Enhanced ethanol production from tree trimmings via microbial consortium pretreatment with selective degradation of lignin
Graphical abstract
Introduction
The developing world's energy demand mostly depends on fossil fuels. However, fossil fuels are well known for being nonrenewable, source-limited and environmental-polluted, which can pose risk to human beings and can't satisfy their tremendous demand of chemicals for long [1,2]. Hence, more and more researchers have focused on looking for a new cleaner, renewable and sustainable energy resource [3,4].
Lignocellulosic biomass obtained from living organisms such as plants, animals, and microorganisms is the most abundant organic material with the production of 15-17 × 1010 million tons annually, evenly distributed throughout the world [5,6]. Tree trimmings, as a principal constituent of lignocellulosic biomass, have imposed an increasing pressure on environment during the rapid urbanization, especially when landscaping has gained more and more favors in modern cities. On the other hand, bioethanol is a renewable and sustainable liquid fuel that is expected to have a promising future in tackling today's global energy crisis and the worsening environment. Lignocellulosic biomass based ethanol is one of the most suitable alternative fuels due to its cost-effectiveness and eco-friendliness [7,8]. Tree trimmings, mainly composed of cellulose, hemicellulose, phenolaldehyde polymer lignin, are an abundant feedstock that could be used for bioethanol production.
However, the structure of lignocellulose in tree trimmings is recalcitrant. Before the lignocellulose is subject to enzymatic hydrolysis to produce sugars, and subsequent fermentation to produce second-generation bioethanol, it needs to undergo pretreatments to decrease the natural recalcitrance [9]. However, to date, the conversion technologies of lignocellulose are still costly and ineffective [[10], [11], [12]]. Aggressive pretreatments functioned as deformation of the rigid components which are structured of lignin, cellulose and hemicellulose, are important for the efficient bioconversion of tree trimmings due to the degradation of the crystallinity and the enhancement of bioaccessibility [13,14].
Varied effects on the structure of lignocellulose were found via different pretreatments. In general, physical pretreatment increases the surface areas of materials by reducing the biomass size and breaking the crystal structure. Chemical pretreatment disrupts the linkages between hemicelluloses and lignin, unraveling the lignocellulosic matrix. Physico-chemical pretreatment combines the physical and chemical effects on lignocellulosic feedstocks [15]. These methods could lead to an increased number of undesirable compounds when they enhanced the solubilization of the lignocellulosic biomass with high operational costs [[16], [17], [18]]. In terms of biological pretreatment, it serves as an attractive option that is both energy-saving and environmentally friendly even though it is restricted by the living conditions and varying substrates for microbes [19,20]. Fungi, especially a majority of white-rot species (e.g. Phanerochaete chrysosporium), have been investigated for several decades in lignocellulose degradation even though the efficiency has been limited by the long cultivation period of fungi and their varied sensitivities to the growing conditions [21,22]. Whereas there is no option of a microbial consortium that is merely capable of lignin degradation or performs effectively in selective lignin degradation so far. Moreover, the capability of lignin degradation by a single bacterium has been reported to be much weaker than using a microbial consortium and it was easily contaminated [23], and a long period of lignocellulose pretreatment was always required [24]. Herein, the microbial consortia with efficient capability of selective lignin degradation had been designedly screened in our lab by using liquid culturing mediums [25], featured by a low function in cellulose degradation which could significantly promote the cellulose recycling in lignocellulose and benefit the subsequent production of bioethanol.
Since pretreatment of biomass in a solid state has been known for being more efficient, space-saving and convenient for further use, the microbial consortia with a capability of selectively degrading lignin screened in our lab by using liquid culturing mediums were applied to pretreat solid tree trimmings in this work, and two strains of commercial fungal isolates which have been both reported on effectively degrading lignin, were taken in comparison. The changes of lignocellulose in tree trimmings via different microbial pretreatments were investigated, revealing the selectivity of lignin degradation at the solid-state pretreatment stage. And the pretreated tree trimmings were offered in the subsequent SSF process for the bioethanol production, elucidating the microbial pretreatment effects on the subsequent bioethanol yields, and revealing the bioconversion efficiency of lignocellulose in SSF as well.
Section snippets
Experimental materials
Tree trimmings were collected from the campus of South China Agricultural University, grounded and screened with a 0.5 cm mesh sieve, and subsequently dried in an oven (Keelrein DGG-9140A, Shanghai, China) at 60 °C for 48 h for further use. The groups of microbial consortia named DM (decomposed mixture) -1and DT (decomposed trunk) −1 were obtained in our lab, and both were screened and isolated after generations of subculture with samples source from rotten trunks, rotten stumps and its
Variations on the properties of substances
As shown in Table 1, the lignin content of raw tree trimmings (RTT) in this work was over 35%, which represents one of the typical green wastes in South China. Owing to the recalcitrant structure of lignocellulose in tree trimmings, in which lignin and cellulose are conjugated in a mosaic structure via various bonds including ether bond and ester bond [32,33], getting rid of the obstruction of lignin has become the point to promote utilization of cellulose in tree trimmings.
Being pretreated by
Conclusions
Tree trimmings pretreated via the microbial consortium DM-1 in a soild state were noted with the highest selectivity of lignin degradation compared to the pretreatment with two commercial fungal isolates and another microbial consortium DT-1, which also displayed the most stable pH value of around 7.0 during pretreatment and a lower fiber crystallinity in lignocellulose after pretreatment. The subsequent SSF assay with pretreated tree trimmings obtained a higher ethanol yield. Moreover, the
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.
Acknowledgements
This work was supported by the Science and Technology Planning Project of Guangdong Province [grant numbers 2015A010106012]; and the Science and Technology Planning Project of Guangzhou [grant numbers 201904010318]; and the Natural Science Foundation of Guangdong Province [grant numbers 2020A1515010748].
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