Interactive effects of ivermectin and nitrogen levels on phytoplankton biomass and stress responses

Ivermectin (IV), a commonly used antiparasitic drug for livestock and humans, eventually finds its way into aquatic environments, primarily through fecal matter and livestock carcasses. The presence of IV in water bodies may pose a risk to non-target aquatic organisms, such as phytoplankton, whose primary habitat is the aquatic environment. The adverse effects of IV may be exacerbated by its interactions with organic matter and nutrients, such as nitrogen, in aquatic ecosystems. Here, the combined effects of IV with low and replete nitrogen on the biomass and oxidative stress responses of Chlorella vulgaris, Selenastrum capricornutum, and Microcystis flos-aquae were studied. Additionally, the microcystins production of M. flos-aquae under the treatment conditions was also measured. The results show that the interaction between IV and nitrogen levels mostly did not affect the biomass of C. vulgaris, S. capricornutum, and M. flos-aquae. IV interacted with replete nitrogen to increase oxidative stress parameters, and also stimulated the three species to produce higher cell densities. Similarly, the interaction between IV and low nitrogen stressed M. flos-aquae and increased its production of microcystins (MC). These findings provide valuable insights into the complex interactions between IV, nitrogen levels, and phytoplankton physiology, highlighting the adaptive mechanisms used by phytoplankton in response to these conditions.