Determination of soil fungal biomass from soil ergosterol analyses

Determination of fungal biomass (FB) and FB-carbon (FB-C) from soil ergoste rol concentration is difficult because of unknown ergosterol-to-fungal biom ass (E-to-FB) conversion factors and inefficient ergosterol extraction meth ods. We applied a microwave-assisted extraction (MAE) and high performance liquid chromatographic (HPLC) procedure to measure ergosterol in soil sampl es. The E-to-FB conversion factors were determined in six species of fungi grown in vitro. The MAE method was fast and extracted up to nine times more soil ergosterol than a classical refluxing saponification method. Soil erg osterol was separated and quantified rapidly (<10 min) by HPLC, Alternaria alternata, Chaetomium globosum, Fusarium oxysporum, Penicillum chrysogenum, Rhizopus stolonifer and Trichoderma harzianum isolated from soil and plant matrices were grown in batch. Ergosterol and biomass content were determin ed in mycelia harvested during the stationary and exponential phases of gro wth. Total mycelial ergosterol ranged from 180 to 2178 mu g, and total dry biomass ranged from 17 to 595 mg. Total ergosterol and fungal dry biomass w ere strongly associated (r(2) = 0.95). The C content in mycelial mats avera ged 43% (+/-1.1, SD), and was similar among fungal species and growth phase s. The analyses of variance showed that the E-to-FB ratio was similar among fungal species or growth phase. An average ergosterol concentration of 4 m u g mg(-1) dry biomass was determined for the six species of fungi, which g ave a conversion factor of 250 mu g dry biomass mu g(-1) ergosterol. The MA E method recovered an average of 62% (+/-11%, SD) of the ergosterol added i n mycelial mats to soils prior to extraction, and its recovery was independ ent of soil properties. The E-to-FB ratio and percent recovery of mycelial ergosterol helped establish for the first time relationships determining so il FB and FB-C from soil ergosterol concentration. The amount of FB ranged from 155 to 4745 mu g g(-1) and that for FB-C ranged from 67 to 2040 mu g g (-1) for different soils, and was higher in samples taken from native undis turbed land than in samples taken from adjacent cultivated fields. Measurem ent of soil ergosterol concentration is a useful estimate content of the li ving soil FB. (C) 2000 Elsevier Science Ltd. All rights reserved.