## 7 Microbial Processive Cellulase Properties of Hydrolyzing Natural Crystalline Substrates
_Clostridium Phytorementans_ is also regarded as biofuel bacterium and a model microorganism for consolidated bioprocessing (CBP), since it can utilize a broad range of carbon sources for generation of ethanol and other metabolites in one step [78,135]. However, the _Clostridium Phytorementans_ GH9 processive endoglucanase Cel9 is a thermophilic cellulase, because it has an optimal peak temperature at 65 \({}^{\circ}\)C and its activity drops down sharply at 40 \({}^{\circ}\)C [135]. The researchers believed that the mesophilic _Clostridium Phtofermentans_ might have acquired the thermophilic GH9 processive endoglucanase Cel9 via the horizontal gene transfer mechanism [78]. [PERSON] et al. [76] further reported that the _Clostridium Phytorementans_ endoglucanase Cel9 only had a specific activity at 0.3464 umol D-glucose/mg protein.min determined at 37\({}^{\circ}\)C with Avicel, which may limit its potential industrial applications in some areas such as exogenous enzyme additives. Furthermore, the _Clostridium Phytorementans_ endoglucanase Cel9 has a multi-modular structure and thus a relatively large molecular weight and dimensional size for penetration into internal microfibril surface of raw and pretreated biomass materials, which may be the additional disadvantage associated with the GH5-tCel5A1 and the GH5-p4818 Cel5_2A processive endoglucanas for their industrial applications. [PERSON] et al. [103] cloned and characterized a multimodular free cellulase CeIA from the extremely thermophilic cellulolytic bacterium _Caldicellulosiruptor bescii_ (previously called _Anareocellular thermophilum_) with a specific activity at 0.239 umol D-glucose/mg protein.min with Avicel, and 1.6174 umol D-glucose/mg protein.min with xylan, respectively, determined at 72 \({}^{\circ}\)C driven by its bi-modular endo-acting and exo-acting processivity and the bi-functional hydrolytic property. The _Caldicellulosiruptor bescii_ CeIA was determined to have an optimal peak temperature at 85 \({}^{\circ}\)C for hydrolyzing Avicel and this activity dropped down sharply to zero at about 45 \({}^{\circ}\)C [103], which may limit its potential industrial applications in some areas such as serving as an exogenous feed enzyme additive in livestock feeding. The _Caldicellulosiruptor bescii_ CeIA had a large molecular weight of about 230 kDa [103], and an estimated effective size of 10-35 nm [77], thus potentially limiting its effective diffusion into microfibril internal surface for biomass hydrolysis. It was shown that the _Caldicellulosiruptor bescii_ CeIA was effective in excavating extensive cavities on the surface of cellulosic substrates, thus increasing its effective hydrolytic areas [77]. In their _Science_ paper, [PERSON] et al. [77] demonstrated that the _Caldicellulosiruptor bescii_ CeIA was effective in enzymatically converting both pretreated and raw biomass materials at 75 and 85 \({}^{\circ}\)C. [PERSON] [88] commented that hydrolytic activities of this _Caldicellulosiruptor bescii_ CeIA showing that now there were no enzymatic barriers for effective cellulose breakdown. [PERSON] [88] further viewed that two other advantages associated with hydrolysis of biomass through the _Caldicellulosiruptor bescii_ CeIA activities at the high temperatures of 75 and 85 \({}^{\circ}\)C included the inhibition of potential bacterial contamination and fluidity of fermentation broth. However, maintaining the high temperatures of 75 and 85 \({}^{\circ}\)C to achieve the optimal _Caldicellulosiruptor bescii_ CeIA activities will likely be too expensive at the large-scale commercial level for manufacturing lignocellulosic ethanol. On the other hand, the _Caldicellulosiruptor bescii_ CeIA cellulase was originally characterized by [PERSON] et al. [103] and the _Clostridium Phytorementans_ bacterium expressing the endoglucanase Cel9 was originally isolated and patented by [PERSON] et al. [136].