Microbiological examination have been carried within the comprehensive hydrobiological investigation of Grosnica reservoir (Serbia, Yugoslavia) during 1997 and 1998. The aim of investigation was to establish the dynamics and fluctuation of specific physiological group. Grosnica reservoir with its microbial features belongs to quality class 1st-2nd according to Kohl. The comparison of bacteria community varies during the year depending of physicochemical features of a environment and the state of other facts in biocenosis. Dominant bacteria are facultative oligotrophic. The larger number of heterotrophic bacteria is in spring time only. Phosphomineralizers and proteolytic bacteria are present in the large number at the beginning of year, amylolytic bacteria are numerable during the summer and autumn. Number of nitrogen fixing bacteria and aerobic cellulolytic bacteria are changed depending on a season, too.

The effect of soil treatment with brominal (a herbicide) and theinsecticide selecron (the equivalent field rates and five-fold) on population counts of bacteria, actinomycetes and celluloyticfungi in soil was tested throughout 10 weeks incubation at28 °C. Also, tested their effect on four soil enzymes:cellulase, acid phosphatase, alkaline phosphatase andarylsulphatase. Bacterial and actinomycetes populations in soiltreated with the two pesticides were promoted at fieldapplication rates and inhibited at higher levels. The twopesticides significantly decreased the total number ofcellulolytic fungi and most fungal species after most incubationperiods either by one or the two used levels but the effect ofselecron was more pronounced.Cellulase activity in soil treated with brominal and selecronwas inhibited after most incubation periods. The effect of soiltreatment with the two pesticides on acid phosphatase waspromotive at field application rates after some incubationperiods but the enzyme activity was delayed at the higherapplication doses. Alkaline phosphatase activity in treated soilwas accelerated with both pesticides even at the higherapplication rates, suggesting a direct role of alkaline soil pHin increasing resistance of alkaline phosphatase to pesticides.The effect of soil treatment with pesticides on arylsulphataseactivity fluctuated between promotion and inhibition, butinhibition was predominant.

Mercury (Hg) methylation was studied in water,sediment and Eichhornia crassipesroots of a freshwater lake, in Rio de Janeiro(Brazil). Samples were incubated with²⁰³HgCl₂ and the Me²⁰³Hg producedwas measured by liquid scintillation.Methylmercury (MeHg) production was <10⁻³% in water, low in sediment (up to5.8%) and high in E. crassipesroots (21–27%). Higher MeHg formation wasfound in aerobic conditions for the roots and inanaerobic conditions for the sediment.Methylation increased with incubation time, upto 5 days. A 3-day incubation period was used inthe majority of the assays, to avoid large scalephysico-chemical changes inside the incubationflasks. Methylation was not detected inheat-sterilized root samples. Sodium sulphatestimulated Hg methylation while sodium molybdateinhibited the process in samples incubated for3, 6, 12, 24, 48 and 72 hr. This suggeststhat sulphidogenic bacteria are responsible forthe methylation process. Experiments with rootsstored at 5 and 25 °C fordifferent periods before incubation, indicatethat methylation is modified by storage time and temperature.