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Wednesday, February 6, 2019 (11:00 am, Paris Time Zone)
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Florent ROSSI (Université Clermont-Auvergne, France)
Leaf litter decomposition in streams subjected to global change: the role of heterotrophic microbial communities
Leaf-litter decomposition is a key mechanism in headwater streams, allowing the transfer of nutrients and energy into the entire food web. However, many streams receive a large variety of chemical compounds, including pesticides and pharmaceuticals that may exercises a high pressure on aquatic organisms, such as heterotrophic microbial communities and their organic matter decomposition activity. In this thesis, different experimental approaches were used to assess the effects of a complex and realistic chemical contamination, including nutrients and xenobiotics, on the structure, biomass and activity (decomposition ability and related extracellular enzyme activities) of microbial communities (fungi and bacteria) colonizing Alnus litter, and their response to cope with this stress.
Firstly, a field experiment was performed on six streams, presenting different gradients of contamination over four seasons. Results showed that nutrients (especially nitrogen) were the main parameter controlling microbial activity, partially compensating the effects of xenobiotics on Alnus microbial decomposition. To go further, a microcosm experiment was performed, testing the effects of nutrients (mesotrophic versus eutrophic) and pesticides (tebuconzaole and s-metolachlor, alone or in mixture) on leaf-microbial decomposition. The interaction between high nutrients and xenobiotics led to more active microbial communities for leaf decomposition and extracellular ligninolytic activities that can be explained by changes in community structure. Moreover, a stimulation in laccase and phenol oxidase activities were 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) suggests that the interaction between these two molecules impaired the ability of microbial communities to display proper stress response. Finally, the resistance and resilience abilities of fungal communities associated with submerged leaf-litter were evaluated in vitro using a translocation experiment between an upstream (less contaminated) and downstream (more contaminated) sites of an agricultural stream. As previously observed, leaf-associated fungal communities appeared sensitive to variations in water chemical contamination, and especially in terms of extracellular laccase activity and fungal community structure. Interestingly, removing water contamination stress through the translocation of microbial communities from the more contaminated to the less contaminated channels resulted in a full recovery in terms of laccase activity and fungal community structure that can be explained by species transfer from the less contaminated to the more contaminated communities. Overall, these results highlight the importance of headwater stream fungal communities in supporting diversity for fungal communities in the more downstream sections.
In conclusion, the results of this thesis work showed that xenobiotics (including pesticides and pharmaceuticals) at environmentally relevant concentrations only have a limited effect on microbial decomposition of leaf-litter in stream ecosystems. Such low effect of the xenobiotics can be explained by the high plasticity and resilience capacity of microbial communities, coupled with their potential to produce extracellular enzymes that can be deployed for stress defense purposes.
Key words: Chemical contamination, nutrients, xenobiotics, microbial communities, leaf-litter, organic matter decomposition, laccase activity.
Thesis supervisors: Joan Artigas and Clarisse Mallet