Comparative sensitivity to the fungicide tebuconazole of biofilm and plankton microbial communities in freshwater ecosystems.

Stream and lake ecosystems in agricultural watersheds are exposed to fungicide inputs that can threaten the structure and functioning of aquatic microbial communities. This research analyzes the impact of the triazole fungicide tebuconazole (TBZ) on natural biofilm and plankton microbial communities from sites presenting different degrees of agricultural contamination. Biofilm and plankton communities from less-polluted (LP) and polluted (P) sites were exposed to nominal concentrations of 0 (control), 2 and 20 µg TBZ L(-1) in 3-week microcosm experiments. Descriptors of microbial community structure (bacterial density and chlorophyll-a concentration) and function (bacterial respiration and production and photosynthesis) were analyzed to chart the effects of TBZ and the kinetics of TBZ attenuation in water during the experiments. The results showed TBZ-induced effects on biofilm function (inhibition of substrate-induced respiration and photosynthetic activity), especially in LP-site communities, whereas plankton communities experienced a transitory stimulation of bacterial densities in communities from both LP and P sites. TBZ attenuation was stronger in biofilm (60-75%) than plankton (15-18%) experiments, probably due to greater adsorption on biofilms. The differences between biofilm and plankton responses to TBZ were likely explained by differences in community structure (presence of extracellular polymeric substances (EPS) matrix) and microbial composition. Biofilm communities also exhibited different sensitivity levels according to their in-field pre-exposure to fungicide, with P-site communities demonstrating adaptation capacities to TBZ. This study indicates that TBZ toxicity to non-targeted aquatic microbial communities essentially composed by microalgae and bacteria was moderate, and that its effects varied between stream and lake microbial communities.

Photoreduction of chlorothalonil fungicide on plant leaf models.

Photodegradation is seldom considered at the surface of vegetation after crop spraying. Chlorothalonil, a broad-spectrum foliar fungicide with a very widespread use worldwide, was considered. To represent the waxy upper layer of leaves, tests were performed within thin paraffin wax films or in n-heptane. Laser flash photolysis together with steady-state irradiation in n-heptane allowed the determination of the photodegradation mechanisms Chlorothalonil ability to produce singlet oxygen was measured; noteworthy its efficiency is close to 100%. Additionally, chlorothalonil photodegradation mainly proceeds through reductive dechlorination. In these hydrophobic media, a radical mechanism was evidenced. Photochemical tests on wax films under simulated solar light show that formulated chlorothalonil is more reactive than pure chlorothalonil. The field-extrapolated half-life of photolysis on vegetation was estimated to 5.3 days. This value was compared to the half-lives of penetration and volatilization available in the literature. It appears that chlorothalonil dissipation from crops is ruled by both photodegradation and penetration. The relative importance of the two paths probably depends on meteorological factors and on physicochemical characteristics of the crop leaf cuticle.

Compositional differences between soil humic acids extracted by various methods as evidenced by photosensitizing and electrophoretic properties.

Humic acids (HAs) were isolated from Elliott soil provided by the International Humic Substances Society (1BS102M) by three commonly used methods: (i) 0.1M NaOH (EI-HA), (ii) neutral 0.1M Na(4)P(2)O(7) (L-HA) and subsequent 0.1M NaOH (S-HA), and (iii) 0.1M NaOH+0.1M Na(4)P(2)O(7) (NP-HA). The objective was to evaluate the impact of these extractants on the photosensitizing properties of the isolated HAs. HAs were analyzed for their elemental composition, functional acid groups content, absorption and emission properties, electrophoretic characteristics and ability to produce singlet oxygen using furfuryl alcohol (FFA) as a scavenger. L-HA was slightly more aromatic and oxygenated than the other HAs and contained a higher portion of long-wavelength fluorophores and macromolecules showing low molecular size (MS) and high electrophoretic mobility. L-HA also gave a rate of FFA photooxygenation between 1.25- and 1.6-fold higher than the other HAs. This suggests that the free humic macromolecules ionized at pH 7 and/or weakly bounded on mineral surfaces via cation bridges are of relatively low MS and contribute significantly to the photosensitizing and long-wavelength emitting properties. Differences among the other HAs were more subtle, but the parallel evolution of the reactivity and electrophoretic characteristics was observed. Photochemical and electrophoretic measurements seem to be sensitive indicators to evaluate differences among the extraction procedures of HAs.