Initial melanin and nitrogen concentrations control the decomposition of ectomycorrhizal fungal litter

Background/Question/Methods

The turnover of ectomycorrhizal (EM) fungal biomass is now recognized as a large and important input into ecosystem carbon (C) and nutrient cycles. Fungal tissues may decompose differently from plant tissues due to their unique chemical components and are an important key to understanding ecosystem biogeochemical cycles. Melanin is a recalcitrant, complex cell wall polymer commonly found in fungi.  Melanin concentration varies significantly across species, which may contribute to a wide variation in decomposition rates previously observed across different taxa.  In a comparative study we examined decomposition of EM fungal litter of varying melanin concentrations. In addition, we examined the effect of inhibiting melanin biosynthesis on decomposition of litter from the highly melanized ectomycorrhizal fungus Cenococcum geophilum. 

Results/Conclusions

Across species, initial melanin concentration was negatively correlated with % mass loss (P= 0.0003; R²=0.48) while initial N concentration was positively correlated with % mass loss (P=0.0064; R²=0.33). Together, both factors explained 53% of the variation in percent mass loss of the EM fungal litter (Multiple Linear Regression; P=0.0011). When the decomposition rate constants were plotted against the initial melanin : initial N ratios we found a significant nonlinear negative relationship (P< 0.0001; R²=0.74). Finally, when melanin synthesis was inhibited in C. geophilum, it decomposed significantly faster than the controls (ANOVA; P=0.0061).  Together, these results suggest that melanin may control decomposition of EM fungal litter as does lignin in plant material, and thus may have significant consequences for terrestrial biogeochemical cycling.