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Theses

Leaf Litter decomposition in streams subjected to global change : the role of heterotrophic microbial communities

Abstract : Human activity through industry, urbanization and agriculture, has led to the production and release of a large amounts of chemical compounds (including pesticides and pharmaceuticals) into the biosphere. One of the problematic related to the xenobiotic compounds fate is their transfer to aquatic ecosystems and the alteration of diversity and activity of microbial communities. Microbial communities associated with immersed leaf-litter can be impacted by these compounds, and in turn, alter global processes such as the carbon and nutrient cycling in the stream ecosystem. Accordingly, this thesis work aims to assess the effects of realistic chemical contamination on microbial leaf-litter decomposition process in streams.The first chapter of this thesis was focused on the comparison of microbial decomposition activity in alder leaves in six watersheds presenting different land uses (agricultural, urbanized, forested) over four seasons (spring, summer, autumn, winter). The effect of the gradient of contamination on microbial organic matter processing from upstream to downstream sections in each watershed was also assessed. Monitoring revealed that microbial decomposition of leaves was slightly higher in contaminated watersheds (agricultural and urbanized) in comparison with control ones (forested), probably because of the compensation effect by nutrients over xenobiotics. However, this compensation mechanism was partial since fungal biomass accumulated in leaves was greatly reduced in contaminated watersheds. Overall, this highlights microbial communities being more efficient for leaf decomposition in polluted watersheds than in the less contaminated ones, which is probably explained by changes in microbial community structure.The second chapter of this thesis aimed to evaluate in vitro the specific interactions between nutrients (nitrogen and phosphorus) and pesticides (herbicide and fungicide, alone or in mixture) exposure on microbial communities during leaf-litter decomposition. High nutrient concentrations (eutrophic conditions) tended to exacerbate the effects of pesticides on leaf decomposition rates suggesting that the compensatory mechanism of nutrients over pesticides observed in the previous part is probably concentration dependent and does not always apply to aquatic microbial communities. Moreover, a stimulation in laccase activity was observed when microbial communities were exposed to the fungicide, suggesting a role of this enzyme in detoxification mechanisms. However, the fact that such stimulation was not observed when exposed to the mixture of both pesticides (herbicide and fungicide) suggest that the interaction between these two molecules impaired the ability of microbial communities to display properstress response. These results constitute the first evidence of the potential interaction between an herbicide and a fungicide on leaf-associated microbial communities functioning. (...)
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Florent Rossi. Leaf Litter decomposition in streams subjected to global change : the role of heterotrophic microbial communities. Ecology, environment. Université Clermont Auvergne, 2018. English. ⟨NNT : 2018CLFAC101⟩. ⟨tel-02895270⟩

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