Degradation Mechanism Of Biodegradable Plastics By Thermophilic Bacteria And Its Application In Aerobic Composting Of Kitchen Waste

Biodegradable plastics(BDPs),as a kind of degradable plastics,are widely used in various fields.Although aerobic composting is a general BDPs treatment,there is still a problem of low plastic degradation efficiency during the composting cycle.BDPs containing poly butylene adipate-co-terephthalate(PBAT),poly lactic acid(PLA)and starch(ST),which have the majority of the market share at present,were selected as the research object.It is of great significance to screen strains for their efficient degradation.Molecular biological technologies such as amplification sequencing and whole genome sequencing were used in combination with polymeric material detection methods such as Attenuated Total Reflection Flourier Transformed Infrared Spectroscopy(ATR-FTIR),X-ray Photoemission Spectroscopy(XPS)and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry(FT-ICR-MS)to study their efficient biodegradation performance and mechanism.Subsequently,screened strain was added to aerobic composting system of kitchen waste to verify its enhanced degradation effect of BDPs.The main research results are as follows:(1)Using BDPs as the only carbon source at 55℃,four strains of thermophilic bacteria with potential degradation ability were screened out,and their plastic degradation rate was more than 10% within 30 days.Bacillus thermoamylovorans,Aeribacillus pallidus,Bacillus licheniformis and Chelatococcus thermostellatus were identified by strain sequence alignment.The hydrophobicity(6.47±0.19%,67.82±2.30%,27.58±0.82% and 79.33±0.88%)and the content of cell membrane long chain saturated fatty acids(95.82%,97.00%,96.42% and 98.98%)of the 4 strains were significantly higher than those of the control group(p < 0.05).Meanwhile,their lipase activity(299.01±0.03 U/L,411.36±0.08 U/L,232.35±0.01 U/L and 351.17±0.04 U/L)and esterase activity(54.28±0.08 U/L,33.67±0.06 U/L,16.79±0.04 U/L and65.36±0.01 U/L)were also significantly higher than those of the control group(p <0.05).These characteristics made the thermophilic bacteria have stronger plastic colonization ability and the ability to break the ester bond of PBAT and PLA.(2)The degradation effect of BDPs by thermophilic bacteria in Luria-Bertani(complex carbon sources)and MSM medium(unique carbon source)showed that C.thermostellatus(LB: 59.61±1.84%/30 d)and A.pallidus(MSM: 21.03±0.9%/30 d)was significantly better than that of B.thermoamylovorans and B.licheniformis(11.25±2.44% ～ 13.87±2.61%).A.pallidus and C.thermostellatus had greater hydrophobicity,richer cell membrane content of long chain saturated fatty acids,and higher lipase and esterase activity.ATR-FTIR and XPS analysis showed that the peak intensity of-OH increased obviously and the peak intensity of C=O decreased significantly after treatment by A.pallidus and C.thermostellatus.Combined with FTICR-MS analysis,it was found that PLA and PBAT chains were broken to form oligomers such as BT,BAB,ABA,BTB,2(LA)and 5(LA),which indicated that the plastic ester bond was broken.Furthermore,whole-genome sequencing of A.pallidus and C.thermostellatus showed that both bacteria had metabolic pathways associated with BDPs degradation(e.g.,benzoate degradation(ko00362),butyrate degradation(ko00650),propionate degradation(ko00640),etc.)and encoded esterase and lipase genes with BDPs degradation potential(e.g.,carboxylesterase(K06999)),which verified its high efficiency for BDPs degradation.(3)The addition of thermophilic bacteria could significantly enhance the degradation of plastics in the actual aerobic composting system of kitchen waste.Specifically,compared with blank control group,molecular weight decreased significantly(weight-average molecular weight decreased by 64.43% ～70.2%,numberaverage molecular weight decreased by 14.93% ～ 29.04%),and hydrophilicity increased significantly(water contact Angle decreased by 26.7%)after treatment by thermophilic bacteria.Microbial community analysis showed that the addition of thermophilic bacteria increased the relative abundance of Comamonas,Bacillus and Thermobifida on the plastic surface.Notably,the relative abundance of Aeribacillus,Chelatococcus and Bacillus added on the plastic surface peaked in the high temperature period of compost.Therefore,it was suggested that the best adding time of thermophilic bacteria was the high temperature period of compost.In addition,the addition of thermophilic bacteria could prolong the high temperature stage of compost for 1.5 ～ 5days,and improve the rot degree and low germination rate of compost products caused by the presence of BDPs,which was conducive to the practical application of kitchen waste and plastic co-compost products.In conclusion,this study screened out thermophilic bacteria with high BDPs degradation ability,analyzed the mechanism of BDPs degradation by thermophilic bacteria,and verified the enhancement effect of thermophilic bacteria on plastic degradation in aerobic compost.These results provide a reference for biodegradation of BDPs in aerobic compost enhanced by thermophilic bacteria.