Species-specific sensitivity of marine phytoplankton to selected herbicides and antibiotics

The toxicity of two herbicides (diuron and bromacil) and three antibiotics (clarithromycin, azithromycin, and clindamycin) was evaluated for on four marine phytoplankton species: two diatoms, Skeletonema costatum and Chaetoceros lorenzianus, a dinoflagellate, Prorocentrum shikokuense, and a raphidophyte, Heterosigma akashiwo. The 50 % effective concentrations (EC50-μ) for growth of the herbicides (approximately 2.3–24.3 μg L−1) were lower than those of the antibiotics, indicating their higher toxicity. The EC50-μ of diuron was close to its reported environmental concentrations. The EC50-μ values for the antibiotics substantially differed by species, ranging from 19.5 to > 1000 μg L−1, with diatoms showing higher sensitivity than flagellates. Herbicides inhibited the photosynthetic yield (φII) of all tested species at concentrations similar to or lower than those affecting growth, while antibiotics affected φII at higher concentrations. Under high-light conditions, photosynthesis in S. costatum was substantially inhibited by all chemicals except clindamycin, suggesting enhanced chemical toxicity under intense light. Overall, these findings indicate that these herbicides and antibiotics can alter phytoplankton abundance and composition in coastal areas and that environmental factors, such as increased solar radiation, can potentially enhance their toxicity.