Lignocellulosic biodegradation of coir fibre by microorganisms.
Coir is a lignocelluloses hard fibre with a very high content of lignin 40%. Coir has remarkable stretching properties besides possessing high resistance to attack from microorganisms because of its impervious nature. Lignin is the main constituent of coir fibre responsible for its harshness, dark color and branching patterns. Lignin is closely associated with cellulose and hemicellulose in hardening and strengthening. It is the most abundant renewable aromatic material on earth & its degradation is a rate limiting step of carbon recycling. Nature is programmed to recycle these resources in a timely way, back to basic building blocks of CO2 and water through biological, thermal, aqueous, photochemical, chemical and mechanical degradation. Irregular arrangement of phenylpropanoid polymer that resists chemical or enzymatic degradation to protect cellulose.Thus lignin forms a barrier against microbial destruction by protecting the readily assimilable polysaccharides.
As lignin itself is extremely resistant to chemical and biological degradation, only a few microorganisms are able to mineralize it. Microbes biologically degrade these polymers have highly specific enzyme system capable of hydrolyzing these polymers in to digestible units. Microorganisms that degrade plant lignin via an oxidative process, are fungi, actinomycetes and to a lesser extent, bacteria. Both fungi and aerobic bacteria play an important role in degrading holocellulose and lignin to lower molecular weight products,some of which are then further metabolized by facultative and obligate anaerobic soil bacteria and actinomycetes. Bacterial lignin degradation systems consist of specific enzymes with the ability to convert lignin into intermediate metabolites.The bulky nature of the heterogeneous lignin polymer forming a complex three dimensional network represents an additional limitation for biodegradation since the enzyme accessibility is strongly reduced. To overcome this difficulty, two main strategies have been developed by ligninolytic organisms based on: (i) presence of catalytic residues widely exposed at the surface of ligninolytic peroxidases, and (ii) use of redox mediators participating in the enzymatic attack. The activity of two enzymes extracellular peroxidase and phenol oxidase (laccase) was found to correlate with both solubilization and mineralization rates of lignin.
The importance of lignolytic bacteria raised, because lignin degrading bacteria have wider tolerance of temperature, pH and oxygen .Identification of bacteria having lignin oxidizing enzymes would be of significant importance. Decomposition of lignin in nature occur by the action of wood rot fungi mostly basidiomycetes class eg.Phanerochaete chrysosporium. the first basidiomycete whose genome was sequenced due to the interest on biological degradation of lignin. Lignin peroxidase is the key enzymes in lignin biodegradation by white rot fungi. Research has been centered on other fungi such as Streptomyces viridosporus ,Pleurotus eryngii,Fusarium proliferatum.Either mixed or pure culture of bacteria can grow on lignin as a carbon source eg Pseudomaonas spp ,Serratia marcescens . S. marcescens produce laccase and its activity correlated positively with lignin mineralization and solubilization..Actomycetes also participate in lignin degradation eg Thermomonospora mesophila , Micromonospora .The sophisticated and well co-ordinated co operation between the termites and the fungi enables efficient utilization of lignocellulose.
Potential application utilizing lignin degrading microorganisms and their enzymes have become attractive ,because first they may maximize the utilization of crop wastes, second they may provide environmentally friendly technologies. Biotechnology based on lignin-degrading microbes and their enzymes can contribute to more efficient and environmentally sound use of renewable lignocellulosic materials for sustainable production of chemicals,biofuels and energy.
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