Mycorrhizal vs saprotrophic status of fungi: the isotopic evidence

Relative abundances of carbon and nitrogen isotopes in fungal sporocarps ar e 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 Wo ods Creek, OR, USA, and isotopically compared with foliage, litter, soils a nd wood collected from the same site. Possible trophic strategies were then isotopically assessed in archived specimens of the Pezizales of known mole cular phylogeny from the western United States. At Woods Creek, mycorrhizal fungi were 3.5 parts per thousand +/- 0.6 parts per thousand depleted in sigma C-13 and 5.7 parts per thousand +/- 0.4 par ts per thousand enriched in sigma N-15 compared with saprotrophic fungi. By contrast, fungi from four genera of uncertain mycorrhizal status (Clavulin a, Helvella, Otidia, and Ramaria) were only 0.4 parts per thousand +/- 0.4 parts per thousand enriched in sigma C-13 and 1.2 parts per thousand +/- 1. 1 parts per thousand depleted in sigma N-15 relative to mycorrhizal fungi. In archived samples, the sigma C-13 measurements appeared to be a better in dicator of trophic strategy than sigma N-15 measurements. The sigma C-13 me asurements suggested that mycorrhizal or saprotrophic status was conserved within families of the Pezizales (as determined by molecular phylogeny), wi th the Helvellaceae and Tuberaceae mycorrhizal and Discinaceae and Morchell aceae being largely saprotrophic. (C) New Phytologist (2001).