Microbial Sources for lignocellulosic enzymes

Several sources have been recognized as active xylanase producers including fungi, bacterial, yeast, marine algae, protozoans, snails, crustaceans, insect, seeds etc., but principal commercial sourse is filamentous fungi. Most of fungal and bacterial xylanases studied so far were found to be optimally active at, or near to mesophilic temperatures (approximately 40-60^o C) and neutral (in particular for bacterial xylanase) or slightly acidic pHs (in particular for fungal xyalanses). Nevertheless, xylanases have also been reported which are not only stable, but also active, at the extremes of pH and temperature. Indeed, xylanases active at temperatures ranging from 5 to 105^o C, pHs from 2 to 11 and NaCl concentrations as high as 30% have also been reported.

Of the extremophilic xylanases, the thermophiles, alkaliphilic and acidophiles have been the most extensively studied. These are produced by microorganisms which have colonized environments that may be said to be extreme from an anthropocentric point of view and which produce enzyme adapted to these extreme habitats. A number of thermophilic (optimal growth at 50-80^o C) and hyperthermophilic  (optimal growth at >80^o C) xylanase producing microorganisms have been isolated from a variety of sources, including terrestrial and marine solfataric fields, thermal springs, hot pools and self-heating decaying organic debris. A few thermophilic fungi belonging to Ascomycetes (Chaetomium thermophile, Thermoascus aurantiacus, Dactylomyces thermophilus, Melanocarpus albomyces, Talaromyces thermophilus, T. emersonii), Basidiomycetes (Phanerochaete chrysosporium) and Hyphomycetes (Acremonium alabamensis, A. thermophilum, Myceliophthora thermophila, Thermomyces lanuginosus, Scytalidium thermophilum, Malbranchea cinnamomea) have been isolated from composts, soils, nesting materials of birds, wood chips and many other sources. Hyperthermophilic eubacteria have been isolated that grow anaerobically at temperatures above 80^o C. These microbes include Thermotoga maritima MSB8 , Thermotoga sp FjSS3-B.1, Caldocellum saccharolyticum . In addition to above mentioned xylanase producing bacteria a number of xylanase producing hyperthermophilic archaea have been reported: Thermococcus zilligii, Pyrococcus furiosus , Pyrodictium abyssi  and Sulfolobus solfataricus .

While the majority of natural environments on earth are essentially neutral, with pH values of between 5 and 9, habitats with extreme pHs are also common, in particular in geothermal regions, carbonate laden soils, soda deserts and soda lakes. Xylanase producing alkaliphilic microorganisms, which typically grow optimally at pH values above 9, and acidophiles, which grow optimally between pH 1 and 5, have been isolated from these environments and also from such sources as Kraft pulp, pulp and paper industry wastes, decomposing organic matter, faeces , plant sources , soils  and even from neutral environments where they are found coexisting with neutrophilic microorganisms . The first report of a xylanase produced by an alkaliphilic microorganism was as early as 1973 for a xylanase from Bacillus sp. C-59-2. Since this initial finding a number of acidophilic and alkaliphilic bacillus have been isolated which includes  Acidobacterium  sp.  , Pseudomonas G6-2 , Clostridium absonum CFR-702 , Bacillus sp. , Bacillus firmus , Bacillus circulans  and  Enterobacter sp. . Xylanases isolated form various acidophilic and alkaliphilic fungi include, xylanases from Trichoderma sp. , Aspergillus sp.   Penicillium sp. , Aureobasidium pullulans , A. fischeri  and A. fumigatus .