Nutrient uptake by boreal forest trees depends upon the symbiotic ectomycorrhizal fungi that colonise more than 95% of the fine roots. These fungi are often considered as a single functional group in ecological studies despite being derived from multiple evolutionary lineages. The limiting nutrients for plant growth in these ecosystems are N and P with the majority sequestered in organic compounds, with levels of mineral N usually only becoming significant due to anthropogenic influences. The enzymatic capabilities of the majority of these fungi to utilise organic nutrients are largely unknown due to problems with their culturability. We have investigated the abilities of a wide taxonomic range of ectomycorrhizal fungi to use protein, three different P sources and nitrate as nutrient sources. To evaluate protein utilisation, we used milk powder plates where protein degradation was indicated by cleared zones around the fungal colony. P use was assessed by growing fungi on orthophosphate, organic P and apatite. The production of phosphomonoesterases was then investigated using an overlay gel with a fluorescing substrate. Nitrate utilisation was compared between isolates by growing them on nitrate as the single N source and, in addition, screening genomes for the nitrate reductase gene.
Results/Conclusions
All the isolates in the study could use protein by the excretion of extracellular proteases and were also able to use nitrate as sole N source. In addition, the nitrate reductase gene was found to occur widely among ectomycorrhizal fungi. All isolates grew on various P sources although different patterns of use of orthophosphate, organic P and apatite were observed. The results demonstrate that enzymatic capabilities of ectomycorrhizal fungi are continuously distributed among species but that abilities vary greatly between species. Our investigations therefore support the view that ectomycorrhizal fungi share many attributes and that a unifying concept of a functional group could be applicable. However, it is also very clear that the expression of traits is highly variable among species indicating limited complementarity. These new data showing the continuous rather than discrete distribution of traits among ectomycorrhizal fungi also raises questions concerning attempts to allocate ectomycorrhizal taxa to ecological groups (protein/ non-protein fungi) depending on functional traits.
