Biochemistry of ectomycorrhizal fungi: From functional traits to ecosystem processes

Ectomycorrhizal (EM) fungi are a group of cosmopolitan symbiotic soil fungi that colonize the finest roots of tree species and play an essential role in plant nutrition and ecosystem function. These fungi account for the majority of microbial biomass found in forest soils and as a consequence the turnover of this biomass represents a large litter input into carbon and nutrient cycles. The factors that control the decomposition of ectomycorrhizal fungi will strongly control forest litter decomposition as a whole and, thus, ecosystem nutrient and carbon cycling. Unfortunately, our understanding of these factors is poor.;It has often been suggested that chitin, a fungal cell wall polysaccharide, is a recalcitrant compound and thus a major controller of the decomposition of fungal litter. This, however, has not been explicitly examined. In Chapter 2 we examined the role of chitin in the decomposition EM fungal tissues. The study shows that chitin concentrations declined rapidly over the course of decomposition and a significant positive relationship between initial chitin concentration and decomposition was found. Together these results suggest that chitin is a labile compound relative to other compounds found in EM fungal tissue and probably does not explain the differences in decomposability of necromass across EM fungal species.;Melanin is a complex aromatic polymer found in fungal cell walls. Its concentration varies widely across fungal species. In Chapter 3 we hypothesize that variation in melanin concentration across fungal species explains a significant amount of variation in decomposition rate. To test this we examined the decomposition of EM fungal necromass of species with varying melanin concentrations in a comparative experiment. In addition, we manipulated melanin biosynthesis by inhibition in the highly melanized EM fungal species Cenococcum geophilum and examined the effect on decomposition. Melanin concentration of the EM fungal necromass was negatively correlated with percent decomposition after 3 months. The inhibition of melanin in C. geophilum was found to increase the decomposability of its tissues. Together this suggests that melanin is likely a major biochemical control on the decomposition of EM litters and may have significant consequences on C and nutrient cycles in ecosystems.;The highly melanized and common ectomycorrhizal fungus, C. geophilum , is drought tolerant and abundant in water stressed habitats, yet the responsible functional traits have not been identified. In Chapter 4 we examined the role of melanin in the EM fungus C. geophilum under water stress by devising a series of experiments that tested the effect of the melanin biosynthesis inhibition on osmotic and desiccation stress tolerance. Melanin inhibition only had negative effects on growth when C. geophilum isolates were subjected to water stress but not under control conditions. This suggests that melanin production is an important functional trait that contributes to water stress tolerance of this cosmopolitan ectomycorrhizal fungus and, given the results presented in Chapter 3, likely has implications for ecosystem function.;In Chapter 5 I synthesize the research presented herein and place it in the context of prior work in a research review examining the factors influencing the decomposition dynamics of ectomycorrhizal fungal litters.