Enrichment of thermophilic and mesophilic microbial consortia for efficient degradation of corn stalk

https://doi.org/10.1016/j.jes.2018.07.010Get rights and content

Abstract

Six different environmental samples were applied to enrich microbial consortia for efficient degradation of corn stalk, under the thermophilic and mesophilic conditions. The consortium obtained from anaerobic digested sludge under thermophilic condition (TC-Y) had the highest lignocellulose-degrading activity. The CO2 yield was 246.73 mL/g VS in 23 days, meanwhile, the maximum CO2 production rate was 15.48 mL/(CO2·d), which was 28.75% and 52.27% higher than that under mesophilic condition, respectively. The peak value of cellulase activity reached 0.105 U/mL, which was at least 34.61% higher than the other groups. In addition, 49.5% of corn stalk was degraded in 20 days, moreover, the degradation ratio of cellulose, hemicellulose and lignin can reach 52.76%, 62.45% and 42.23%, respectively. Microbial consortium structure analysis indicated that the TC-Y contained the phylum of Gemmatimonadetes, Acidobacteria, Chloroflexi, Planctomycetes, Firmicutes, and Proteobacteria. Furthermore, the Pseudoxanthomonas belonging to GammaProteobacteria might be the key bacterial group for the lignocellulose degradation. These results indicated the capability of degrading un-pretreated corn stalk and the potential for further investigation and application of TC-Y.

Introduction

Lignocellulosic biomass is the most abundant source of renewable organic compound, which mainly consists of cellulose, hemicellulose and lignin. More than 600 million tons of crop lignocellulosic biomass, such as rice straws, wheat straws, and corn stalks, are produced yearly in China (Guo et al., 2010). However, the complex composition and structure of natural lignocellulosic made it difficult to be utilized (Hamid et al., 2015).

Microbes play a critical role in the utilization of lignocellulosic biomass. The microbiological treatment and enzyme treatment are green in environment and efficient in degradation (Bilal et al., 2017c). However, the lignin can prevent the contact between enzymes and cellulose or hemicellulose, therefore, the degradation of lignin is generally regarded as rate-limiting step (Asgher et al., 2016). Recent studies have demonstrated that the degradation of lignin could be enhanced under the aerobic condition (Phongpreecha et al., 2017). As a consequence, the aerobic condition revealed higher degradation efficiency for lignocellulosic materials. Moreover, increasing studies have been focused on the biodegradation of lignocellulosic biomass by microbial co-cultures or consortia during recent years (Maki et al., 2009, Wongwilaiwalin et al., 2010, Wushke et al., 2015). For example, Wang et al. (2011) established a lignocellulosic microbial consortium from the compost by successive enrichment cultures, which could degrade lignocellulosic biomass efficiently in a short period of time. Besides, Guo et al. (2010) obtained an efficient lignocellulose-degrading composite microbial system XDC-2 from the compost in 2010, and their further study indicated that XDC-2 could degrade 17.6% of un-pretreated corn stalk, meanwhile, degradation ratios of cellulose, hemicellulose and lignin were 10.4%, 16.5% and 9.6% respectively (Wang et al., 2013a). Furthermore, Wang et al. (2016) enriched a microbial consortium from compost, which could degrade 31.5% of rice straw in 30 days. However, most of the consortia were enriched from compost, which might limit the abundances and diversities of the microbial consortia. In addition, the degradation efficiencies of previously described microbial consortia were unsatisfactory when the substrates were un-pretreated. Especially, the degradation ratio of lignin is required to be increased.

In the present study, a novel microbial consortium thermophilic condition (TC-Y) that can efficiently degrade un-pretreated corn stalk was enriched. The microbial consortium revealed high degradation ratio for the lignin. In order to ensure the diversity of the consortia, a variety of samples were collected from soil and sludge. Additionally, TC and MC (mesophilic condition) were adopted to achieve the optimal conditions for enrichment. The structures of microbial consortia were also analyzed to reveal the changes of the microbial consortia in the process of acclimatization. Meanwhile, the underlying reasons for the differences of corn stalk degradation performance between TC and MC were figured out by the microbial consortium structure analysis.

Section snippets

Substrate and inoculums

Corn stalk obtained from corn field of Pingdu (Shandong Province, China) was used as a substrate. The total solid (TS) and volatile solid (VS) of corn stalk were (88.00 ± 0.50)% and (82.27 ± 0.38)% (based on TS), which were determined according to standard methods (APHA, 2006). The proportions of hemicellulose, cellulose and lignin are 28.40%, 41.90% and 14.80%,respectively (based on TS). The corn stalk was chopped and sieved between 10 to 40 meshes and dried at 105°C before usage.

Six types of

CO2 productions of TC and MC groups

The mechanism of lignocellulose degradation indicated that the lignocellulose can be hydrolyzed to small molecule compounds. Among which the lignin can be metabolized into vanillic acid by variety of pathways. Afterwards, the vanillic acid was transformed into pyruvic acid and α-ketobutyric acid. These products then participate in the TCA cycle to produce CO2 in the aerobic condition (Bugg et al., 2011). The cellulose and the hemicellulose can be depolymerized to cellobiose and xylobiose.

Conclusion

Several microbial consortia were enriched from six inoculums under the different conditions. The TC showed higher lignocellulose-degrading activity for un-pretreated corn stalk than the MC. The TC-Y enriched from the anaerobic activated sludge had the highest lignocellulose-degrading activity. The phylum Proteobacteria (especially class Gammaproteobacteria, genus Pseudoxanthomonas) had the highest abundance in TC-Y, which enables higher CO2 production, cellulase activity, weight loss and

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 41773102), the Key Research & Development Project of Shandong (No. 2017GSF217007), and the Key Technological Innovation Project of Shandong (No. 2017CXGC0305).

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