A study of changes in microbial biomass during the decontamination of diesel oil was conducted in five microcosms (M1–M5). The rate of removal of diesel oil varied as follows: M2 (fertilized) 35.63% > M3 (penicillin–streptomycin treatment) 33.0% > M4 (cycloheximide treatment) 31.33% > M5 (treat-ment with a mixture of antibiotics) 23.4% > M1 (the control treatment, where a poison was applied) 14.5%. The fungal count, ergosterol content, biomass C and CO2 efflux were all measured. All of these properties increased gradually in the M2, M3 and M5 samples, and decreased after the begin-ning of the experiment in the M1 and M4 microcosms. Seventeen fungal species were isolated during 105 days of incubation. Species present in abundance were: A. flavus, A. niger, A. flavipes and P. chrysogenum, all of which were high in biomass (33.0, 25.3, 5.0 and 22.7 mg dry weight) and ergosterol content (31.5, 49.3, 19.0 and 23.4 mg/g dry weight) respectively. On average, the ergosterol to microbial biomass ratio was 0.004 in the M2 and M3 microcosms. Significant correlations between ergosterol and microbial biomass C, and between microbial biomass C and CO2, were observed. Selective inhibition methods indicated that both fungal and bacterial respiration increased over three months, except in the M1and M4 microcosms.

Key words: biostimulation, biomass, ergosterol, fungi, microcosms, sandy beaches