Oxalate regulation by two brown rot fungi decaying oxalate- amended and non-amended wood

Oxalic acid secretion by brown rot wood-degrading fungi has been proposed to function in pH control and non-enzymatic biodegradation. Although oxalate production in liquid cultures of brown rot fungi commonly correlates with glucose oxidation, excess oxalate accumulation in wood during oxidative decay could impede Fe 3+ reduction by fungal-derived chelators and thus inhibit brown rot. In this study, we pre-treated spruce wood with various oxalate concentrations and subjected it to brown rot decay by Fomitopsis pinicola and Meruliporia incrassata in agar- and soil-block trials. In agar-block microcosms containing wood pre-treated with 0, 1, 10 or 100 mM sodium oxalate, test fungi equalized wood oxalate and pH at week 12 of decay by either increasing or reducing wood oxalate, depending on the pre-treatment. Oxalate reductions in wood were not accompanied by increases in agar oxalate. During soil-block decay of wood pre-treated with 0 or 50 mM oxalate, oxalate and pH regulation were time-dependent and more variable. Wood oxalate levels did not increase with increasing fungal biomass (per ergosterol); however, decreases in oxalate were not explained by enhanced oxalate catabolism activity, Ca 2+ import, or translocation of oxalate into the soil. Our results suggest that brown rot fungi may optimize extracellular oxalate during wood decay, and that soil characteristics may influence this dynamic.