating the physiological state of P.chrysosporium and enhancing the wastewater treatment efficiency.P.chrysosporium was pretreated with sodium hydrosulfide?NaHS,a hydrogen sulfide donor?.After pretreatment of NaHS,removal efficiency of AgNPs and heavy metals by P.chrysosporium was investigated.The results demonstrated that an enhancement in Pb????removal with an increase in NaHS concentration was observed,and the maximum removal efficiencies increased by 31%and 17%under 100 and 200 mg/L Pb????exposure,respectively,in the presence of 500?M NaHS.Increase in total Ag uptake and cell survival was also elicited by NaHS in a concentration-dependent manner under AgNP stress.Activities of SOD and CAT were significantly enhanced with the introduction of NaHS under stresses of Pb?II?,Cd?II?,Cu?II?,Zn?II?,Ni?II?,and AgNPs,while the inhibition in lipid peroxidation and oxidative stress caused by these toxicants was observed following NaHS pretreatment.Hydrogen sulfide played roles in the process,and can alleviate heavy metals and AgNP-induced toxicity to P.chrysosporium,improving the removal efficiency of these toxicants from wastewater.The study clarified the synergistic effect of AgNPs on the degradation of2,4-DCP by P.chrysosporium,and impacts of exogenous Ag⁺and cysteine on AgNP toxicity;revealed the interaction mechanisms between P.chrysosporium and toxicants;and explored an effective method to mitigate oxidative stress elicited by toxicants in P.chrysosporium,further enhancing the efficiency of wastewater treatment.Related research results contribute to deeply understand toxicity mechanisms of AgNPs,and provide theoretical basis and technical support for the application of AgNPs in wastewater treatment.