The impact of copper on filamentous fungi and yeasts present in soil

[摘要] ENGLISH ABSTRACT: Numerous workers studied the impact of pollutants and agriculturalchemicals, containing heavy metals such as copper (Cu), on soil microbes. Itwas found that elevated soil Cu levels do have a detrimental effect on soilbacterial populations however the filamentous fungi seemed to be lessaffected. Most of these studies were conducted in soils containing alreadyrelatively high Cu levels and the effect of this heavy metal on the nonfilamentousfungi (i.e. yeasts) was never investigated. The aim of this studywas therefore to determine the impact of elevated Cu levels on filamentousfungi and yeasts occurring in soils containing relatively low natural Cu levels.A synthetic selective medium containing glucose as carbon source, thymineas nitrogen source, vitamins, minerals and chloramphenicol as anti-bacterialagent (TMV-agar), was used to enumerate ascomycetous andbasidiomycetous Cu resistant yeasts in a sample of virgin soil containing ~2ppm Cu. Media that were used to enumerate Cu resistant filamentous fungiwere malt extract agar, malt extract agar with streptomycin sulfate, maltyeast-extract-peptone agar with chloramphenicol and streptomycin sulfate,benomyl–dichloran-streptomycin medium for the enumeration ofhymenomycetous fungi and two selective media for the isolation ofmucoralean fungi. Cu resistant fungi able to grow on all of the abovementioned solid media supplemented with 32 ppm Cu occurred in the soilsample. To obtain an indication of the level of Cu tolerance of fungi present inthis soil sample, a number of fungal isolates were screened for the ability togrow on a series of agar plates, prepared from glucose-glutamate-yeastextract agar, containing increasing concentrations of Cu. It was found thatfilamentous fungi and yeasts that were able to grow on this agar mediumcontaining up to 100 ppm Cu were present in the soil. A series of soilmicrocosms was subsequently prepared from the soil sample byexperimentally contaminating the soil with increasing amounts of copperoxychloride, were after fungal populations in the microcosms, including Curesistant fungi, were monitored using plate counts. At the end of theincubation period, after 245 days, fungal biomass in the microcosms wascompared by determining the concentrations of the fungal sterol, ergosterol, inthe soil. Generally, Cu had little impact on the numbers of filamentous fungalcolony forming units on the plates, as well as on the ergosterol content of thesoil. The numbers of filamentous fungi in the soil, including the Mucoralesand hymenomycetes, seemed to be less affected by the addition of copperoxychloride than the numbers of soil yeasts able to grow on TMV-agar. Thefocus of the next chapter was on the response of yeasts in different soils toelevated levels of Cu in the soil. TMV-agar was used to enumerate yeasts insoil microcosms prepared from four different soil samples, which wereexperimentally treated with copper oxychloride resulting in Cu concentrationsof up to 1000 ppm. The selective medium supplemented with 32 ppm Cu wasused to enumerate Cu resistant yeasts in the microcosms. The resultsshowed that the addition of Cu at concentrations ≥ ~1000 ppm did not have asignificant effect on total yeast numbers in the soil. Furthermore, it was foundthat Cu resistant yeasts were present in all the soil samples regardless of theamount of Cu that the soil was challenged with. At the end of the incubationperiod, yeasts in the microcosms with zero and ~1000 ppm additional Cuwere enumerated, isolated and identified using sequence analyses of theD1/D2 600-650bp region of the large subunit of ribosomal DNA.Hymenomycetous species dominated in the control soil, while higher numbersof the urediniomycetous species were found in the soil that received Cu.These observations suggest that urediniomycetous yeasts may play animportant role in re-establishing overall microbial activity in soils followingperturbations such as the addition of Cu-based fungicides.

[发布机构] Stellenbosch University