This contrasts neutral lipid fatty acids (NLFA), which are storage components, especially in fungi, and give interesting information on the nutritional status of fungi (Bååth 2003). 1991 Joergensen and Emmerling 2006), largely consisting of viable but non-culturable organisms (Zelles 1999). Consequently, the total PLFA concentration is used as an indicator for microbial biomass in soil (Frostegård et al. PLFA are rapidly synthesized during microbial growth and do not accumulate in soil organic matter (Zelles 1999 Zhang et al. Phospholipid fatty acids (PLFA) are the main components of cell membranes in all organisms other than archaea and do not occur in storage components (Zelles 1999). PLFA analysis will still be of interest over the next decades as an important independent control of DNA-based methods. The reliability and accuracy of gas chromatographic separation need to be regularly checked against unintentional variations. Soil handling and extraction conditions should be further optimized. This problem might be intensified by recycling processes during decomposition to an unknown extent. The majority of indicator PLFA are not fully specific for a certain microbial group. In the presence of plants, ergosterol should be used instead of the PLFA 18:2ω6,9 and 18:1ω9 as fungal indicators for Mucoromycotina, Ascomycota, and Basidiomycota. Fungal PLFA should embrace the AMF-specific 16:1ω5. The PLFA 16:1ω5 should be used as an indicator for the biomass of arbuscular mycorrhizal fungi (AMF) as there is no experimental evidence that they occur in marked amounts in Gram-negative bacteria. If the phylum Actinobacteria is excluded from the group of Gram-positive bacteria, all remaining bacteria belong to the bacterial phylum Firmicutes, which should be considered. The straight-chain PLFA 14:0, 15:0, 16:0, and 17:0 occur in all microorganisms, i.e., also in fungi and not only in bacteria. The current opinion and position paper highlights (1) correct assignation of indicator phospholipid fatty acids (PLFA), (2) specificity and recycling of PLFA in microorganisms, and (3) complete extraction and detection of PLFA.

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