Effects And Mechanism Of Substrate Concentrations On Tetracycline Resistance Genes During Anaerobic Digestion Of Dairy Manure

Tetracycline resistance genes(TRGs)were detected in quantity in manures,which pose a threat to the environment or human health.Anaerobic digestion,as one of the treatment methods for resource utilization of feces,can also effectively reduce TRGs in manure.The intervention of wastewater in the process of manure may cause different substrate concentrations.To better understand the changes of TRGs in anaerobic digestion of different substrate concentrations,we conducted a substrate concentrations of dairy manure control experiment(100%,75%,50%,and 25%)and the dairy manure was used as the substrate.Analyzed the effects of substrate concentration on digestive performance,TRGs and changes in microbial communities at three temperature(35℃,55℃ and 65℃).Explored the main factors affecting the changes in TRGs abundance,and revealed the microbial mechanism of changes in their abundance.The main results and conclusions of the study are as follows:(1)As the substrate concentrations decreases,the total biogas production decreased by 0.94%～5.85% at 35℃ and increased by3.79%～11.72% and 1.55%～24.55% at 55℃ and 65℃.The total biogas production was also affected by temperature,and the total biogas production was the largest at 55℃,which was 5.50%～24.5% higher than35℃ and 65℃.The cumulative amount of total ammonia nitrogen(TAN)was the largest at 25% substrate concentration,reaching102.23%～304.83%.The concentration of soluble chemical oxygen demand(SCOD)and volatile acids(VFAs)were not significant with the substrate concentration.There was a little changes of pH between 6.8～8.3under different substrate concentrations.(2)The substrate concentrations significantly affected the changes in the abundance of TRGs.The total abundance of TRGs all increase and first increases and then decreases with the decrease of the substrate concentrations at 35℃.At the substrate concentration with 25%,the increase is the least,which is 1.24 logs.At 55℃,the trend of the total abundance was same with the changes at 35℃,and at the substrate concentration with 25%,TRGs increased by 1.26 logs.At 35℃ and 55℃,the increase in total abundance was related to the increase in tetW,which with the highest abundance.At 65℃,the total abundance first decreases and then increases with the decrease of the substrate concentration.At the substrate concentration with 50%,the total abundance decreases by 0.54 log,and the abundances of tetQ,tetT and tetW all decrease.(3)As the substrate concentrations decreases,the first dominant bacteria phyla changed from Firmicutes to Proteobacteria at 35℃ and55℃,but it was stil Firmicutes at 65℃.Pearson correlation analysis between microbes and TRGs abundance showed that substrate concentration had an effect on the species and quantity of TRGs potential host bacteria.As the decrease of substrate concentration,the number of potential host bacteria of TRGs increased first and then decreased at 35℃,and reached 24 at the substrate concentration with 75%.There was no obvious change of potential host bacteria at 55℃,but the tetQ,tetT and tetX had more common potential host bacteria,including Oscillospira,Proteiniclasticum and vadin HB04.At 65℃,the number of potential host bacteria gradually increased,and the substrate concentration with 25%reached the maximum of 15.(4)The effects of physicochemical factor on TRGs potential host bacteria were analyzed by redundancy analysis.The redundancy analysis demonstrated that physical and chemical factors affected the changes of TRGs potential host bacteria to a certain extent,and the explanation rate was between 54.99% and 59.24%,of which SCOD and VFAs were the main influencing factors.The changes of SCOD and VFAs were the main influencing factors.The changes of SCOD and VFAs caused by different substrate concentrations affected the microbial community structure and indirectly led to the changed in the abundance of TRGs and MGEs.