The utility of ergosterol as a bioindicator of fungi in temperate soils

In this paper we evaluate the utility of ergosterol as a measure of fungal biomass in temperate soils. We summarise published findings and compare them with data from our own broad-scale assessment of the relationship between ergosterol and ATP in a range of temperate soils. Two hundred and ninety five plots (three cores taken from each 10 × 10 m plot) in seven ecotypes were sampled. Soils ranged from entirely mineral to entirely organic (0.01–46% Corg) and sites comprised two primary successions, one on shingle ridge on the south coast of England and one in the slack of a dune blow-out on the south coast of Wales, various meadow, pasture (some restored after opencast mining) and ancient woodland soils throughout England and acid forest soils in Central Europe. We found a strong relationship between ergosterol and ATP (r2=0.80), which was largely unaffected by the key soil properties of Corg, C/N ratio, moisture and pH. The sources and implications of the 20% of residual variance were explored by assuming that the error was compounded from three sources: the inaccuracies in methods of analysis of ergosterol and ATP, the failings of each of the variables to estimate their underlying populations (i.e., fungal and total biomass, respectively) (evar), and the non-equivalence of these populations (i.e., their incomplete overlap) (epop). By partitioning the residual variance into components corresponding to the levels of sampling, we estimated that the sum of the systematic portions of evar and epop formed as much as three quarters of the 20% of residual variance in the ATP–ergosterol correlation, leaving just 5% mostly due to random error. Despite this close relationship, the attainment of a universal conversion factor between ergosterol and fungal biomass, applicable to all temperate soils, remains elusive and problematic. Many problems are caused by a lack of comparability between the various measures of fungal and total biomass used and the reliability, or otherwise, of extrapolations based on measures of axenic cultures (in contrast to in-situ measurements). The issue is further complicated by the non-linearity of the relationship between fungal biomass and fungal surface area; ergosterol is more correctly an index of the latter since it is a principal membrane sterol. We conclude that ergosterol is likely to be a reliable indicator of the extent of fungal membranes in temperate soils, if not an accurate measure of fungal biomass.