Interactions between Pesticides and Microorganisms in Freshwater Sediments Toxic Effects and Implications for Bioavailability

Widenfalk, A. 2005. Interactions between pesticides and microorganisms in freshwater sediments – Toxic effects and implications for bioavailability. Doctoral dissertation. ISSN 1652-6880, ISBN 91-576-7022-6. In aquatic ecosystems sediment microbial communities provide many important functions, such as organic matter decomposition and by constituting a major food source for organisms at higher trophic levels. Sediments are also sites were pesticides have been frequently detected. In this thesis, laboratory microcosm experiments on the interactions between pesticides and microorganisms in freshwater sediments were performed. Natural microbial communities were exposed to both environmentally relevant and high concentrations of different pesticides. In short-term exposures, pesticides decreased overall microbial activity at concentrations that are predicted to be environmentally safe. Surprisingly, short-term exposure to the high pesticide concentrations did not always affect the microbial activity. Long-term exposures (one month), were observed to induce different shifts in the microbial community composition, detected by using molecular methods, depending both on the type of pesticide and the concentrations applied. Hence, toxic effects of pesticides in microorganisms are not always straightforward and easy to interpret. The microbial community of an artificial sediment used in toxicity tests was shown to have a low microbial activity, biomass, diversity as well as a different community composition compared to natural sediments. This could have implications for the fate of the test compound and the outcome of the toxicity tests, which may need to be considered when interpreting the toxicity test results. Sediment bacteria and their extracellular polymeric substances (EPS) in biofilms were observed to increase the uptake and bioaccumulation of the hydrophobic pesticide chlorpyrifos to midge larvae, Chironomus riparius. This indicates that microorganisms and EPS may increase the bioavailability and be important vectors for the uptake of sediment-associated contaminants in aquatic food webs. Hence, the quality of the sediment organic carbon may need to be taken in consideration in toxicity tests and risk assessments of these pollutants. This thesis shows that sediment microorganisms can be affected by pesticide exposure and that they can affect pesticide bioavailability. Therefore, the role of microbial processes should be regarded in risk assessments of pesticides.