Xenobiotics modify vertical energy transfer from photoautotrophic biofilms to grazers

At the base of stream food webs, photoautotrophic biofilms supply essential polyunsaturated fatty acids (PUFAs) to higher trophic levels. Xenobiotics can alter biofilm structure and function, potentially impairing this vertical interaction. Here we assess how biofilms respond to the herbicide propyzamide and the antibiotic ciprofloxacin, individually and in mixture, and whether effects of these xenobiotics are transferred to higher trophic levels, represented by the grazing snail Potamopyrgus antipodarum. Biofilms were grown in indoor stream microcosms, exposed to xenobiotics for 14 days and subsequently used in a 21-day feeding assay. Biofilm responses were ash-free dry weight, chlorophyll content, and fatty acid (FA) composition, whereas grazer responses were determined as biomass and FA composition as fitness proxies. Xenobiotic exposure had negligible effects on biofilm biomass and chlorophyll content, but FA analyses revealed treatment-specific reductions. In particular, PUFAs decreased by up to 39% under xenobiotic exposure. Correspondingly, grazers biomass was reduced across all xenobiotic treatments. Grazer FA composition differed markedly, with control individuals exhibiting the highest PUFA levels. Propyzamide and the mixture caused the strongest PUFA reductions in grazers (-30% and -45%, respectively), while ciprofloxacin exposure led to increased saturated fatty acids (+25%) and a pronounced PUFA decline (-24%). These results demonstrate that even when biofilm functions appear unaffected, subtle biochemical shifts seem to propagate across trophic levels, reducing consumer growth and modifying energy allocation. Our findings highlight the importance of evaluating direct and indirect effects of xenobiotics when assessing ecological risks in aquatic ecosystems.