In 2020, worldwide pesticide use in agriculture was estimated at 2.7 million tons (FAO, 2022). The application of these compounds on the landscape has resulted in the detection and persistence of pesticides in aquatic ecosystems. Even at low concentrations, pesticides can interact with other compounds and represent a serious risk to aquatic and terrestrial organisms. Pesticides that target autotrophs (i.e., herbicides) represent about 48% of the pesticides used globally. Atrazine and S-metolachlor are two herbicides, commonly applied for grain, legume and cereal crop production. Resultantly, these herbicides are frequently detected in nearby aquatic ecosystems with atrazine concentrations reaching upwards of hundreds µg.L− 1 in agricultural regions of the USA (Hansen et al., 2019). S-metolachlor is also commonly applied for corn and soybean production, and can reach concentrations between 5 µg.L− 1 and 50 µg.L− 1 in agricultural regions of Europe (Griffini et al., 1997; Kapsi et al., 2019; Roubeix et al., 2012; Székács et al., 2015; Vryzas et al., 2011), and up to 100 µg.L− 1 in agricultural regions of the USA (Battaglin et al., 2003, 2000).
Atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] is a triazine compound marketed in the late 1950s but was subsequently banned in Europe in 2003 due to its toxicity to humans and aquatic organisms. However, atrazine is still used in several countries worldwide, including Canada and the USA, albeit under increased regulation (e.g., Quebec, see Fortier, 2018). Atrazine is a photosynthesis inhibitor herbicide that binds the D1 protein of photosystem II and blocks electron transport (Vallotton et al., 2008). Blocking electron transport, thus photosynthesis, leads to an imbalance of reactive oxygen species (ROS) causing oxidative stress, lipid peroxidation of cell membranes, and ultimately the senescence of non-crop plant species (de Albuquerque et al., 2020). When present in aquatic ecosystems, atrazine can be harmful for aquatic plants (Gao et al., 2019), micro-algae (Baxter et al., 2016), as well as non-phototrophic organisms such as bacteria (DeLorenzo et al., 1999). Moreover, atrazine is known to cause negative physiological effects to higher level organisms (e.g., amphibians, Hayes et al., 2002), and has been classified as a confirmed or probable endocrine disruptor (Pesticides Action Network (PAN), 2005).
S-metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-[(1S)-2-methoxy-1-methyethyl] acetamide) is an extensively used chloroacetamide herbicide available since the 1990s. S-metolachlor inhibits very long chain fatty acids (VLCFAs) biosynthesis by binding with a synthase involved in fatty acid elongation. VLCFAs are an important component for the well functioning of biological membranes. For example, Böger et al. (2003) found that S-metolachlor inhibited 68% of VLCFAs biosynthesis in the green algae Scenedesmus acutus compared to control. Similarly, Debenest et al. (2009) found that this compound can directly affect cellular density of periphytic diatoms. In addition, S-metolachlor is highly soluble, mobile, can bioaccumulate in non-target organisms (Zemolin et al., 2014), and it is suspected to be an endocrine disruptor for certain fish species (Ou-Yang et al., 2022; Quintaneiro et al., 2017).
Freshwater biofilms or periphyton is a heterogeneous assemblage of algae, bacteria, fungi, archaea and viruses as well as micromeiofauna trapped in a matrix of extracellular polymeric substances that develop on various submerged substrates (Wetzel, 1983). Periphyton is an integral part to the function of aquatic ecosystems and provides services in nutrient cycling. In addition, it is the basal resource of aquatic food webs providing essential compounds such as proteins, lipids and fatty acids needed for the growth and metabolism of higher trophic levels (Thompson et al., 2002). Fatty acids (FAs), in particular, are an important compound transferred along the food chain from prey to consumers (Gladyshev et al., 2011). Polyunsaturated fatty acids (PUFAs) are involved in physiological processes and maintain membrane structure (Huggins et al., 2004). While vegetal cells can synthesize PUFAs de novo, consumers must obtain them through dietary pathways (Brett and Müller-Navarra, 1997). In particular, certain essential FAs such as linoleic acid (LIN; C18:2n6) and α-linoleic acid (ALA; C18:3n3) are almost exclusively produced by vegetal cells; therefore, algae represent an essential source of these molecules for animal consumers (Brett and Müller‐Navarra, 1997). In aquatic ecosystems, long-chain PUFAs (LCPUFAs) such as arachidonic acid (ARA; C20:4n6), eicosapentanoic acid (EPA; C20:5n3) and docosahexanoic acid (DHA; C22:6n3) are also mainly produced by microalgae (Li et al., 2014) and are transferred to consumers with high efficiency (Gladyshev et al., 2011). There is some evidence that herbicides may affect the FAs composition of microalgae by interfering with vegetal lipid metabolism (Demailly et al., 2019; Gonçalves et al., 2021). Herbicides may also induce changes in microorganism community structure of periphyton by selecting for more tolerant species that differ in FA composition (Konschak et al., 2021). For example, diatoms are known to be rich in EPA, while green algae are characterised by high content of ALA and bacteria by C18:1n9, C16:0 and C18:0. Thus, there is considerable risk that herbicides reaching aquatic ecosystems may affect the structure of periphyton assemblages and their FA profiles consequently altering the nutritional quality of this basal resource to higher consumers (Müller-Navarra et al., 2000).
In this study, we conducted a laboratory experiment to (1) determine the effects of atrazine and S-metolachlor on periphyton FA composition and to (2) relate possible modifications in FA profiles to changes in the community structure of autotrophic organisms monitored by biomass measurements. For this purpose, we exposed cultured periphyton in microcosms to either atrazine or S-metolachlor along an environmentally relevant concentration gradient.