Herbicide Contamination of Freshwater Ecosystems: Impact on Microbial Communities

Human society relies on surface freshwater ecosystems for many goods and services (drinking water, recreational facilities...), which places these systems at the center of a web of ecological, economic and political interests (Wilson & Carpenter, 1999). They are also being subjected to increasing pressure resulting from anthropogenic activities, including contamination by a variety of mineral and organic pollutants. Most of the organic pollutants are herbicides (e.g. Kreuger, 1998; Dorigo et al., 2007), which are used not only in agriculture but also for many other purposes (ranging from domestic use in gardens to maintaining railway tracks weedfree, for example). These herbicides can enter aquatic ecosystems as a result of terrestrial runoff, and to a lesser extent, of direct application and aerial spraying (e.g. Carter, 2000). Microbial communities in freshwater ecosystems are not directly targeted, but these communities are exposed to herbicides and can be directly or indirectly affected by these compounds. For instance, many commercial herbicides act by binding to Photosystem II (PSII), which is a pigment-protein membrane complex (see e.g. Schuler & Rand, 2008). PSII inhibitors have a direct impact on photosynthetic aquatic microorganisms that contain the same PSII apparatus as the terrestrial weeds targeted by these herbicides. In addition to this direct impact on photosynthetic microorganisms, as a result of the strong interactions that occur between all these aquatic microorganisms, herbicides can also have an indirect impact on non-photosynthetic species that are not susceptible to PSII inhibitors. These effects on microbial communities can have a critical impact on the overall functioning of freshwater ecosystems, because prokaryotic and eukaryotic microorganisms are major players in these systems. Indeed, microorganisms contribute to most of the primary production in these systems, which is closely related to the total productivity (Aoki, 2003). They are also involved in nutrient cycling and decomposition, via the microbial loop (see, for example, the review of Fenchel, 2008). In lentic ecosystems (lakes, reservoirs, ponds...), most of these processes (primary production and nutrient cycling/decomposition) occur in the water column, and involve planktonic communities. On the other hand, in lotic ecosystems (rivers, streams...), these processes occur mainly in the biofilms located on the surface of sediments and plants, which involve benthic microbial communities. Depending on the kind of freshwater ecosystem concerned, both these communities must be considered when attempting to evaluate the impact of herbicides on microbial communities.