Triazine herbicide prometryn impaired trophic link between phytoplankton and zooplankton Brachionus plicatilis by diminishing polyunsaturated fatty acids contents

The intensive use of herbicides, such as prometryn in agricultural and aquaculture, causes increasing environmental risks to aquatic organisms. However, current ecotoxicological studies primarily focus on the direct toxicity to single organisms in freshwater, neglecting their effects on marine plankton, particularly interspecific relationships and nutritional linkages. This study investigated both direct and indirect effects of the triazine herbicide prometryn on a marine zooplankton species, Brachionus plicatilis , through trophic transfer from phytoplankton. Results showed that while prometryn exhibited low direct toxicity to rotifers, exposure to environmentally relevant concentrations (5 and 50 μg/L) significantly suppressed rotifer population growth. This effect was driven by a bottom-up mechanism, in which prometryn reduced both the biomass and nutritional quality of the phytoplankton food sources Phaeodactylum tricornutum and Chlorella pacifica . Prometryn exposure significantly decreased the content of carbohydrates and fatty acids (FA), including saturated (SFA), monounsaturated (MUFA), and polyunsaturated fatty acids (PUFA) in phytoplankton, especially with a severe reduction in the essential eicosapentaenoic acid (EPA, C20:5n-3) observed at 50 μg/L. Consequently, rotifers consuming this nutritionally deficient phytoplankton exhibited reduced ingestion rates, decreased body size, and significantly lower levels of SFA, MUFA, and PUFA. The analysis also revealed a significant positive correlation in PUFA content between phytoplankton and zooplankton within the simplified planktonic food chain. Therefore, this indirect bottom-up effect of prometryn on zooplankton, mediated by affecting phytoplankton and causing a substantial deterioration in their nutritional profile, would likely pose a threat throughout the marine food chain.