The Effect Of Different Volatile Fatty Acids And Light-oxygen Conditions On The Growth And Value-added Products Accumulation Of Purple Bacteria

Purple non-sulphur bacteria（PNSB）wastewater treatment technology has great potential in wastewater treatment and biological resource recovery.PNSB can effectively remove pollutants from wastewater,but also assimilate these pollutants into bacterial biomass to produce high value-added substances.PNSB cells have high content of protein,coenzyme Q10,carotenoid and polyhydroxyalkanoates（PHA）.PNSB cells are good substitutes for fertilizer,fish feed or feed additives due to its features of non-toxic and have high nutritional value.However,the development and application of PNSB resource recovery are restricted by the low biomass,coenzyme Q10 and protein yield in PNSB and high cost of cultivation.In this study,a strain of Rhodobacter sphaeroides isolated and purified by our lab was used as the experimental strain,and the medium based on volatile fatty acid（VFA）was used as the degradation substrate,which explored the effects of VFA（Acetate,propionate,butyrate and mixed VFAs）and light-oxygen conditions（light-anaerobic,natural light-microaerobic and dark-aerobic）on the growth of Rhodobacter sphaeroides,pollutants removal efficiency,and accumulation of protein and coenzyme Q10.The relevant metabolic mechanisms of PNSB wastewater treatment and resources recovery were revealed by VFA dynamics and key enzyme activities of metabolic pathways.The results are as follows:（1）PNSB can effectively remove pollutants from wastewater under different VFAs and light-oxygen conditions,with the removal rates of chemical oxygen demand（COD）,total nitrogen（TN）and total phosphorus（TP）are up to 85.5%,84.6%and 91.2%,respectively.However,the wastewater treatment efficiency varied under different growth conditions.Compared to butyrate,the group of mixed VFAs showed increased removal rates,with COD,TN and TP increasing by 14%～39%,20%～46%and 10%～15%,respectively.The pollutants removal rates under light-anaerobic condition were lower than that under aerobic condition.The appropriate amount of oxygen enhanced the oxidative phosphorylation pathway,increased the metabolic rate of Rhodobacter sphaeroides,and then promoted the degradation of organics in wastewater.（2）Further investigation on VFA dynamics indicated that acetate was the substrate with the fastest uptake rate(0.76 g COD g^-1dry cell weight（DCW）d^-1)in the individual VFA feeding.While propionate was most rapidly consumed by PNSB in the mixed VFAs feeding,up to 0.75 g COD g^-1DCW d^-1.The uptake rates of propionate and butyrate in mixed VFAs feeding were 2.7 times and 1.3 times higher than those in individual VFA feeding,respectively.Acetate as a co-substrate enhancing the utilization of propionate and butyrate through providing additional cell metabolism precursor acetyl-Co A.（3）The biomass yields and specific growth rates of PNSB were in the ranges of0.57～1.08 g biomass g^-1COD_removedand 0.48～0.71 d^-1,respectively.There was no significant effect between individual and mixed VFAs on biomass yield,while the specific growth rates of PNSB grown on acetate and mixed VFAs were 1.2～1.5 times higher than that of propionate and butyrate.（4）Coenzyme Q10 and protein content followed the same trend as biomass.Under natural light-microaerobic conditions,acetate or mixed VFAs as carbon source was beneficial to the accumulation of Co Q10 and protein,up to 13.21 mg g^-1DCW and 609 mg g^-1DCW,respectively.Proper growth conditions activated cell activity,increased the biomass,thus promoting the resources recovery and the accumulation of value-added products.（5）The activities of succinate dehydrogenase（SDH）and fructose-1,6-bisphosphatase（FBP）were highest under natural light-microaerobic conditions(2018.93 U g^-1DCW and 1789.81 U g^-1DCW).The tricarboxylic acid cycle of respiratory metabolism was more active at this time,compensating for the energy required for biosynthesis.Photo-pigment contents were highest under light-anaerobic conditions(16.41 mg g^-1DCW)affected by photosynthetic efficiency.The growth of PNSB and accumulation of intracellular value-added substances are related to their metabolic pathways.The changes of key enzyme activities were consistent with the changes of PNSB biomass and value-added products under different VFAs and light-oxygen conditions.VFA and light-oxygen conditions increased the activities of key enzymes in the energy metabolism and substance synthesis pathway of PNSB to promote pollutant removal,cell growth and accumulation of high-value substances.