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The “quality” and “quantity” of microbial species drive the degradation of cellulose during composting

https://doi.org/10.1016/j.biortech.2020.124425Get rights and content

Highlights

  • The degradation degree of cellulose could be explained by cellulases activities.

  • The cellulose degradation needed the cooperation of various microorganisms.

  • The microbial evenness and richness were found to be the primary driving factors.

  • The richness symbolized the “quality” of microbial species.

  • The evenness symbolized the “quantity” of microbial species.

Abstract

The aim of this study was to explore the contribution of microbial community to cellulose degradation during cellulosic wastes composting. Three raw materials with different cellulose content were employed, including rice straws (RS), leaves (L) and mushroom dregs (MD). The cellulose degraded by 92.09%, 56.68% and 40.03% during RS, L and MD composting, respectively, which could be explained by cellulases activity. Besides, each cellulase were only linked to a specific group of bacteria, thus cellulose degradation needed the cooperation of various microorganisms. Ultimately, structural equation models verified that the richness and evenness of microbial community were the primary driving factors of cellulose degradation. The richness symbolized microbial functionality, which was equivalent to the “quality” of microbial species. The evenness symbolized the scope of function, which was equivalent to the “quantity”. Therefore, the “quality” and “quantity” of microbial species drove cellulose degradation during RS, L and MD composting.

Introduction

The biomass of plant fiber can reach billions of tons every year, of which lignocellulose accounts for 90%. Lignocellulose contains cellulose (35%–50%), hemicellulose (20%–30%) and lignin (20%–30%). Cellulose is the main component of plant cell and the most abundant organic matters (Rebière et al., 2017), but it is wrapped in lignin, which is one of the most difficult substances in natural products. The unreasonable utilization will cause resource loss and place a burden on environmental affordability. Therefore, its effective utilization has also become a long-standing problem. Composting is considered as an effective way to treat these cellulosic wastes, so as to make them resourceful and harmless (Liu et al., 2020, He et al., 2019, Zhang et al., 2021). In the process of cellulose biodegradation, cellulases produced by microorganisms play the main roles during composting. The cellulases can be roughly classified into endo-β-1,4-glucanase, exo-β-1,4-glucanase and β-1,4-glucosidase (Bayitse et al., 2015). The endo-β-1,4-glucanase can attack the middle part and cut into shorter chain at will (Rahman et al., 2014). However, exo-β-1,4-glucanase can only cut off one cellobiose from the non-reducing terminals (Abe et al., 2017). The β-glucosidase can catalyze the decomposition of low molecular dextran into glucose (Ahmed et al., 2015). Finally, cellulose is degraded into glucose under the action of various cellulases. The biodegradation process is a complex enzymatic reaction; thus, it also needs the cooperation of many microorganisms (An et al., 2020, Zang et al., 2020, Zhao et al., 2019). Many studies have shown that exogenous microorganisms could accelerate the degradation of cellulose during composting. On the one hand, the addition of exogenous microorganisms changed microbial community (Hu et al., 2019), on the other hand, it promoted enzyme activity (Zhang et al., 2019). Meanwhile, cellulose degradation could be effectively improved by regulating the external environmental conditions during composting (Wu et al., 2020). However, the contributions of microbial richness and evenness to cellulose degradation are rarely studied during composting. The microbial community composition and structure directly affect microecological function and determine composting quality (Pan et al., 2021). Therefore, the contributions of microbial community to cellulose degradation are worth further analysis. A better understanding of cellulose degradation characteristics will help to find effective ways to promote cellulose degradation.

In this study, rice straws (RS), leaves (L) and mushroom dregs (MD) were used to represent the wastes with different cellulose content. The aims of this study were: (1) to investigate the relationships of cellulose content, related enzymes activity and microbial species during different cellulosic wastes composting; and (2) to reveal the contributions of the richness and evenness of microbial community to cellulose degradation. This paper provides theoretical supports for accelerating cellulose degradation and composting process during composting.

Section snippets

Composting materials and experimental design

The RS, L and MD were acquired from Xiangfang farm, Northeast Agricultural University and surrounding village, Harbin, China, respectively. The properties of raw materials are shown in Supplementary Information. The C/N ratio was adjusted to 20–25 with urea, and moisture content was adjusted to about 65%. The composting device was a self-made reaction tank with a usable volume of 10 L in the laboratory. Each process had three parallel repeats. The detailed composting process and microbial

The changes of cellulose content and cellulases activity

The initial cellulose content was quite different, and the order was RS > MD > L (Fig. 1a). The cellulose content presented a decrease trend during composting. The cellulose degraded by 92.09% during RS composting, while that of MD composting only degraded by 40.03%. Although the initial cellulose content of L was the least, the content was reduced by 56.68%. Therefore, a greater ability to cellulose degradation was found during RS and L composting compared with MD composting. The FPase

Conclusion

The cellulose degraded well during RS composting, while cellulose degradation of MD composting was poor, which could be explained by cellulases activities. The evenness and richness of microbial community were different obviously during composting. Meanwhile, the evenness and richness were found to be significantly related to cellulose content. Ultimately, SEMs verified that the richness and evenness were important driving factors of cellulose degradation. The richness symbolized the “quality”

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work is funded by the National Natural Science Foundation of China (grant numbers: 51978131 and 51878132), National Key Research and Development Project (No. 2019YFC1906400) and National Natural Science Foundation of China (grant numbers: 51778116).

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