Fluopyram SDHI pesticide alters fish physiology and behaviour despite low in vitro effects on mitochondria

Pollution from pesticides is an increasing concern for human health and biodiversity conservation. However, there is lack of knowledge about some emerging molecules such as SDHI fungicides (succinate dehydrogenase inhibitors) that are widely used but potentially highly toxic for vertebrates. Boscalid, fluopyram, and bixafen are 3 frequent SDHI molecules commonly detected in surface waters, which may pose risks to aquatic species. This study aimed to (1) test the in vitro effects of SDHI on mitochondrial activities (inhibition of succinate dehydrogenase SDH, also named respiratory chain complex II) and (2) assess the in vivo effects of sublethal SDHI concentrations on fish physiology and behaviour over 96 hours of exposure, using Carassius auratus fish as a model species. Results show that bixafen and boscalid inhibited complex II activities in vitro as expected (bixafen > boscalid), while fluopyram had no in vitro effects. In contrast, in vivo studies showed that fluopyram strongly altered fish behaviour (enhanced activity, social and feeding behaviours), likely explained by reduced AChE enzymatic activity. In addition, fluopyram increased muscle lipid content, suggesting metabolic disruption. These findings raise serious concern about the toxic effects of SDHI pesticides, especially fluopyram, although its underpinning molecular mechanisms remain to be explored. We thus encourage further research on the long-term impacts of SDHI pesticides to improve existing regulation and prevent adverse effects on wildlife.