Ectomycorrhizal fungi are ubiquitous symbionts in many terrestrial systems. Functional aspects of the fungal symbiont have been relatively neglected compared to the tremendous explosion of information on fungal community composition brought on by genetic techniques of the last 20 years. Fungal functionality is necessarily linked to how fungi interact with the soil environment.  Here, I propose that the hydrophobicity or hydrophilicity of the fungal sheath covering the fine roots of host plants (ectomycorrhizae) is a key characteristic that is linked to a suite of important functional attributes or biological markers. These attributes include fungal carbon demand, enzymatic capabilities, exploration strategy, capabilities for long-distance transport, growth on oligopeptides, and ability to acquire insoluble or soluble nutrients.  One example of a biological marker would be nitrogen isotope patterns in ectomycorrhizal fungi, which have been correlated with proteolytic capabilities and with the partitioning of nitrogen between plants and fungal symbionts.  I compare the hydrophobicity against other functional attributes or fungal characteristics as reported in the literature, with a focus on carbon and nitrogen dynamics of ectomycorrhizal fungi.

Results/Conclusions Ectomycorrhizal fungi with hydrophobic mycorrhizae are more important at low nitrogen availability than at high nitrogen availability.  These fungi are also more sensitive to nitrogen deposition and more likely to have proteolytic capabilities than hydrophilic mycorrhizae.  Nitrogen isotopes are clearly differentiated between hydrophobic and hydrophilic mycorrhizal fungi, with hydrophobic fungi averaging 3-4‰ higher in ¹⁵N than hydrophilic mycorrhizae. Taken together, these patterns suggest two main ectomycorrhizal strategies for growth and nitrogen acquisition, one focusing on uptake of labile nitrogen forms such as amino acids, ammonium, and nitrate, and one focusing on patchily distributed resources. This second strategy requires hydrophobic rhizomorphs to prevent leakage of solutes during long-distance transport. Such long distance exploration cannot rely on labile substrates under conditions of low nutrient availability, as such substrates are too scarce (e.g., free amino acids) to make the exploratory investment worthwhile. Therefore, this strategy also requires hydrolytic capabilities to facilitate access to insoluble substances such as proteins. The strong correlation between mycorrhizal hydrophobicity and a range of important functional attributes, combined with the high taxonomic fidelity within taxa of hydrophobic status, suggests that this marker could prove useful in bridging the current gap between community-scale, functional, and ecosystem-scale views of mycorrhizal fungi.