Algal toxicity of antibacterial agents applied in fish farming was investigated. The growth-inhibiting effects of amoxicillin (A), flumequine (F), oxolinic acid (OA), oxytetracycline hydrochloride (OT), sarafloxacin hydrochloride (SF), sulfadiazine (SD), and trimethoprim (T) were investigated by a modified test procedure based on the procedure described in the ISO 8692 (1989) protocol on three algal species: the freshwater cyanobacteria Microcystis aeruginosa, the freshwater green alga Selenastrum capricornutum, and the marine cryptophycean Rhodomonas salina. Algal growth was measured as increased chlorophyll concentration by extraction with ethanol followed by measurement of fluorescence. Results were quantified in terms of growth rates using the Weibull equation to describe the concentration response relationship. M. aeruginosa showed higher sensitivity compared to both R. salina and S. capricornutum, whereas the results for the latter two were more or less identical. The toxicity (EC50 value, mg/L) in decreasing order were A (0.0037), SF (0.015), SD (0.135), F (0.159), OA (0.180), OT (0.207), and T (112) for M. aeruginosa; OT (1.6), OA (10), T (16), F (18), SF (24), SD (403), and A (3108) for R. salina; and OT (4.5), F (5.0), SD (7.8), OA (16), SF (16), T (130), and A (NOEC > 250) for S. capricornutum. Applying this test procedure the toxicity of antibacterial agents, being mono- or polyprotic compounds, may be underestimated because of partitioning between ionized and unionized forms.
