The significance of ericoid mycorrhizal mycelium for carbon turnover in heathland

Ericaceous plants (Calluna, Vaccinium etc) are major components of boreal forest, heathlands and peatlands. These biomes contain enormous quantities of carbon and so play a key role in global biogeochemical cycles. In nature, most plants are mycorrhizal and the autotrophs allocate large amounts of photosynthate very rapidly to their fungal partners, and in return the fungi provide host plants with increased quantities of nutrients. Ericaceous plant roots associate with ericoid mycorrhizal fungi but, unlike other mycorrhizal types, virtually nothing is known of their functional importance in the carbon cycle. Because ericaceous plants have low primary productivity, it has been suggested that their fine 'hair' roots and the external mycelium of the ericoid mycorrhizal fungi that colonise them are one of the most important inputs of relatively labile carbon to heathland soils, although this hypothesis remains to be tested. This lack of knowledge represents a major gap in our understanding of soil biodiversity and ecosystem functioning; these are two priority areas of NERC's research strategy. In addition to its likely role as an important carbon source to heathland soils, external ericoid mycorrhizal mycelium also has an active role in regulating nutrient cycles. The nature of litter produced by ericaceous plants is highly recalcitrant and comprises large quantities of phenolic compounds. The fungi forming ericoid mycorrhizas (e.g. Hymenoscyphus ericae) have adapted to these oligotrophic conditions and it is thought that they are crucial components of the microbial biomass for regulating decomposition rates. Simplified laboratory experiments have demonstrated that the breakdown of complex phenolic compounds by H. ericae is facilitated by production of extracellular enzymes, such as laccases, by the external mycelium. While direct evidence for laccase production by ericoid mycorrhizal fungi when grown in symbiosis with a host plant still remains to be obtained, it is thought that such enzymes are released to facilitate nutrient acquisition by the fungal partner from soil organic matter. There is some evidence for considerable diversity in laccases among potentially ericoid mycorrhizal ascomycetes, yet little is known about the genetic diversity, mode of action, and modulation of laccases and other phenol oxidases in such systems. This project will test the hypothesis that ericoid mycorrhizal mycelium has a key role in carbon turnover in heathlands. We will 1) determine rates of turnover of ericoid mycorrhizal mycelium in a range of field conditions, 2) determine the rates of photosynthate transfer from host plant to hair roots and external mycelium in microcosm systems and in the field, 3) determine the potential for different ericoid mycorrhizal fungi to produce laccases and determine how the production of laccases is regulated, and 4) establish if there is a link between carbon supply to ericoid mycorrhizal fungi, laccase gene expression and resource transfer to the host plant. This project will use a range of techniques in routine use in our laboratories including stable and radioisotopes, molecular biology and synthesis of microcosm systems to manipulate combinations of plant and ericoid mycorrhizal fungi. Mycelial turnover will be studied using 13C labelled mycelial material from ericoid mycorrhizal fungi. This will be introduced into microcosms containing plants grown in the mycorrhizal and non-mycorrhizal condition, and into the field in litterbags. The fungal assemblages colonising decomposing carbon sources in the field will be characterised using molecular fingerprinting techniques targeting the ribosomal internal transcribed spacer (ITS) region. The genetic diversity and activity of polyphenol oxidase (PPO) enzymes involved in the degradation of litter will be studied using PPO enzyme activity assays and molecular fingerprinting techniques.