Oxalate Production and Cation Translocation during Wood Biodegredation by Fungi

Wood biodegradation is primarily caused by Basidiomycetous white or brown rot fungi. White rot fungi are unique in degrading lignin, while brown rot fungi circumvent lignin to degrade holocellulose via iron-dependent oxidative chemistry. Both groups of fungi produce oxalate during wood metabolism, and oxalic acid secretion may promote wood decay by reducing pH, mobilizing iron, detoxifying copper, and immobilizing calcium. The function of oxalate during wood decay remains unclear, however, primarily due to difficulties in extracting bound oxalate and to inconsistencies among analytical techniques. This work aims to improve oxalate quantification during wood biodegradation and to better characterize fungal oxalate production in relation to cation availability. Accurate and repeatable soluble and acid-extractable oxalate quantification was achieved with an improved high-performance liquid chromatography (HPLC) method. This procedure was verified in fungal liquid cultures by demonstrating a decrease in soluble/acid-extractable oxalate ratio with increasing filtrate calcium, due to calcium oxalate crystallization. For wood material, HPLC analyses consistently demonstrated that wood oxalate dynamics could not be inferred from data generated in artificial culture. An agar-block trial also established that several brown rot fungi optimized oxalate levels in wood, unlike in agar, suggesting excess oxalate in wood may impede brown rot, perhaps by limiting iron availability. Effects of iron, as well as aluminum and copper, on brown rot oxalate dynamics were tested in agar-block microcosms containing metallic or hydroxide metal treatments. The effect of calcium on oxalate was similarly tested among several decay fungi, including “dry rot” species theorized to use calcium to neutralize excess oxalate. In both trials, test fungi mobilized cations from treatment sources and enriched decaying wood with the respective cations; however, oxalate and wood weight loss were unaffected in either case. These studies suggest that wood-degrading fungi, notably brown rot species, may regulate oxalate in wood during degradation, but perhaps not simply as a function of Fe, Ca, or Cu availability, as previously theorized. This work has implications on the function of oxalate in wood decay and the role of wood-degrading fungi in forest biogeochemistry, and it provides analytical means for better exploring these dynamics in the future.