In the intricate world of microbial biomass degradation, bacteria and fungi play a crucial role in breaking down lignocellulose present in plant cell walls. Unlike organisms capable of engulfing large particles, these microorganisms employ a strategy involving the secretion of enzymes, such as cellulases, to facilitate the degradation process.
Divergent Approaches in Aerobic and Anaerobic Microorganisms
The approach to deconstructing plant cell walls varies significantly between aerobic and anaerobic microorganisms. In anaerobic conditions, cellulases and hemicellulases are not just secreted individually; instead, they form large multienzyme complexes known as cellulosomes. These intricate complexes, often exceeding 2 MDa, play a pivotal role in lignocellulose breakdown (Bégum and Lemaire, 1996; Bayer et al., 1998, 2004; Shoham et al., 1999; Gilbert, 2007; Smith and Bayer, 2013). On the other hand, aerobic microorganisms release cellulases and hemicellulases as free enzymes, often accompanied by carbohydrate-binding modules or CBMs (Wilson, 2008).
The Marvel of Cellulosomes
Cellulosomes, fascinating supramolecular assemblies, are typically anchored to the exterior surface of microorganisms (Smith and Bayer, 2013). These complexes boast an array of cellulose-degrading enzymes that collaborate synergistically, proving highly efficient in deconstructing even the most resilient crystalline lignocellulosic substrates (Fierobe et al., 2002, 2005).
C. thermocellum’s Cellulosome Saga
Among the myriad of cellulosomes, one of the most extensively studied is that of the anaerobic thermophilic bacterium Clostridium thermocellum. First described in the 1980s, the C. thermocellum cellulosome stands out not only for its cellulases but also for a diverse arsenal of hemicellulases and pectinases (Bayer et al., 1983; Lamed et al., 1983a, 1983b; Morag et al., 1990; Kosugi et al., 2002; Tamaru and Doi, 2001). This intricate machinery includes polysaccharide lyases, CEs, and glycoside hydrolases, showcasing the complexity of lignocellulose breakdown.
Enigma of Free Enzymes in Anaerobic Bacteria
Interestingly, certain anaerobic bacteria express both cellulosomes and free cellulases, leaving the function of these free enzymes in cellulose degradation shrouded in mystery (Gilad et al., 2003; Berger et al., 2007).
Anaerobic Fungi’s Cellulosome Chronicles
The world of cellulosomes extends beyond bacteria, with anaerobic fungi also constructing these enzymatic complexes. Species like Neocallimastix, Piromyces, and Orpinomyces have been reported to harbor fungal cellulosomes, adding a fascinating layer to the microbial strategies for lignocellulose deconstruction (Wilson and Wood, 1992; Ali et al., 1995; Fanutti et al., 1995; Li et al., 1997; Fillingham et al., 1999; Steenbakkers et al., 2001, 2003; Nagy et al., 2007; Haitjema et al., 2013; Wang et al., 2014b).
Conclusion
As we unravel the microbial intricacies of lignocellulose deconstruction, the dual narratives of cellulosomes and free enzymes captivate us with the versatility and complexity that nature employs in breaking down one of the Earth’s most abundant organic compounds. The interplay between aerobic and anaerobic strategies further enriches our understanding of microbial contributions to sustainable bioconversion processes.