Limnetica 39

Contamination by uranium mining activity may lead to harmful effects on freshwater biota, and can affect the reproduction, activity and diversity of aquatic fungi. Here we investigate uranium inhibition of fungal growth in solid medium, using (1) four species of aquatic hyphomycetes and (2) six strains of Heliscus lugdunensis. We also measured (3) fungal sporulation, (4) fungal biomass and (5) litter decomposition in laboratory microcosms exposed to uranium. The uranium concentration causing 50% growth inhibition (EC₅₀) ranged from 12.5 to 45 mg/l, with Articulospora tetracladia the most sensitive and Varicosporium elodeae the most tolerant species. Strains sampled from reference and uranium polluted waters differed in their tolerance, but the tolerance was independent of the uranium concentration in the streams where fungi were isolated.  The EC₅₀ for the six strains ranged from 9 to 25 mg/l. Sporulation was inhibited in microcosms at uranium concentrations ≥ 1 mg/l, and the minimum concentration inhibiting litter decomposition and biomass standing crop over 24 days was 16 mg/l. Leaf-litter exposed to uranium accumulated the metal up to 89 mg/kg (in 262 mg/l of U). Overall, the amount of uranium in many streams receiving discharges from abandoned or recovered mining sites is high enough to impair fitness of some aquatic hyphomycete species.