13 C pulse labeling indicates differing dynamics for two pools of amino acids in ectomycorrhizal fungi

Background and aims Ectomycorrhizal fungi synthesize carbohydrates and amino acids from recent photosynthate but can also assimilate soil-derived amino acids. The dynamics of different amino acid pools within fungi may differ because of differences in biosynthetic complexity and energetics. Methods To study carbon dynamics between Pinus sylvestris and ectomycorrhizal fungi, 13 CO 2 pulses were traced for 97 days post-labeling into amino acids and structural carbon of Cortinarius semisanguineus and Lactarius rufus sporocarps. Results Without 13 C labeling, amino acid δ 13 C was higher in Cortinarius than Lactarius , which suggested uptake of older, soil-derived amino acids by Cortinarius . In stepwise regression, 13 C labeling in amino acids was 59 ± 2% of that in structural carbon. This first amino acid pool derived from recent plant sugars and peaked 16 days after 13 CO 2 labeling. The other 41% of amino acid carbon peaked on day 30, at 23% of the 13 C enrichment of structural carbon, indicating two amino acid pools differing in 13 C dynamics. Conclusion I hypothesized that amino acids derived from recent plant sugars mostly had simple synthesis pathways (such as alanine, aspartic acid and glutamine), whereas the second pool reflected soil-derived or slow-turnover amino acids (such as lysine, leucine, and isoleucine) with multiple enzymatic steps to synthesis. Studies of amino acid use by ectomycorrhizal fungi should account for differences in dynamics and energetic costs between these two groups of amino acids.