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17 Utilization of Microbial Potential for Bioethanol Production from Lignocellulosic Waste

17.3.1.2

Lignin-Degrading Bacteria

Bacteria play a major role in nutrient cycling and plant biomass degradation in the

terrestrial ecosystem. Many bacterial species belonging to the group of Actinobac-

teria, Proteobacter, and firmicutes play an essential role in lignin degradation in

feedstock. Streptomycetes, a genus in the Actinobacteria group, are most common

and largest contributors in the biological treatment process. Over 500 species of

Streptomycetes, mainly Streptomycetes viridosporus, Streptomycetes flavirens, and

Streptomycetes Cyaneus, have been reported to actively participate in the degrada-

tion of synthetic lignin, kraft lignin, aromatic dyes, and plastic with decrease up to

52% lignin content with 21 days of treatment in some cases. Studies on Psudomonas

spp. and Thermospora spp. have shown similar results using popular wood and

barley straw, respectively [39]. Bacterial strains often act selectively on different

lignocellulosic substrates but have greater potential for pretreatment owing faster

growth rate as illustrated in Table 17.4.

17.3.2

Mechanism Involved in Delignification

White-rot fungi are capable of disintegrating complex carbon–carbon bonds and

mineralize lignin by depolymerizing enzymes. Various experiments conducted to

study the action of fungi on ¹⁴C-labeled lignin measure the ¹⁴CO2 generated after its

disintegration. Pyrolysis gas chromatography–mass spectroscopy (GC–MS) analysis

revealed potential change in the ratio between p-hydroxyphenyl (H), guaiacyl (G),

and syringyl (S) lignin units by action of the fungal-derived lignolytic enzymes [31].

It was reported that 10 U of fungal peroxidase per mg of straw reduced the quantity

of phenolic H units compared to 31% in control to 3% in the treated straw, the G

units from 40% to 4%, and completely eliminated the minute quantity of phenolic S

present in the treated straw. This revealed that the sensitivity of lignin units to fungal

degradation is in the following order: S > G > H.

The mechanisms involved in the delignification of the lignocellulosic substrate

by fungi can be broadly classified into two groups: hydrolytic and oxidative types.

Oxidative mechanisms involve action of free radicals of reactive oxygen species, pri-

marily hydroxyl ions generated as a result of iron reacting with hydrogen peroxide,

from fungal enzymes like aryl-alcohol oxidase, glyoxaline oxidase, and pyranose-2

oxidase on lignin, breaking it into low molecular weight products. Oxidative mech-

anism is also achieved by hydrolytic breakdown of hydrogen peroxide mediated

by manganese peroxidase (MnP) and laccases which leads to oxidation of Mn²⁺ to

Mn³⁺. Hydrolytic type mechanism involves hydrolytic enzymes that aid in breakage

of the glycosidic linkage in the complex lignocellulose structure.

17.3.3

Enzymes Involved Biological Pretreatment

Microorganisms are commercially important due to their versatility in enzyme pro-

duction. Complex and multiple enzyme production systems permit them a higher

level of delignifying capacity. The following sections deal with different enzymes and

mechanisms of their action involved in the pretreatment process.