Mycorrhizal vs saprotrophic status of fungi: the isotopic evidence

Relative abundances of carbon and nitrogen isotopes in fungal sporocarps are useful in assessing mycorrhizal or saprotrophic status, and might provide insights into the evolutionary history of these traits. Sporocarps of known mycorrhizal or saprotrophic genera were collected at Woods Creek, OR, USA, and isotopically compared with foliage, litter, soils and wood collected from the same site. Possible trophic strategies were then isotopically assessed in archived specimens of the Pezizales of known molecular phylogeny from the western United States. At Woods Creek, mycorrhizal fungi were 3.5‰ ± 0.6‰ depleted in δ13C and 5.7‰ ± 0.4‰ enriched in δ15N compared with saprotrophic fungi. By contrast, fungi from four genera of uncertain mycorrhizal status (Clavulina, Helvella, Otidia, and Ramaria) were only 0.4‰ ± 0.4‰ enriched in δ13C and 1.2‰ ± 1.1‰ depleted in δ15N relative to mycorrhizal fungi. In archived samples, the δ13C measurements appeared to be a better indicator of trophic strategy than δ15N measurements. The δ13C measurements suggested that mycorrhizal or saprotrophic status was conserved within families of the Pezizales (as determined by molecular phylogeny), with the Helvellaceae and Tuberaceae mycorrhizal and Discinaceae and Morchellaceae being largely saprotrophic