In nature, degradation of lignocellulose is coordinated by various active enzymes secreted by various microorganisms. Pure-cultured single bacterial strains produce fewer enzymes types or unbalanced ratios of enzymes. Therefore, using single lignocellulose-degrading bacteria in combination with composite floraos an effective way to degrade lignocellulose18. Many studies have shown that by simulating the decomposition process of lignocellulose under natural conditions (i.e., taking the original environmental samples as the inoculum and adopting restrictive culture techniques), composite flora that can efficiently degrade filter paper, rice straw, and pulp waste can be identified19,20,21. However, there are limited reports on composite flora that can efficiently degrade lignocellulose under low temperature conditions. In this study, microbial consortium M44 was cultured at 15°C for 21 days, and the rate at which corn straw was degraded was found to be 35.90%.
The microbial consortium M44 is a complex microbial mix composed of aerobic bacteria, anaerobic bacteria, and strict anaerobic bacteria, and its microbiome structure changed considerably during degradation of straw. Through an analysis of alpha and beta diversity of complex microorganisms in different culture periods, we found that although the dominant microorganisms were similar in different culture periods, the overall community structure was still different, and the microbiome of the original sample was the richest, probably because the microbial composition of the original sample itself was rich. However, during the culture process, carbon and other nutrients were limited in the culture medium, and some bacteria that mainly used the straw degradation products did not grow and reproduce sufficiently. At the phylum level, the abundance of Proteobacteria and Firmicutes in the F11 generation was significantly higher than that in the F1 generation. These types of bacteria are common in rice straw compost22, decaying wood23, and rumen24, which suggests the importance of these types of bacteria for the degradation of lignocellulolytic materials. For example, among Firmicutes, most bacilli have been shown to degrade lignin. Bacillus atrophaeus and Bacillus pumilus isolated from tropical rainforest soil in Peru were shown to produce laccase and degrade Kraft lignin25. Among Proteobacteria, Sphingobium paucimobiliz SYK-6 and Pseudomonas putida are capable of degrading lignin monoaryls, biaryls, and phenolic intermediates using extracellular laccases and peroxidases26,27. Based on our genus-level analysis, in the process of subculturing, because the oxygen in the medium was sufficient at an initial stage of culture, aerobic microorganisms Pseudomonas, Devosia,and Azospirillum used the nutrients in the medium for their own growth, and oxygen was consumed rapidly, which created a good growth environment for facultative anaerobic and anaerobic bacteria such as Trichococcus, Acinetobacter, Rihizobium, and Dysgonomonas. In later stages of culture, their abundance increased, organic acids and some intermediate metabolites were consumed, and they became the main microorganisms degrading the lignocellulose. Across the subculture process, the species and abundance of microorganisms change with changes in the environment, so microbial consortium M44 is able to efficiently and stably degrade straw at a low temperature.
Acinetobacter, Azospirillum, Pseudomonas, Brevundimonas, Devosia, Achromobacter, and Chryseobacterium play major roles in this process. Acinetobacter is found in cellulose-containing agricultural waste as the only carbon source, and efficiently secretes extracellular cellulase and hemicellulose enzymes28,29; Azospirillum has been shown to produce hydrogen peroxide enzymes, oxidase, methyl cellulase, and produce acetic acid, butyric acid, and lactic acid, and to participate in straw degradation metabolism30,31; Dye-decolonizing peroxidases (DYPs) secreted by Pseudomonas have the ability to degrade lignin and lignin model compounds32,33 and have high laccase and lignin peroxidase activities34. They are believed to be important functional bacteria for degradation of straw lignin. In this study, lignin enzyme activity and VFA content of the microbial consortium fermentation broth at generation F5 were considerably increased, which may be the result of massive growth of the above bacteria. Studies have shown that Brevundimonas secretes oxidase and catalase to promote the decomposition of cellulose35; Devosia decomposes catalase and utilizes xylose, glyceraldehyde, cellulose, etc36; and Achromobacter can oxidize xylose, secrete oxidase and xylanase, which effectively degrade cellulose and hemicellulose37. These may be the basic functional bacteria for degrading straw cellulose. Our correlation analysis showed that the activities of endoglucanase and filter paper enzyme were significantly positively correlated with the abundance of Brevundimonas, Devosia, and Achromobacter. With subculture, their relative abundance decreased, resulting in a reduction in straw degradation by the microbial consortium. The degradation characteristics stabilized at the F11 generation and were not significantly different from those in the F8 generation. Chryseobacterium decomposes cellulase and protease, degrading cell walls, and can cooperate with Pseudomonas to degrade cellulose and hemicellulose38,39, which correlates with xylanase activity. This is speculated to be a functional bacterium for straw hemicellulose degradation. This microbial consortium is rich in microbial diversity that continuously changes through subculture. The microorganisms in M44 cooperate for efficient straw degradation.