Lignin

LIGNIN or lignen is a complex polymer of aromatic alcohols known as monolignols. It is most commonly derived from wood, and is an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood. It is one of the most abundant organic polymers on Earth, exceeded only by cellulose, employing 30% of non-fossil organic carbon, and constituting from a quarter to a third of the dry mass of wood. As a biopolymer, lignin is unusual because of its heterogeneity and lack of a defined primary structure. Its most commonly noted function is the support through strengthening of wood (xylem cells) in trees.

Global production of lignin is around 1.1 million metric tons per year and is used in a wide range of low volume, niche applications where the form but not the quality is important.

Biological function

Lignin fills the spaces in the cell wall between cellulose, hemicellulose, and pectin components, especially in xylem tracheids, vessel elements and sclereid cells. It is covalently linked to hemicellulose and, therefore, crosslinks different plant polysaccharides, conferring mechanical strength to the cell wall and by extension the plant as a whole. It is particularly abundant in compression wood but scarce in tension wood, which are types of reaction wood.

Lignin plays a crucial part in conducting water in plant stems. The polysaccharide components of plant cell walls are highly hydrophilic and thus permeable to water, whereas lignin is more hydrophobic. The crosslinking of polysaccharides by lignin is an obstacle for water absorption to the cell wall. Thus, lignin makes it possible for the plant’s vascular tissue to conduct water efficiently. Lignin is present in all vascular plants, but not in bryophytes, supporting the idea that the original function of lignin was restricted to water transport. However, it is present in red algae, which seems to suggest that the common ancestor of plants and red algae also synthesised lignin. This would suggest that its original function was structural; it plays this role in the red alga Calliarthron, where it supports joints between calcified segments. Another possibility is that the lignin in red algae and in plants are result of convergent evolution, and not of a common origin.

Biodegradation

Biodegradation of lignin by brown rot, soft rot, or white rot fungi leads to destruction of wood on the forest floor and man-made structures such as fences and wooden buildings. However biodegradation of lignin is a necessary prerequisite for processing biofuel from plant raw materials. Current processing setups show some problematic residuals after processing the digestible or degradable contents. The improving of lignin degradation would drive the output from biofuel processing to better gain or better efficiency factor.

Lignin is indigestible by animal enzymes, but some fungi (such as the Dryad’s saddle) and bacteria are able to secrete ligninases (also named lignases) that can biodegrade the polymer. The details of the biodegradation are not yet well understood and the pathways depends on the type of wood decay. The enzymes involved may employ free radicals for depolymerization reactions. Well understood ligninolytic enzymes are manganese peroxidase and lignin peroxidase. Because it is cross-linked with the other cell wall components, lignin minimizes the accessibility of cellulose and hemicellulose to microbial enzymes such as cellobiose dehydrogenase. Hence, in general lignin is associated with reduced digestibility of the overall plant biomass, which helps defend against pathogens and pests.

Lignin degradation is done by micro-organisms like fungi and bacteria. Lignin peroxidase (also “ligninase”, EC number 1.14.99) is a hemoprotein from the white-rot fungus Phanerochaete chrysosporium with a variety of lignin-degrading reactions, all dependent on hydrogen peroxide to incorporate molecular oxygen into reaction products. There are also several other microbial enzymes that are believed to be involved in lignin biodegradation, such as manganese peroxidase, laccase.

Lignin-related chemicals can be further processed by bacteria. For instance, the aerobic Gram-negative soil bacterium Sphingomonas paucimobilis is able to degrade lignin-related biphenyl chemical compounds.

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