Ecological engineering for the bioremediation of aquatic systems : effects of the combined bioturbation and phytoremediation on cadmium and atrazine removal.

The development of efficient bioremediation techniques to reduce pollutant loads in aquatic ecosystems is a challenging research question for ecological engineering. The accuracy of phytoremediation processes has been primarily demonstrated by individual applications on soils or water sediments. The present Ph.D. aims to demonstrate the interest of additional bioturbation combined to phytoremediation processes for the improvement bioremediation efficiency of aquatic sediments. This strategy benefits are tested experimentally in controlled laboratory conditions with a serie of microcosms reproducing each a portion of water/sediment interface such as in wetland areas. In our experiments, bioturbation was carried out by a conveyor-belt invertebrate population, the tubificidae oligochaetes Tubifex tubifex, well known as an active ecological engineer. The phytoremediation was conducted by the riparian plant Typha latifolia known for its ability to remove organic and inorganic pollutants from sediments by accumulation into its biomass. The experiments were managed to demonstrate the effects of this biological influence (plant and inveterbrate) on the mass balances and fluxes of one metal and one pesticide as models of pollutants. Cadmium as a heavy, inorganic and conservative metal pollutant was introduced as a pulse input in the overlying water of the contaminated microcosms, with a cadmium concentration of 20 µg.L-1 in at the initial time of the experiment that lasted one month. In a second experiment, atrazine was mixed in the whole sediment column at the initial time in order to reach a concentration of 5 µg.g-1 of fresh sediment as a source of organic micropollutant and herbicide in the microcosms. The pesticide was radiolabeled with 14C. Fluxes from water to sediment, and from sediment to plants were assessed in experimental conditions with several treatments (+/- plants, +/- bioturbation, +/- pollutants) allowing to demonstrate the effects of the biological influence. Our results indicated that the tubificids and the related bioremediation influences are still efficient under cadmium and atrazine contaminations in aquatic systems. Biotransport due to tubificids changed the distribution of cadmium across the sediment column and enhanced the pumping of cadmium from the water to surface sediment and then to the anoxic underlying sediment surrounding the plant roots. Thereby this biotransport increased the bioaccumulation of cadmium in the root system of Typha latifolia as demonstrated by the roots enrichment coefficients (ECR). Combining Cd contamination and bioturbation optimized the transport of the metal (flux) from sediment to plants with fluxes estimated to 0.02+/-0.00 and 0.07+/-0.03 µg Cd per day without and with bioturbation respectively. In the case of atrazine contamination, bioturbation influence on the chemical properties of sediment (pH, porosity, Organic matter) is explaining the adsorption-desorption behavior of atrazine in sediment, resulting in the acceleration of atrazine mobility and bioavailability of the pesticide. The transfert of pollutant that passed from attached forms onto sediment particles into a free fraction in pore water in the sediment column was enhanced under bioturbation. Consequently higher values of atrazine were measured in the roots system with ECs of 7.16 ± 0.66 with bioturbation compared to 5.99 ± 0.64 in the treatment without bioturbation. The metabolization of the pesticides was also significantly increased under the effects of bioturbation on the sediment microbiome as demonstrated by enhanced number and quantity of metabolites in Typha latifolia roots in this treatments. Our study, therefore, highlights the potential of bioturbation addition of the phytoextraction for integrated bioremediation strategies of metallic and organic polluted sediments of aquatic ecosystems. Further researches need to take into account the bioturbation influence on the microorganism communities in the relationship with organic compounds degradation.