Discrepancies between ergosterol and the phospholipid fatty acid 18:2ω6,9 as biomarkers for fungi in boreal forest soils

Ergosterol and the phospholipid fatty acid (PLFA) 18:2ω6,9 are frequently used as fungal biomarkers in studies on soils, and in accordance with the ideal for biomarkers of microorganisms they are thought to turn over rapidly after cell death and lysis. These biomarkers should also show the same patterns and responses to perturbations of the studied system. Here, I report strong correlations, in natural boreal forests of contrasting fertility, between free ergosterol and PLFA 18:2ω6,9 (r=0.821, P=0.007, n=9). Surprisingly, ergosterol, but not PLFA 18:2ω6,9, appears non-responsive to both large-scale tree girdling, which interrupts tree belowground C allocation to ectomycorrhizal fungi, and to long-term N-loading, which may have negative effects on both mycorrhizal and saprotrophic fungi. These results, therefore, question the use of ergosterol to monitor effects of soil perturbations on fungi in the field, but do not put into question the use of the biomarker in natural forest ecosystems.