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 the m 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 f ive plots (three cores taken from each 10 x 10 m plot) in seven ecotypes we re sampled. Soils ranged from entirely mineral to entirely organic (0.01-46 % C-org) 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 th e south coast of Wales, various meadow, pasture (some restored after openca st mining) and ancient woodland soils throughout England and acid forest so ils in Central Europe. We found a strong relationship between egosterol and ATP (r(2) = 0.80), which was largely unaffected by the key soil properties of C-org, C/N ratio, moisture and pH. The sources and implications of the 20% of residual variance were explored by assuming that the error was compo unded from three sources: the inaccuracies in methods of analysis of ergost erol and ATP, the failings of each of the variables to estimate their under lying populations (i.e., fungal and total biomass, respectively) (e(var)), and the non-equivalence of these populations (i.e., their incomplete overla p) (e(pop)). By partitioning the residual variance into components correspo nding to the levels of sampling, we estimated that the sum of the systemati c portions of e(var) and e(pop) formed as much as three quarters of the 20% of residual variance in the ATP-ergosterol correlation, leaving just 5% mo stly due to random error. Despite this close relationship, the attainment o f a universal conversion factor between ergosterol and fungal biomass, appl icable to all temperate soils, remains elusive and problematic. Many proble ms are caused by a lack of comparability between the various measures of fu ngal and total biomass used and the reliability, or otherwise, of extrapola tions based on measures of axenic cultures (in contrast to in-situ measurem ents). The issue is further complicated by the non-linearity of the relatio nship between fungal biomass and fungal surface area; ergosterol is more co rrectly 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 fung al biomass. (C) 2000 Elsevier Science Ltd. All rights reserved.