Photocatalytic Methane Production From Carbon Dioxide By A Newly Constructed Rhodopseudomonas Palustris/CdS Biohybrid System

In recent years,with the rapid development of industrial production and the large-scale use of fossil fuels,carbon dioxide has been a large number of emissions,leading to the increasingly serious problem of climate change.Therefore,how to reduce carbon dioxide emissions has become an urgent problem in the world.Biomethanation of carbon dioxide has attracted much attention because it can convert carbon dioxide into methane under environmental conditions.In particular,the genetic engineering bacteria of Rhodopseudomonas palustris can use solar energy to achieve one-step reduction of carbon dioxide to produce methane,which has great attraction for carbon dioxide fixation and solar energy storage.However,due to the poor response of Rhodopseudomonas palustris to visible light,Rhodopseudomonas palustris has low utilization of visible light.In order to solve this problem,Cd S quantum dots with good visible light response were prepared,and Rhodopseudomonas palustris /Cd S hybrid system was constructed.Cd S quantum dots can be used to enhance the absorption of visible light energy by Rhodopseudomonas palustris,and further improve its methane production efficiency under visible light conditions.The main results are as follows Two amino acid residues in the nitrogenase active center of Rhodopseudomonas palustris were replaced by site directed mutagenesis.It is proved that Rhodopseudomonas palustris with mutant nitrogenase can reduce carbon dioxide to produce methane by light energy.The main results are as follows:(1)The absorption range of visible light and wavelength utilization range of Rhodopseudomonas palustris were enhanced by Cd S quantum dots because of its wide absorption range and good absorption performance.The methane production of the hybrid system after photocatalytic 24 h was 1.7 times of that of pure bacteria.(2)The hybrid system of Rhodopseudomonas palustris / Cd S was constructed to further improve the methanogenic capacity of Rhodopseudomonas palustris.Cd S quantum dots and Rhodopseudomonas palustris were combined on the cell surface by electrostatic adsorption to form a hybrid system.X-ray diffraction(XRD),Raman spectroscopy(Raman),energy dispersive X-ray spectroscopy(EDS)and atomic absorption spectrometry(AAS)were used to characterize the structure and composition of the hybrid system.In addition,the morphology of the hybrid system was observed by scanning electron microscope(SEM)and transmission electron microscope(TEM).It was further confirmed that the binding site of Cd S QDs and Rhodopseudomonas palustris was on the cell surface.The activity of Rhodopseudomonas palustris in the hybrid system was analyzed by fluorescence microscope,fluorescence spectrophotometer and CFU counting.The results showed that the binding quantum dots had no significant effect on the activity of cells.Compared with pure bacteria,the survival rate of cells in the hybrid system was 98.9± 0.5%.(3)The effects of OD660,substrate concentration,L-Cys concentration and light intensity on the methane production of the hybrid system were investigated.Based on the effects of different parameters,the methane production from RP/Cd S hybrid cells was tested under the optimum conditions(OD660 = 0.1,sodium bicarbonate = 2 m M,L-Cys = 14 m M,light intensity = 5000 lx).It was found that the methane production steadily increased upon inoculation and reached the highest production of 171 ± 10nmol/mg total protein.In conclusion,in order to solve the problem of low visible light utilization efficiency of Rhodopseudomonas palustris,a hybrid system of Rhodopseudomonas palustris / Cd S was constructed by combining visible light responsive Cd S nanomaterials with cells.The photocatalytic performance of the hybrid system was analyzed.It was found that the hybrid system could greatly improve the methane production efficiency of Rhodopseudomonas palustris under visible light.