Effect of culture conditions on ergosterol as an indicator of biomass in the aquatic hyphomycetes

Ergosterol is a membrane component specific to fungi that can be used to es timate fungal biomass using appropriate factors of conversion. Our objectiv es were to determine the limits of use of ergosterol content as a measure o f biomass for aquatic hyphomycetes, and to evaluate a previously establishe d ergosterol-to-biomass conversion factor. We varied inoculum quality, grow th medium, and degree of shaking of four aquatic hyphomycete species. In cu ltures inoculated with homogenized mycelium, we found a significant effect of shaking condition and culture age on ergosterol content. In liquid cultu res with defined medium, ergosterol content reached 10 to 11 mug/mg of myce lium (dry mass) and varied by factors of 2.2 during exponential growth and 1.3 during stationary phase. The increase in ergosterol content during expo nential phase could be attributed, at least in part, to rapid depletion of glucose. Oxygen availability to internal hyphae within the mycelial mass is also responsible for the differences found between culture conditions. Erg osterol concentration ranged from 0.8 to 1.6 mug/mg in static cultures inoc ulated with agar plugs. Ergosterol content varied by a factor of 4 in two m edia of different richnesses, For different combinations of these parameter s, strong (r(2) = 0.83 to 0.98) and highly significant (P much less than 0. 001) linear relationships between ergosterol and mycelial dry mass (up to 1 10 mg) were observed, Overall, the ergosterol content varied by a factor of 14 (0.8 to 11 mg/g). These results suggest that care must be taken when th e ergosterol content is used to compare data generated in different field e nvironments.