Epsilon Open Archive

Abstract

Nutrient uptake by boreal forest trees depends upon the symbiotic ectomycorrhizal (ECM) fungi that colonise more than 95% of the fine roots. In these ecosystems, nitrogen (N) is the most important growth-limiting nutrient for plants, followed by phosphorus (P). In the absence of anthropogenic influences, soil nutrients mainly occur in the form of organic compounds that are usually inaccessible to the plants. However, ECM fungi are able to utilise many of these organic resources via the production of a wide range of enzymes. What is known about the enzymatic capabilities of ECM fungi comes from experiments with a very small number of fast growing, easily culturable species. Very little is known about the functional capabilities of the most frequently occurring taxa that form ECM on fine roots. In this thesis, the functional diversity of ECM fungi in nutrient acquisition was examined by assessing the ability of a wide range of ecologically important ECM fungi to use protein, nitrate and different P sources as nutrients. The ability to use protein by the excretion of extracellular proteases was widespread, which supports the theory that this trait is of considerable significance in the boreal forest. The ability to use mineral N was tested by assessing biomass production by a wide range of fungal isolates when grown on nitrate as the single N source. In addition, the genetic potential to reduce nitrate was examined by screening ECM fungal genomes. All isolates grew on nitrate and the nitrate reductase gene was found to occur widely among ECM forming taxa. All isolates grew on various P sources and differential patterns of nutrient use with respect to orthophosphate, organic P and apatite could be observed. Organic P sources were not necessary for phosphomonoesterase expression and, in addition, this enzyme expression was not correlated to growth rate. The results from this study demonstrate that the enzymatic capabilities of ecologically important ECM fungi are continuously distributed between species rather than discrete. All the isolates had some ability to use all of the examined nutrient sources but the degree to which nutrients were utilised and converted to biomass varied considerably among taxa.