Biodegradation of wood by fungi offers an example of how lignocellulose can be efficiently converted from a recalcitrant mixture of complex biopolymers into readily metabolized sugars. Representing a diverse group, brown rot fungi utilize a unique, yet incompletely understood cellulolytic decay process, believed to involve the generation of hydroxyl radicals by Fenton chemistry, which rapidly depolymerize cellulose. Thermodynamically driven in the absence of enymzatic catalysis, this process can be chemically mimicked, allowing for accelerated bioconversion as fungal growth and colonization rates often bottleneck biological pretreatment times. Furthemore, brown rot fungi are able to circumvent lignin, leaving behind a potentially value-added byproduct in the form of an oxidized lignin-rich residue. This dissertation expands our understanding of brown rot decay through a set of studies with differing approaches. First, decay residues were compared across phylogenetic groupings of brown rot fungi to explore decay variability. As substrate can dictate decay rates, three distinct and representative substrate types were used. Noting significant differences in the Antrodia clade on corn stover, a separate study was conducted to explore the relationship between membership in this clade and the extent of decay they can cause in Poales grasses. Next, a survey of wood-degrading fungi was conducted to assess their ability to improve saccharification yield, the differences in variability across decay types, and to determine the relationship between chemical changes within the substrate and yield improvement. Lastly, hydroquinone-driven Fenton oxidation was both chemically mimicked and theoretically modeled to discern the efficacy of this mechanism in improving cellulose accessibility, its compatibility with cellulases, and the potential role that other redox active chemical species might have in the brown rot mechanism. Saccharification potential in relation to chemical and compositional changes in various substrates was used as a metric in most studies, allowing for consideration of the applied biotechnological benefit of brown rot, while furthering our fundamental understanding of this remarkable decay mechanism. The progression of substrate chemical component losses on a mass loss basis was found to be consistently identical among all known clades of brown rot fungi in all relevant studies. This contrasted with white rot decay, which displayed notably greater variation among tested species in how decay progressed. Despite this consistency in how brown rot decay progressed, there were notable differences between clades in their ability to initiate decay. Where a Gloeophyllum-clade representative was capable of degradation rates similar to those observed on wood substrates, Antrodia clade brown-rotters were found to have a limited ability to degrade Poales grasses. Meta-analysis indicated that this finding was consistent with previous studies. Lastly, the direct use of the Fenton reaction resulted in chemical composition changes that were consistent with brown rot. Despite this, improvement in saccharification yield was difficult to realize because of the reactivity of hydroxyl radical with the desired monosaccharide product. This suggests that if the mechanism for brown is dependent on Fenton chemistry, the manner in which hydroxyl radicals are produced by this reaction must be highly controlled.
University of Minnesota Ph.D. dissertation. December 2015. Major: Bioproducts/Biosystems Science Engineering and Management. Advisor: Jonathan Schilling. 1 computer file (PDF); xiii, 193 pages.
Informing Mechanism through Application: Chemical Mimicry and Comparative Decay Studies of Brown Rot Fungi.
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