Submitted to: Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/14/2008
Publication Date: N/A
Interpretive Summary: Lignin comprises roughly 15% of all terrestrial biomass and must be continuously broken down to maintain the global carbon cycle, yet is one of the most recalcitrant to biodegradation of all natural polymers. Few organisms possess the machinery to break down lignin; efficient “ligninolysis” is apparently confined to certain filamentous fungi that grow on the cellulose and hemicelluloses of dead plant cell walls. Brown rot fungi cause a rapidly destructive decay that is the primary cause of failure in wooden structures, but also provide an essential source of carbon cycling and humus formation in coniferous forests. The lignin remaining after brown rot is still polymeric, which has led most researchers to conclude that brown-rot fungi modify lignin without degrading it significantly. However, there are several lines of reasoning to suggest that this view might be incorrect. To address this dilemma, we have adopted a new technique for lignocellulose solubilization that permits, for the first time, a closer look at brown-rotted lignin via two-dimensional solution-state nuclear magnetic resonance (NMR) spectroscopy. After decay by a brown-rot fungus, our spruce samples had lost 64% of their dry weight and were 36% deficient in methoxyl groups, had lost 85% of their cellulose and over 90% of their other polysaccharides, but only 16% of their lignin. However, degradative methods released only 18% as much monomers as the intact wood (on a lignin basis). After dissolution of the intact and brown-rotted material by our recently developed dissolution method, NMR analysis showed that, although the lignin remained essentially at the same level, the structure of the lignin had changed. Most strikingly, all of the normally recognizable sidechains in lignin had been obliterated. Therefore, it is clear that the aromatic polymer remaining after extensive brown rotting is no longer recognizable as lignin. Elucidating mechanisms of degradation enhance our knowledge of carbon cycling, and provide potential avenues into improving biomass utilization.
Technical Abstract: Biodegradation by brown-rot fungi is quantitatively one of the most important fates of lignocellulose in nature. It has long been thought that these fungi do not degrade lignin significantly, and that their activities on this abundant aromatic biopolymer are limited to minor oxidative modifications. Here we have applied a new technique for the complete solubilization of lignocellulose to show, by one-bond 1H-13C correlation (HSQC) solution-state NMR spectroscopy, that brown rot of spruce wood by Gloeophyllum trabeum resulted in a marked depletion of all intermonomer sidechain linkages in the lignin. The resulting polymer retained most of its original aromatic residues and was probably interconnected by new quaternary linkages that are silent in HSQC spectroscopy. Additional work is needed to characterize these linkages, but it is already clear that the aromatic polymer remaining after extensive brown rot is no longer recognizable as lignin.