| Summary: | Aquaculture facilities are a potential source of antibiotics to the aquatic ecosystems. The presence of these compounds in the environment may have deleterious effects on non-target aquatic organisms such as microalgae, which are often used as biological indicators of pollution. Therefore, the aim of the present study was to evaluate the toxicity induced by chloramphenicol (CHL), florfenicol (FLO) and oxytetracycline (OTC), three antibiotics widely used in aquaculture, on the marine microalgae <i>Tetraselmis suecica</i>, a species also used in aquacultural practices. Toxicity was evaluated taking into account alterations on growth and cellular viability and activity, being these parameters monitored using flow cytometry technique. Results showed that all three antibiotics assayed inhibit growth of <i>T. suecica</i> with 96 h IC50 values of 11.16, 9.03 and 17.25 mg l-1 for CHL, FLO and OTC, respectively. After 24 hours of exposure, the integrity of the cell membrane, related with cellular viability and assessed by propidium iodide staining (PI), was not altered; therefore cells remained viable. However, FLO and OTC were found to significant reduce the metabolic activity at higher concentrations assayed, as indicated the fluorescein diacetate assay (FDA). Since growth inhibition and significant physiological alterations were observed, it can be concluded that T. suecica was sensitive to the three antibiotics tested, thus the use of these antibiotics should be carefully monitored to reduce the potential risk of contamination of the marine environment.
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