Beyond Single-Pollutant Toxicity: How Erythromycin–Metolachlor Mixtures Affect the Physiology of Raphidocelis subcapitata

This work aimed to evaluate the combined effects of an antibiotic (erythromycin, ERY) and an herbicide (metolachlor, MET), at environmentally relevant concentrations, on the physiology of the freshwater microalga Raphidocelis subcapitata. For this purpose, firstly, the alga was exposed to each toxic compound to determine the respective effect concentrations after 72 h (72h-EC values). Subsequently, the alga was exposed to mixtures composed of 72h-EC10, 72h-EC25, and 72h-EC50 values of each toxicant, and interaction analysis was conducted using the independent action (IA) model. Overall, the mixtures of ERY and MET inhibited algal growth synergistically. Regarding photosynthetic performance, exposure to the mixtures of toxicants, generally resulted in a reduction of the maximum photochemical efficiency of photosystem II (PSII) (Fv/Fm), the effective photochemical yield of PSII (ΦPSII), and the electron transport rate (ETR). However, MET antagonizes the negative effects of ERY on photosynthesis, particularly at lower concentrations of the antibiotic (72h-EC10 and 72h-EC25). The exposure to ERY–MET mixtures resulted in an overall increase in photosynthetic pigments (chlorophyll a and carotenoids), likely reflecting a compensatory response by the alga to counterbalance impaired photosynthetic function. Nonetheless, overall pigment analysis indicated an antagonistic interaction between the two toxicants, as the magnitude of this increase was lower than predicted by the IA model. The results presented here provide new insights into the toxicological interactions between ERY and MET in microalgae and contribute to addressing the critical knowledge gap regarding the ecotoxicity of pharmaceutical–herbicide mixtures in aquatic environments.