Microbial degradation of lignin: how a bulky recalcitrant polymer is efficiently recycled in nature and how we can take advantage of this

Abstract

SummaryLignin is the second most abundant constituent of the cell wall of vascular plants, where it protects cellulose towards hydrolytic attack by saprophytic and pathogenic microbes. Its removal represents a key step for carbon recycling in land ecosystems, as well as a central issue for industrial utilization of plant biomass. The lignin polymer is highly recalcitrant towards chemical and biological degradation due to its molecular architecture, where different non‐phenolic phenylpropanoid units form a complex three‐dimensional network linked by a variety of ether and carbon–carbon bonds. Ligninolytic microbes have developed a unique strategy to handle lignin degradation based on unspecific one‐electron oxidation of the benzenic rings in the different lignin substructures by extracellular haemperoxidases acting synergistically with peroxide‐generating oxidases. These peroxidases posses two outstanding characteristics: (i) they have unusually high redox potential due to haem pocket architecture that enables oxidation of non‐phenolic aromatic rings, and (ii) they are able to generate a protein oxidizer by electron transfer to the haem cofactor forming a catalytic tryptophanyl‐free radical at the protein surface, where it can interact with the bulky lignin polymer. The structure–function information currently available is being used to build tailor‐made peroxidases and other oxidoreductases as industrial biocatalysts.

A novel approach for application of white rot fungi in wastewater treatment under non-sterile conditions: immobilization of fungi on sorghum

“…They facilitate organic matter decomposition and nutrient recycling in favour of own and other organisms growth [2]. Among these fungal species, white rot fungi (WRF) are of particular interest, due to their capability to efficiently mineralize lignin [3,4]. The extracellular enzymes of WRF have been reported to be responsible for the degradation of lignin [5,6].…”

The fungal ability for biobleaching/biopulping/bioremediation of lignin-like compounds of agro-industrial raw material

“…106 Sugarcane is one of the several species of tall perennial true grasses of the genus Saccharum, native to the warm temperate to tropical regions of South Asia and Melanesia, used for sugar and ethanol production. In 2014, The Food and Agriculture Organization (FAO) estimates that in the 5 major producer countries, it was cultivated on about 20.484 ×10 6 hectares, with a production of 1.443×10 9 tons, being Brazil the largest producer in followed by India, China, Thailand, Pakistan and Mexico. 107 It is one of the plants with the highest bioconversion, being able to efficiently fix solar energy, yielding some 55 tonnes of dry matter per hectare of land annually.…”

“…It is highly recalcitrant towards both chemical and biological degradation. 6 Lignin provides rigidity for stems to growth on land, and waterproofing vascular tissues for From the literature on fungal bioremediation, c.a. 30% focus on white-rot basidiomycetes -the main organisms able to extensively mineralize lignin, 5,13,14 and the ideal model for this is the white-rot fungus Phanerochaete chrysosporium.…”

“…[69][70][71][72] These initiators may be phenolates or other iron-chelating compounds (siderophores), oxalate, and simple aromatic compounds. 6,[73][74][75][76][77] Although some brown-rot fungi accumulate oxalic acid, which causes a significant pH-decreasing, this do not happens with G. trabeum, probably due to enzymes that degrade oxalate, 75 once this fungus may use oxalic acid as a proton donor for enzymatic and non-enzymatic hydrolysis of polysaccharides and as a chelator for a Fe(II)-H 2 O 2 system, generating hydroxyl radicals. 71 G. trabeum showed an unusual ability to rapidly degrade an aliphatic polyether via extracellular one-electron oxidation, 76 and it produces the simple aromatic compounds 4,5-dimethoxy-catechol and 2,5-dimethoxyhydroquinone, 77 which may serve as ferric chelators, oxygen-reducing agents, and redox-cycling compounds.…”

“…4 In turn, polyoses or hemicelluloses are branched polymers formed by various monosaccharides (glucose, mannose, galactose, xylose and small amounts of arabinose, uronic acids and acetyl groups), with a molecular weight and percentage (20-25%) lower than cellulose in the lignocellulosic material (Table 1). [4][5][6][7][8][9][10] After cellulose, lignin -a complex aromatic polymer (Figure 1), is the second main constituent of plant biomass, typical of the middle lamella of cell wall (the most external layer) of vascular plants acting as a cementing agent between fibers. It is highly recalcitrant towards both chemical and biological degradation.…”