Effects of the carotenoid inhibiting herbicide diflufenican on the photosynthesis of benthic algae

Sammanfattning: Throughout the world, the large-scale use of pesticides in agriculture is an area of concern due to known harmful effects on human and environmental health. Mixtures of different pesticides are found in lakes and streams, where their presence endangers aquatic organisms, like benthic algae, and the ecosystem services they provide. Benthic algae are important primary producers and chemical modulators. Since they are photoautotrophs, they are especially threatened by photosynthesis inhibiting herbicides. Studies have shown that exposure to herbicides causes toxic effects on algae, although most chemical risk assessments are performed on planktonic algae. The aim of this study was to evaluate the effects of the herbicide diflufenican (2',4'-difluoro-2-(α,α,α-trifluoro-m-tolyloxy)nicotinanilide) on the photosynthetic efficiency of benthic algal communities. The choice of herbicide is based on the high prevalence of diflufenican above its water quality standard in Swedish surface waters as well as its known toxicity to aquatic organisms. Diflufenican inhibits the synthesis of carotenoids in weeds, which causes photo-bleaching of green tissues due to oxidative degradation of chlorophylls, the main light-harvesting pigments. The hypothesis of this study was that diflufenican would diminish the photosynthetic efficiency of the benthic algal community. The study was conducted using benthic algae grown for six weeks on tiles submerged in Lake Erken (59˚50’N, 18˚35’E). The algae were then exposed to diflufenican for twelve days. To evaluate the effects on photosynthesis throughout the experiment, chlorophyll fluorescence was measured with a Pulse Amplitude Modulated Fluorometer (PAM-FMS 1, Hansatech®). In contrast to my expectations, the photosynthetic efficiency of the algae exposed to the highest test concentration (10 μg/L) increased to a level above the control. Interestingly, the biomass did not seem to differ between the treatments and the control. These results indicate that diflufenican might have induced synthesis of more chlorophylls as a way to increase energy attainment to be able to cope with the toxic stress; a mechanism called the greening effect. However, diflufenican does not affect the existing pool of carotenoids, thus it only affects the growing parts of weeds. Since the algal biomass did not increase over the course of this study, the existing carotenoids might have been sufficient, at least for some of the algae. This could explain the lack of difference in photosynthetic efficiency between the lower treatments and the control. Additionally, it has been shown that diflufenican also inhibits synthesis of fatty acids in plants. This mechanism might have contributed to the induced greening effect despite the lack of growth. Further analysis will be performed to evaluate the effects on species composition and the pigment content, which will bring more insights regarding the effects of diflufenican on the community of benthic algae.