Isolation And Screening Of Cellulose-degrading Bacteria For Crop Straws

Crop straw is the main by-product of food grain production, and is an important biological resource in agriculture. China is a large agricultural country, therefore, crop straw is a abound resource in our country. Crop straw contains abundant cellulose, and cellulose is one type of biodegradable, renewable energy sources and is plentiful in the world. Consequently, the research on biodegradation of cellulose is becoming the hot spot in the field of utilization of agricultural organic wastes nowadays. In recent years, our research group has developed a new strategy to increase CO2 concentration in the greenhouse utilizing crop straw and animal manure composting. However, insufficient gas production in the middle or later fermentation process and short gas releasing-time has emerged as the primary problem limiting the development of this technique. For the purpose of solving the above problem, our research aimed at isolation and screening of high effective cellulose-degrading strains. The results obtained from the experiments are as follows:(1) In the preliminary experiment,26 strains that have efficient cellulose degradation ability were isolated and purified from fermentation residues resulting from the composting in greenhouse for CO2 fertilization, as well as the decayed wood. And 3 strains with strong degradability of cellulose and strong cellulase-producing capability, named B1, F5 and A5, were screened from the previously obtained strains using Congo red differential medium and liquid culture of filter paper method. And a series of comparative studies were conducted between the selected 3 strains and the previously used degrading strain in the composting system for CO2 enrichment.(2) After 14 d culture of the selected 3 strains in peptone culture solution of cellulose (PCS) at 60℃, it is discovered that the pH values of culture medium were reduced to the minimum on the 5th day, ranging from 7.21 to 7.31. In the second week, the pH values of culture medium began to increase, reaching to 8.40 at last. During the culturing process, the activities of carboxymethyl cellulose (CMC) enzyme and filter paper enzyme increased with pH decreasing. The maximum enzyme activity was observed on the 5th day when the pH value was reduced to the minimum, and then it began to decline. And it is discovered that the carboxymethyl cellulose (CMC) enzyme activity and filter paper enzyme activity of F5 were highest in the process of filter paper degradation test.(3) Using rice straw, wheat straw and corn straw as experimental materials, the degradation capabilities of B1, F5 and A5 for cellulose were studied, comparing with the control strain. The results showed that:1) the pH values of culture medium were reduced to the minimum on the 5th day during the degradation test of rice straw and corn straw, while the minimum pH value was observed on the 7th day for wheat straw. And in the series of three experiments, it is found that the activity of carboxymethyl cellulose (CMC) enzyme was the highest when the pH value was reduced to the minimum. The reason for the difference between rice straw, corn straw and wheat straw may be the relatively high contents of cellulose and lignin in wheat straw, which resulted in slower degradation rate. The cumulative degradation rates of cellulose and lignin suggested that the degradation capacities of the selected 3 strains on different kinds of straws are different, following the order of rice straw> wheat straw> corn straw. The carboxymethyl cellulose (CMC) enzyme activities and degradation rates of B1, F5 and A5 were significantly higher than that of the control strain, indicating that the screened 3 strains had better degradation capability for cellulose.2) On the 5th day of the degradation test for rice straw, the activities of carboxymethyl cellulose (CMC) enzyme of the four strains (B1, F5, A5 and CK) reached to the highest levels of 28.64 U/ml,31.05 U/ml,27.85 U/ml and 26.17 U/ml, respectively. While in the degradation test for wheat straw, the highest activities of carboxymethyl cellulose (CMC) enzyme appeared on the 7th day, which were 25.28 U/ml,30.54 U/ml,27.38 U/ml and 21.73 U/ml, respectively. And during the degradation test of the corn straw, the activities of carboxymethyl cellulose (CMC) enzyme of the four strains reached to the highest levels of 28.23 U/ml,30.30 U/ml,26.63 U/ml and 25.24 U/ml on the 5th day.(4) With the peptone culture solution of cellulose (PCS), the carboxymethyl cellulose (CMC) enzyme activities and degradation rates of the four strains (B1, F5, A5 and CK) were studied under various conditions, including different carbon source, carbon source concentration, culture temperature and initial pH. The results showed that:1) the carboxymethyl cellulose (CMC) enzyme activities and degradation rates of the four strains were higher when cellulose was used as carbon source, comparing with lignin.2) For the study on different carbon source concentrations, incubation temperature and initial pH, it is discovered that the carboxymethyl cellulose (CMC) enzyme activities and degradation rates of the four strains reached the highest under the following optimum conditions:filter paper concentration 0.50%, incubation temperature 60℃and initial pH 7 or 8. And the results showed that the screened 3 strains (B1, F5 and A5) had greater abilities of acclimation to high temperature and various acidic-alkalinic conditions than the control strain.