http://www.dzumenvis.nic.in/Biodegradation/pdf/Biodegradation%20of%20lignin.pdf

Biodegradation of lignin in a compost environment: a review

Photosynthetic fixation of carbon dioxide in our biosphere yields approxi­ mately 136 X lOIS g of dry plant material annually, which represents Earth's most abundant form of biomass (88). Two major constituents of such biomass are cellulose and lignin, and hence this material is often referred to as . \ lignocellulosic biomass, or simply lignocellulose. Most of the synthesis (about 2/3) occurs in terrestrial ecosystems where it is balanced, or nearly so, by the decomposition/respiration side of the carbon cycle (63). Decom­ position of lignocellulose is carr ied out primarily by microorganisms, chiefly fungi and bacteria. However, augmenting the activities of microbes is an arr ay of soil macro-invertebrates, whose effects may range from simple comminution and dispersion of plant material to actual dissimilation of the structural polymers of lignocellulose (86, 90, 91). Among the most abundant and important of these invertebrates ' are termites, which, with their associated microbial symbionts, dissimilate a significant proportion of the cellulose

Role of Microorganisms in the Digestion of Lignocellulose by Termites

Lignins are amongst the most studied macromolecules in natural environments. During the last decades, lignins were considered as important components for the carbon cycle in soils, and particularly for the carbon storage. Thus, they are an important variable in many soil–plant models such as CENTURY and RothC, and appeared determinant for the estimation of the soil organic matter (SOM) pool-size and its stabilization. Recent studies challenged this point of view. The aim of this paper was to synthesise the current knowledge and recent progress about quantity, composition and turnover of lignins in soils and to identify variables determining lignin residence time. In soils, lignins evolve under the influence of various variables and processes such as their degradation or mineralization, as well as their incorporation into SOM. Lignin-derived products obtained after CuO oxidation can be used as environmental biomarkers, and also vary with the degree of degradation of the molecule. The lignin degradation is related to the nature of vegetation and land-use, but also to the climate and soil characteristics. Lignin content of SOM decreases with decreasing size of the granulometric fractions, whereas its level of degradation increases concomitantly. Many studies and our results suggest the accumulation and potential stabilization of a part of lignins in soils, by interaction with the clay minerals, although the mechanisms remain unclear. Lignin turnover in soils could be faster than that of the total SOM. Different kinetic pools of lignins were suggested, which sizes seem to be variable for different soil types. The mechanisms behind different degradation kinetics as well as their potential stabilization behaviour still need to be elucidated.

Fate of lignins in soils: A review

Composting is a controlled self-heating, aerobic solid phase biodegradative process of organic materials The process comprises mesophilic and thermophilic phases involving numerous microorganisms In several successive steps, microbial communities degrade organic substrates into more stable, humified forms and inorganic products, gener- ating heat as a metabolic waste product Due to the complexity of substrates and intermedi- ate products, microbial diversity and the succession of populations is a prerequisite to ensure complete biodegradation Due to the dynamic process, both in time and space (microhabi- tats), which is reflected by constantly changing pH, humidity, oxygen partial pressure and temperature it is extremely difficult to detect, albeit isolate, all the microorganisms involved Research on composts is also so difficult because the process can hardly be simulated in the laboratory since all major gas and temperature fluxes are to a large extent determined by the physical extension of the system In this comprehensive survey of literature an inventory of the mesophilic and thermophilic bacteria, actinomycetes and fungi isolated during several phases of composting (including also self-heating organic materials) is presented

A survey of bacteria and fungi occurring during composting and self-heating processes

Introduction
Historical Outline
Bacteria and Microfungi
Actinomycetes
Other Bacteria
Soft-Rot Fungi and Other Microfungi


Brown-Rot Basidiomycetes
White-Rot Basidiomycetes
Mineralization of 14C-Labeled Lignins
Ligninolytic Enzymes


Catabolism of Primary Degradation Products
Outlook and Perspectives
Patents


Keywords:

lignin;
wood;
lignocellulose;
white-rot fungi;
brown-rot fungi;
soft-rot fungi;
actinomycetes;
lignin peroxidase;
manganese peroxidase;
laccase;
molecular biology;
radicals;
mediators;
manganese;
veratryl alcohol;
oxalate;
vanillic acid

Biodegradation of Lignin

Six biodegradative actinomycete strains were grown on a dimeric model lignin compound of the β-aryl ether type. Although only two strains, Thermomonospora mesophila and Streptomyces badius, utilized the compound as a carbon and energy source and produced substantial amounts of monomeric products, all of the strains could demethylate the substrate and oxidize Cα on the phenylpropane side-chain. Streptomyces sp. EC1 produced small amounts of aromatic acids and unidentified lignin-derived products when grown on straw. This organism also produced cell-bound demethylase requiring H2O2 and Mn2+, protocatechuate 3,4-dioxygenase and β-carboxymuconate decarboxylase activity in response to growth on low-molecular-mass aromatic compounds but not lignocellulose or its polysaccharide components. Extracellular peroxidase and catalase activity were detected in all of the strains. These data are used to propose a scheme by which actinomycete attack of the lignin component of plant biomass can be envisaged.

/pdf/towards-elucidation-of-the-lignin-degradation-pathway-in-24dsx85fls.pdf

Towards elucidation of the lignin degradation pathway in actinomycetes

Actinomycetes grown on wheat straw solubilized a lignocarbohydrate fraction which could be recovered by acid precipitation. Further characterization of this product (APPL) during growth of Streptomyces sp. strain EC1 revealed an increase in carboxylic acid and phenolic hydroxyl content, suggesting progressive modification. This was also observed in dioxane-extracted lignin fractions of degraded straw, and some similarity was further suggested by comparative infrared spectroscopy. However, the molecular weight profile of APPL was relatively constant during growth of Streptomyces sp. strain EC1 on straw, while analysis of the dioxane-extracted lignin fractions appeared to show fragmentation followed by repolymerization. Lignocarbohydrate solubilization could be monitored in all cultures by routine assay of APPL-associated protein, which accounted for up to 20% of the extracellular culture protein in some cases. Interestingly, this protein fraction was found to include active hydrolytic and oxidative enzymes involved in the degradation of lignocellulose, and specific enzyme activities were often increased in the acid-insoluble fractions of culture supernatants. This was particularly important for peroxidase and veratryl oxidase activities, which could be readily detected in the acid-precipitable lignocarbohydrate complex but were virtually undetectable in untreated culture supernatants.

P Helvenstein

Papers

Towards elucidation of the lignin degradation pathway in actinomycetes

Lignocarbohydrate Solubilization from Straw by Actinomycetes