Despite indications that S. granulatus and S. luteus release iron-chelating compounds, the exact spectrum of ferric hydroxamates synthesized by these two Suillus species remained unclear. Hence the aim of this study was to identify all of the main siderophores produced by these two ectomycorrhizal fungal species under pure culture conditions. By means of HPLC and LC-MS analyses we show that S. granulatus releases cyclic and linear fusigen, ferrichrome, coprogen and triacetylfusarinine C into the nutrient medium, while S. luteus culture filtrates contain cyclic and linear fusigen, ferricrocin and coprogen. All of the different siderophores were identified on basis of reference compounds and their specific MS spectra which were recorded on a high resolution MS in positive electrospray ionisation mode. Initial HPLC separations were performed on a C-18 stationary phase, using an acidic eluent (0.1% formic acid in water and acetonitrile) in gradient mode. The potential of these two ectomycorrhizal fungal species to produce siderophores representing three different groups of hydroxamates is discussed in relation to its ecological significance.

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A screening for siderophores produced by the ectomycorrhizal fungi Laccaria laccata and Laccaria bicolor in synthetic low iron medium revealed the release of several different hydroxamate siderophores of which four major siderophores could be identified by high resolution mass spectrometry. While ferricrocin, coprogen and triacetylfusarinine C were assigned as well as other known fungal siderophores, a major peak of the siderophore mixture revealed an average molecular mass of 797 for the iron-loaded compound. High resolution mass spectrometry indicated an absolute mass of m/z = 798.30973 ([M + H](+)). With a relative error of Δ = 0.56 ppm this corresponds to linear fusigen (C33H52N6O13Fe; MW = 797.3). The production of large amounts of linear fusigen by these basidiomycetous mycorrhizal fungi may possibly explain the observed suppression of plant pathogenic Fusarium species. For comparative purposes Fusarium roseum was included in this study as a well known producer of cyclic and linear fusigen.