| In the field of wastewater treatment, microbial technology attracts scientists’ attention for its fast reproduction, mild reaction conditions and high treatment efficiency. Nowadays, wastewater pollution is worsening and its components becoming more and more complex; exploring microorganisms which can both decompose organic pollutants and adsorb heavy metals in wastewater is of important guiding significance to improve the utilization of microorganisms and the efficiency of wastewater treatment. Heavy metals and organic pollutants in wastewater usually are high toxicity. They can inhibit the growth, reproduction and metabolic activity of microorganisms, thereby affect the treatment efficiency. Therefore, only by clarifying the internal mechanism of interaction between microorganisms and contaminants, can we improve the performance of microorganisms specifically, create conducive conditions for microbial treatment of wastewater and regulate wastewater treatment efficiency on macroscopical.White-rot fungi have been found to be efficient in wastewater treatment due to its unique degradation to xenobiotics and bioabsorption ability to heavy metals. In this study, white-rot fungus was applied to treatment cadmium and2,4-dichlorophenol (2,4-DCP) co-contaminated wastewater. The influences of factors such as pH, initial concentration and the dosage of biomass on the treatment efficiency were examined. The removal mechanism was also investigated. Systematic study focused on the proton/oxygen fluxes, extracellular enzymes, proteins and cell membrane responses of white-rot fungus under cadmium and2,4-DCP stress were conducted. Finally, we sought a way to improve the physiological state of white-rot fungi and improve its treatment efficiency. Specific research work and the results of this paper can be summarized in the following four sections:The first section describes the research on simultaneous cadmium removal and2,4-dichlorophenol degradation from aqueous solutions by white-rot fungi. Firstly, the white-rot fungus model species-Phanerochaete chrysosporium was inoculated to remove cadmium and2,4-DCP from wastewater. The effects of solution pH, initial concentration of cadmium and2,4-DCP, initial biomass on the removal efficiency were investigated. Variations of pH, extracellular proteins and enzyme activity in the system were monitored. The results can be concluded as follows:(1) the optimal pH for P. chrysosporium in the treatment was about6.5. Under this condition, removal rates of5mg/L cadmium and20mg/L2,4-DCP reached63.6%and98.2%, respectively.(2) Low concentrations of2,4-DCP (<20mg/L) can be used as a carbon and energy source by P. chrysosporium, which was beneficial for the removal of cadmium. The degradation rate of2,4-DCP, extracellular protein content and enzyme activity decreased with increasing in cadmium concentration.(3) Infrared spectroscopy showed amino, carboxyl and hydroxyl functional groups on the cell wall were responsible for cadmium adsorption.The second section focused on the response characteristics of proton flux, oxygen flux and cadmium ion flux in white-rot fungi under2,4-DCP and cadmium stress. Non-invasive techniques were used to measure the real-time and steady fluxes of particles in P. chrysosporium. The results showed that (1)2,4-DCP increased the protons and oxygen flux of P. chrysosporium.(2) Cadmium changed the proton influx of P. chrysosporium into efflux immediately, and decreased the oxygen flow rapidly.(3) A significant influx (-66±2pmol cm-2sec-1) was observed in P. chrysosporium under0.1mM cadmium exposure. The calcium channel inhibitor (GdCl3) reduced the cadmium ions influx by30%, indicating that calcium channels were involved in the cadmium uptake by P. chrysosporium. The results also suggested that the intracellular uptake of cadmium possess at least30%of the total cadmium removal.The third section describes the plasma membrane behavior, oxidative stress and antioxidant detoxification mechanism of P. chrysosporium under cadmium stress.(1) Study on the response mechanism of P. chrysosporium plasma membrane to cadmium stress showed that cadmium reduced the plasma membrane H+-ATPase activity and membrane fluidity. The permeability of mitochondrial membrane increased, resulting in the decrease of mitochondrial membrane potential and release of cytochrome c, inducing apoptosis.(2) The pattern of cell oxidative damage in P. chrysosporium caused by cadmium was explored. It was found that oxidative stress induced by cadmium is implicated by two effects:concentration-course effects and time-course effects. Low concentrations or short-term exposure increased intracellular ROS levels, resulting in plasma membrane oxidation and membrane damage. Under high concentrations or prolonged exposure, the enhanced ROS and malondialdehyde (MDA) level were not observed. Cell viability decreased with the increase of cadmium concentration. Vitamin C alleviated the cadmium-induced oxidative damage to some extent.(3) The antioxidant system facing cadmium toxicity was monitored. The results showed that superoxide dismutase (SOD) activity was well correlated with MDA level. Catalase and peroxidase were stimulated only when the concentration of cadmium was1μM. Low concentrations of cadmium also stimulated reduced glutathione which can eliminate intracellular H2O2. When concentration of cadmium was high, oxidized glutathione levels increased significantly resulting in the decrease of GSH/GSSG.(4) Scanning electron microscopy-energy spectrum and X-ray diffraction were conducted to analysis the fungi treated with cadmium. Extracellular synthesis of cadmium crystal particles was observed after exposure to dissolved cadmiumion, which is probably another detoxification mechanism in which the dissolved metal is precipitated, thus reducing its bioavailability and toxicity.In the fourth section, we focused on seeking way to alleviated oxidative stress in white-rot fungi caused by pollutants, for the purpose to improve the physiological state of white-rot fungi and wastewater treatment efficiency. P. chrysosporium was pretreated with a hydrogen sulfide donor-sodium hydrosulfide. Degradation efficiency of2,4-DCP by treated P. chrysosporium was investigated. The results showed that sodium hydrosulfide pretreatment not only improved the2,4-DCP degradation rate, but also greatly reduced the time required for2,4-DCP completely degradation. Hydrogen sulfide played the improvement role in degradation process. The SOD enzyme activity increased for three times after H2S treatment, which indicated that H2S alleviated the2,4-DCP caused oxidative stress by increasing activities of ROS scavenging enzymes, and played an important role in cell protection, and further accelerating2,4-DCP degradation.The paper clarified the feasibility of using white-rot fungi in the treatment of heavy metal-organic compound co-contaminated wastewater, revealed the interaction mechanism of white-rot fungi with environmental pollutants and showed reason of the low efficiency when dealing with toxic environmental pollutants to a certain extent. An effective method to alleviate oxidative stress caused by toxic environmental pollutants was found for improving the efficiency of wastewater treatment. This study provided basis and reference for reasonable development of related research work in regulating wastewater treatment efficiency. |
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