Fatty acid 16:1 omega 5 as a proxy for arbuscular mycorrhizal fungal biomass: current challenges and ways forward

Fatty acid biomarkers have emerged as a useful tool to quantify biomass of various microbial groups. Here we focus on the frequent use of the fatty acid 16:1 omega 5 as a biomarker for arbuscular mycorrhizal (AM) fungi in soils. We highlight some issues with current applications of this method and use several examples from the literature to show that the phospholipid fatty acid (PLFA) 16:1 omega 5 can occur in high concentrations in soils where actively growing AM fungi are absent. Unless the study includes a control where the contribution of other microbes can be estimated, we advocate for the use of the neutral lipid fatty acid (NLFA) 16:1 omega 5. This biomarker has higher specificity, is more responsive to shifts in AM fungal biomass, and quantification can be conducted along with PLFA analysis without doubling analytical efforts. We conclude by contrasting various methods used to measure AM fungal biomass in soil and highlight future research needs to optimize fatty acid analyses.