Department of Biotechnology, U.I.E.T, Kurukshetra University Kurukshetra G. M. N College Ambala Cantt, Haryana, India

Corresponding author Email: deepmolbio@rediffmail.com

Article Publishing History

Introduction

Lignocellulosic biomass consists mainly of cellulose (35–50%), hemicellulose (25–30%) and lignin (25–30%). The plant biomass is a carbon source for bio-refinery industry, considered as sustainable and environmental friendly substitute to the current petroleum platform (Kamm et al., 2004). The composition of lignocelluloses not only depends upon the species but also on the different parts of the plant, their age and growth conditions (Jorgensen, 2003). The large quantity of lignocellulosic materials create them potentially inexpensive and easily available natural resources for the production of high value compounds and biofuels. The degradation of cellulose, hemicelluloses and lignin are extremely slow. The various microorganisms are capable of growing on lignocellulosic materials and produce a wide range of enzymes that could be of scientific or industrial importance, (Varma et al. 2017 Tolalpa et al. 2018) .

Cellulose

It is homopolysaccharide constituent of the fiber wall, consists of D-glucose linked together by ß-1, 4-glycosidic bonds. Cellulose form intra- or intermolecular hydrogen bonds resulting in the formation of cellulose microfibrils. Due to hydrogen bond it is not soluble in most solvents and resistance against microbial degradation (Jorgensen, 2003). It is hydrolyzed by cellulase, 1, 4-ß-cellobiosidase and ß-glucosidase (Schmidt, 2006). Cellulases hydrolyses cellulose consists of cellobiohydrolases, endoglucanases and ß-glucosidase. The cellobiohydrolase and endoglucanase function together for hydrolyzing 1, 4-ß-D-glycosidic linkages in cellulose, cello-oligomers and other ß-D-glucans and to form cellobiose from the non-reducing ends, which are degraded by ß-glucosidase to glucose units.

Hemicellulose

It consists of monomeric residues, like D-glucuronic acid, D-mannose, D-arabinose, D-glucose and D-xylose. They have lesser degree of polymerization in comparison to cellulose, with side chains that can be acetylated. They are classified according to the monomeric sugar in the backbone of the polymer, e.g. mannan (ß-1, 4-linked mannose) or xylan (ß-1, 4-linked xylose) hemicelluloses. The main chains of glucose and mannose residues are generally associated with ß-(1, 4) bond. The side chain is attached to main chain via a-(1, 6) bonds in galactoglucomannan. Xylan can be degraded by endo-1, 4-ß-xylanase and 1, 4-ß-xylosidase to xylose (Jorgensen, 2003). Due to more heterogenous nature of hemicellulose, a combination of enzymes is necessary for its degradation, such as endoxylanases, ß-xylosidases, endomannanases, ß-mannosidases, a-L-arabinofuranosidases and a-galactosidases.

Lignin

It is very much hydrophobic and forms an amorphous complex with hemicelluloses enclosing cellulose and make unavailable to microbial utilization of available carbohydrates within the wood cell wall. It is jointing component to attach cells and harden the cell wall of xylem, which is accountable for the smooth movement of water from roots to leaves. Its aromatic rings are responsible to create it trickier to degrade (Schmidt, 2006). Ester linkages take place among the free carboxy group in hemicellulose and the benzyl groups in lignin, the lignicarbohydrate complex (LCC), embeds the cellulose, which is resposibe for giving protection against microbial and chemical degradation (Jeffries, 1994). The lignin is not to be utilized by most of the microorganisms due to its phenylpropane units in the structure (Schmidt, 2006). Now days there are huge attention in isolating organisms able to degrade lignin, in the cleaving of the chemical bonds that are present between lignin and hemicelluloses. The continuous utilization of these compounds is extremely slow. The microorganisms isolated from from soil and rumens are capable to degrade in to sugars, which can be utilized as energy and carbon source by a variety of microorganisms for the production of different products.

Role of microbial consortium in utilization of lignocellulose biomass

Kumar et al. (2001) isolated Branhamella catarrhal, Brochothrix sp., Micrococcus luteus and Bacillus firmus from cane sugar factory effluent contaminated soil. All microbes can use cinnamic acid as sole carbon source with significant inhibition after addition of glucose. The B. catarrhalis and Brochothrix sp. were able of metabolizing ferulic acid but not in the presence of glucose. The lignocellulolytic enzyme profiles of five strains of Agaricus and four strains of Pleurotus were determined by Rana and Rana in 2004. The crude enzyme from the colonized substrate was used to analysis the enzymatic activities. All strains of Agaricus showed higher level of cellulases activities in comparison of Pleurotus strains. The Pleurotus sp. exhibits high ligninases activity. The difference in lignocellulolytic enzyme summary was present both at interspecies and intraspecies point. The lignocellulose utilization in solid state fermentation in sugarcane bagasse and rice straw by Aspergillus tamari was isolated by Umasaravanan et al. (2010). The microorganisms isolated from marine environment which can utilize the lignocellulosic biomass were showed by Sethi et al. (2013).

The organisms showing maximum degradation were recognized as Bacillus pumilus and Mesorhizzobium sp., as well two fungal sp. recognized as Aspergillus niger and Trichodermaviride. The organisms showed different levels of utilization at different time point in a diversity of substrates. The thermophilic microbial consortium was isolated by Malik et al. (2015) from sugarcane industry mature compost. The consortium was able to degrade rice straw. The lignin can be productively degraded by white-rot fungi, producing a number of oxidoreductases, enzymes able to attack phenolic structures. Varma et al. (2017) studied about those microbes which were actively involved in lignocelluloses degradation during drum composting of mixed organic waste i.e. saw dust, cattle manure, dry leaves and vegetable waste in a 550 L rotary drum composter.

An anaerobic thermophilic bacterial strains for ethanol production from D-xylose was reported by Sommer et al. (2004). Lu et al. (2004) reported Clostridium phytofermentans, responsible for the maximum number of enzymes for the utilization of lignocellulosic biomass between sequenced Clostridial genomes. The Clostridium thermocellum generate a collection of cellulolytic and hemicellulolytic enzymes in a multienzyme complex system are known as cellulosome. The cellulosome was firstly reported in anaerobic bacteria such as Clostridium thermocellum in 1983 and then in anaerobic fungi in 1992 (Wilson et al., 1992). Abd-Elsalam et al. (2009) isolated bacterial strains KafAH19 degrade synthetic lignin and use as a carbon source. The strain was biochemically characterized as gram-positive rod. By using 16S rDNA sequencing the culture was recognized as Bacillus sp.. The lignocellulose utilizing microbes by using a selective media using lignin, xylan and cellulose as selective substrates was isolated by Wahyudi et al. (2010). The isolates were recognized as Enterococcus casseliflavus/gallinarum sp. Wongwilaiwalin et al. (2010) worked on thermophilic lignocellulose utilizing microbial consortium and multi-species lignocellulolytic enzymatic involvement. Gontikaki et al. (2015) observed that the proficient degradation of terr OC in the marine environment could be fuelled by labile marine based objects by treating coastal sediments with 13C-lignocellulose, as a proxy for terrOC, in the presence and absence of unlabelled diatom detritus that acted as the important inducer. The amount of priming was viewed by the differentiation in lignocellulose mineralisation between diatom-amended treatments and controls in aerobic sediment slurries.

Ransom-Jones et al. (2017) have reported that landfill sites symbolize a repository of uncultivated lignocellulose-degrading Microbes such as Firmicutes, Bacteroidetes, Spirochaetes, and Fibrobacteres, which are rich source for biomass degradation. According to Cortes-Tolalpa et al. (2017), Lignocellulosic biomass (LCB) is a striking source of carbon for the production of sugars and other useful chemicals. Due to its innate density and heterogeneity, proficient biodegradation requires the actions of diverse and various types of hydrolytic enzymes. So, they observed that the wheat straw degradation potential of synthetic microbial consortia composed of bacteria and fungi.

Alessi et al. (2018) explored the enzymatic degradation of lignin which is complicated due to its structural complexity, lack of hydrolysable linkages and insoluble nature and also discuss the developments in the degradation of lignin by microorganisms and the catabolic pathways for degradation of lignin. Mohn et al. (2018) illustrate that by applying stable isotope probing (SIP) coupled with amplicon and shotgun metagenomics, it is possible to recognize and describe the functional attributes of cellulose, hemicelluloses and lignin – degrading bacteria and fungi. The halotolerant lignocellulose degrading microbial consortia was isolated by Cortes-Tolalpa et al. (2018) by feeding a salt marsh soil microbiome on an intractable carbon and energy source, i.e., wheat straw.

Conclusion and future prospects

Lignocellulose compounds are most abundant agricultural residues present in the world. There are various reports that lignocellulose compounds can be utilized by fungi, action mycetes and bacteria. The lingo cellulytic utilizing microbial consortium can be used for the conversion of biomass feedstock to useful bio-based products. The use of farming crop wastes involves a separation of the polymeric compounds – cellulose and hemicelluloses. This approach comes under sustainable “green” biotechnology.

References

Abd-Elsalam H.E. and El-Hanafy A. A. (2009) Lignin Biodegradation with Ligninolytic Bacterial Strain and Comparison of Bacillus subtilis and Bacillus sp. isolated from Egyptian Soil. American-Eurasian J. Agric. & Environ. Sci. Vol. 5 No 1: Pages 39-44.

Alessi A. M., Bird S. M., Oates N. C. et al. (2018) Defining functional diversity for lignocellulose degradation in a microbial community using multi-omics studies. Biotechnology for Biofuels. Vol. 11: Page166.

Cortes-Tolalpa L., Salles J. F. and van Elsas J. D. (2017) Bacterial Synergism in Lignocellulose Biomass Degradation –
Complementary Roles of Degraders As Influenced by Complexity of the Carbon Source. Front. Microbiol. Vol. 8: Pages1628. doi: 10.3389/fmicb.2017.01628

Cortes-Tolalpa L., Norder J., van Elsas J. D. & Falcao Salles J. (2018) Halotolerant microbial consortia able to degrade highly recalcitrant plant biomass substrate. Applied Microbiology and Biotechnology Pages 2913- 2927. DOI: 10.1007/s00253-017-8714-6

Gontikaki E., Thornton B., Cornulier T. and Witte U. (2015) Occurrence of Priming in the Degradation of Lignocellulose in Marine Sediments. PLoS ONE. Vol. 10 No 12: Pages e0143917. doi:10.1371/journal.pone.0143917