Diuron sorption isotherms in freshwater biofilms

Abstract

Biofilms are excellent bioindicators for water quality assessment because of their ability to integrate contamination, and their position at the base of the trophic chain in aquatic environments. Pesticides are ubiquitous in aquatic environments and can constantly interact with aquatic organisms, including those that make up biofilms, at fluctuating concentrations. The aim of this study was to describe pesticide behaviour in biofilms. Previous research highlighted that contaminant sorption was not always linear, but no study considered organic bioaccumulation isotherms and toxic impacts to biofilms concurrently. In order to characterize pesticide sorption isotherms in biofilms and the mechanisms involved in the uptake process, we simultaneously assessed bioaccumulation and toxic impact of diuron (a photosynthesis inhibiting herbicide) at the water-biofilm concentrations equilibrium. Mature biofilms grown on glass slides during one month were subsequently exposed in channels to 7 increasing concentrations of diuron from 1 to 500 μg·L^-1, plus a control condition, for 2 h with a flow velocity of 2 cm·s^-1. Then, a Langmuir isotherm equation was fitted to the bioaccumulation data, and an E_max model to toxic impact results. This study established that diuron bioaccumulation in biofilm is nonlinear, and allowed to calculate the Langmuir constant and maximal concentration of diuron potentially accumulated in biofilm (up to 17,771 μg·g^-1). In turn, we found that photosynthetic inhibition followed classical dose-response patterns with diuron concentrations in the water, and that EC₅₀ could be established at 75 μg·L^-1. A continuous diffusion phenomenon was thus demonstrated but it was not linearly correlated to bioaccumulation, highlighting complex uptake mechanisms operating within the matrix. The coupling of toxicokinetic and toxicodynamic approaches provided original information about pesticide behaviour and impact in periphytic microorganisms.

Keywords: Adsorption isotherms; Bioaccumulation; Pesticides; Photosynthesis inhibition; TK-TD.