*2.3.2 Bacterial lignin degradation*

It has been reported that bacteria are able to degrade lignin through a complex of enzymes, such as extracellular peroxidases, Dye-decolorizing peroxidases (DyPs), and laccases. Among the reported bacterial genus, we found *Rhodococcus*, *Pseudomonas*, *Streptomyces, Novosphingobium*, and *Bacillus* [49]. The bacteria *S. viridosporus* and *N. autotrophica* were able to degrade lignin through extracellular peroxidases, whereas *P. putida*, *Rhodococcus RHA1*, and *Rhodococcus* sp. were active in hydrogen peroxide absence suggesting the presence of extracellular laccases [50]. DyP peroxidases are able to oxidize lignin, aromatic dye, and other phenolic compounds [51]. In spite of finding at first the DyP peroxidases in fungi, recent studies have shown that these enzymes are prominent in bacteria [52]. Bioinformatic analysis showed that *R. jostii* sp. presents two peroxidases members of the DyP peroxidase family, and the deletion mutant gene assay in these genes showed reduced lignin degradation [53]. Bacterial laccases have showed high tolerance to temperature, salt, and acid/alkaline conditions, which make them valuable in the industry,

*Biotechnological Applications of Biomass*

**2.3 Biological pretreatment**

*2.3.1 Fungal lignin degradation*

surface area of the fibers and the compatibility with the polymer matrix. According to Ahmed et al. [11], these physical treatments can be classified as follows: mechanical treatment (stretching, calendaring, or rolling), solvent extraction treatment, and electric discharge (plasma treatment, corona treatment, ionized air treatment, thermal treatment, steam explosion, electron radiation, dielectric barrier, and ultraviolet). The *mechanical treatments* promote the interactions between the natural fibers and the polymeric matrix by increasing the surface area of the fibers and decreasing the density and stiffness; therefore, a better distribution of the fibers in the polymer matrix is achieved [19]. *Solvent extraction* can increase the surface area and remove soluble impurities for natural fibers and fillers. Hence, fibers with high cellulose content are obtained. However, this treatment is not widely used because it generates dangerous stems that pollute the environment [20]. *Electric discharge* improve the compatibility between the hydrophilic fiber and the polymer matrix through roughness of the natural fiber surface and structure [21]. Plasma treatment does not need the use of chemicals, which makes it environmentally friendly and cheaper as well. Fazeli et al. [22] modified cellulose fibers by using plasma treatment for the development of biocomposites using a thermoplastic starch matrix (TPS), obtaining a biocomposite with acceptable mechanical properties due to a good interfacial interaction between cellulose fibers and TPS, verified by scanning electron microscope (STEM). Corona treatment changes the surface of natural fibers (surface energy can decrease or increase and free radicals can be produced) by using different types of gases and cold plasma [23]. The steam explosion and alkaline extraction treatments are the most efficient for the removal of hemicellulose fibers. Ultraviolet rays treatment oxidizes the surface of the natural fibers and improves the mechanical properties due to a good

interfacial adhesion between natural fibers and the polymer matrix [24].

enzymes have an enormous potential to be used for lignin valorization.

The breaking down of lignin by fungi has been reported mainly for white-rot fungi due to their highly efficient enzymatic system. White-rot fungi are able to degrade lignin in such an efficiently and selectively way that gives them utility in the industry. These fungi have been applied by different industries such as paper, biofuels, and biorefinery for delignifying biomass [28]. According to the selected strain, it is possible to obtain 20–100% for lignin removal. Black liquor from a pulp and paper mill, treated with the fungi *Pleurotus ostreatus,* reduced 70% its lignin content [29]. Sugarcane bagasse treated with *Lentinula edodes* and *P. ostreatus* presented, after the treatment, 87 and 85% of lignin, respectively [30]. Biological pretreatment of bamboo culms with *Punctularia* sp. Strain TUFC20056 showed more than 50% on lignin degradation [31]. High ligninolytic capabilities have

Biological pretreatment is based in the extracellular enzymes released by microorganisms in which enzymes degrade the noncellulosic components of the fiber surface. Biological pretreatment of fiber offers relevant advantages, such as low chemical and energy use that make it eco-friendly [25]. A great variety of microorganisms exists in nature, they are able to hydrolyze lignin, being the fungi the most studied [3]. Basidiomycetes white-rot fungi are responsible for lignin degradation in nature; they can break down not only lignin but also hemicellulose and cellulose. It has been reported that these microorganisms degrade lignin in a selective way that is able to offer potential biotechnological application [26]. However, recent studies have shown that many bacteria are able to break down lignin [27]. Likewise,

**40**

being the first bacterial laccases identified in *Azospirillum lipoferum* [54]. Many soil bacteria, actinobacteria, and α-, β-, and γ-proteobacteria have shown bacterial laccase genes [55]. A higher laccase production was reached by the bacteria *Streptomyces* sp. KS1025A compared with white-rot fungi in reduced time [56].
