Antibiotiques apportés seuls ou en mélange (antibiotique+antibiotique et antibiotique+métaux) dans des sols agricoles – Devenir et impacts sur les microorganismes du sol et leurs activités

Abstract : The antibiotics (ATB) used in human and veterinary medicine are largely excreted and can enter agricultural soils through the spreading of organic waste products (OWP), with unassessed risks on human health and the environment. In addition, these ATBs are most often combined with other organic and / or mineral contaminants. However, very few studies have addressed the fate and effects of ATBs interacting with other contaminants in soils.The objective of the thesis was to better understand the fate of ATB applied to soils alone or in combination with other contaminants, and their impacts on soil microorganisms and their activities.The antibiotics selected were sulfamethoxazole (SMX) and ciprofloxacin (CIP), for their contrasting mode of action and behavior, and N-ac-SMX, the main metabolite of SMX. The chosen metals were copper and zinc, which are known to increase in soils with regular applications of certain OWPs. They were brought together, at an environmental dose, and at a dose 5 times higher. The soils are a control soil that has never received OWP and two soils amended since 1998, by manure or compost green waste and sewage sludge, to test the impact of repeated applications of OWP and of the nature of OWP.Microcosms were incubated under controlled conditions for 156 days. For the study of ATB fate, the soils were treated with 14C-labeled ATBs and the 14C distribution was followed in the mineralized, easily and hardly extractable and non-extractable fractions. The impact of ATBs alone or in combinations on microorganisms and their activities was studied using unlabelled ATBs.The fate of ATBs is controlled by :i) The nature and properties of ATBs: CIP is rapidly and strongly adsorbed in soils and is not mineralized. Sulfonamides are rapidly degraded and mineralized up to 10% after 156 days.The fates of the SMX and N-ac-SMX are almost similar when they are brought alone, the N-ac-SMX being transformed into SMX. Whatever the molecules, the non-extractable residues are major at the end of incubation (> 50%).ii) Previous repeated applications of OWP that enhance adsorption of ATBs and reduce their mineralization.iii) The nature of the OWP : the more stable compost promotes the production of hardly extractable residues, the manure, more degradable, stimulates the mineralization of the sulfonamides and the production of non-extractable residues.iv) The presence of other contaminants, their nature and concentration : although the CIP at environmental doses did not affect the fate of SMX and N-ac-SMX in soils, the influence of metals was measurable and increased with their dose. At low doses, metals decrease the mineralization of SMX and N-ac-SMX by a factor of 2, by promoting the adsorption of ATBs through complexation, in particular with copper. At high doses, metals inhibit the mineralization of sulfonamides, through physicochemical reactions (increased adsorption of ATBs via copper) and certainly biological effects (toxic effects especially of zinc).The sulfonamides had no effect on soil microorganisms and mineralization of carbon and nitrogen when applied alone or as a mixture with ciprofloxacin or metals at environmental doses. But mixtures with the high dose of metals induce toxic effects on microbial biomass and mineralization of carbon. These toxic effects seem mainly due to metals and not to pH or ionic strength indirect effects.Therefore under these experimental conditions, the environmental risks associated with ATBs seem limited in the short term. But it would be necessary to study the effects in the longer term and of cumulative applications.
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Aurore Andriamalala. Antibiotiques apportés seuls ou en mélange (antibiotique+antibiotique et antibiotique+métaux) dans des sols agricoles – Devenir et impacts sur les microorganismes du sol et leurs activités. Ecologie, Environnement. Université Paris-Saclay, 2018. Français. ⟨NNT : 2018SACLA022⟩. ⟨tel-02140610⟩

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