Synergistic effects of the antibiotic ciprofloxacin and a simulated heatwave on the Baltic Sea dinoflagellate Apocalathium malmogiense

Climate change-driven heatwaves in the Baltic Sea are becoming more frequent and intense, potentially exacerbating phytoplankton blooms that impact biodiversity and ecosystem functioning. Alongside this, chemical pollutants, such as antibiotics, may compound these effects. This study examined the combined impacts of a simulated heatwave (+5 °C) and the antibiotic ciprofloxacin (0.1 μg L^-1) on the dinoflagellate Apocalathium malmogiense. We assessed cell counts, size, growth rates, Chlorophyll-a (Chl-a) content, and nutrient uptake. The simulated heatwave increased growth and Chl-a content but reduced cell size, while ciprofloxacin alone had no effect on growth response parameters. However, the combination of both stressors significantly reduced cell counts (-17 %), Chl-a content (-34 %), and growth rates (-20 %). Ciprofloxacin also decreased nitrogen uptake by over 40 %, exacerbating the nitrogen deficit caused by the heatwave. This study highlights the importance of testing global change stressors in combination, as synergistic effects may otherwise go undetected if only studied in isolation.