Ergosterol and fatty acids for biomass estimation of mycorrhizal fungi are discussed in a New Phytologist article by Olsson et al. (2003). The findings indicate that arbuscular mycorrhizal fungi (AMF) do not contain ergosterol and thus cannot be assessed using an ergosterol assay. This speaks directly to a recent study in which we used ergosterol analysis to estimate living AMF biomass in the root and soil (Hart & Reader, 2002a,b).
The results of Olsson et al. suggest that our results do not reflect AMF colonization but rather colonization by nonglomalean fungi. We feel that our results do reflect AMF activity for the following reasons. First, our ergosterol measurements were extremely low in nonAMF treatments, thus it is unlikely that there was a high degree of contamination solely in AMF treatments. Second, our ergosterol measurements were almost perfectly correlated with measures of per cent root length colonization (using uniquely AMF structures) and soil hyphal length (Hart & Reader, 2002b). Finally, because our results showed high ergosterol in roots for the Glomaceae and high ergosterol in soil for the Gigasporaceae, the results of Olsson et al. would suggest that contamination in our study mirrored the pattern of AMF colonization. If this were the case, and contaminating fungi were so closely associated with AMF, then ergosterol would still provide a good, if indirect, estimate of AMF activity.
Olsson et al. have shown that, in vitro, two AMF isolates contain little ergosterol. In the future, it will be important to examine more isolates and more species. It remains to be seen how ergosterol production is affected by edaphic conditions because monoxenic cultures do not reflect the way in which AMF behave under natural conditions. More work needs to be done to discover the true origin of ergosterol in such studies. Is the ergosterol contaminant in origin and, if so, how is it so closely linked to AMF colonization? Or do AMF in soil behave very differently to monoxenic cultures?