The sensitivity of Daphnia magna and Daphnia curvirostris to 10 antibacterials and to some of their binary mixtures: a comparison

Daphnia magna is a freshwater cladoceran largely employed in ecotoxicity tests due to its high sensitivity to xenobiotics, relative easiness of culturing, short parthenogenetic cycle and small dimensions. However, various studies have shown that other daphnids may be more sensitive than D.magna to some environmental contaminants. The use of indigenous species in toxicity tests has the advantage of generating ecologically relevant and environmentally realistic data for use in water quality guidelines. Aim of this study was to evaluate the suitability of Daphnia curvirostris for the acute toxicity test usually performed on D.magna, and to compare the sensitivity of the two species toward 10 antibacterials and to some of their binary mixtures. The D. curvirostris clone used in the tests was generated from a specimen collected in Rosolina (RO, coordinate WGS84: 45,138666698153 lat 12,3240131618937 lon), while D. magna was originally obtained from Ecotox LDS S.r.l. (Milano, Italy). Tests were conducted in accordance with the OECD 202 guideline . Daphnids were exposed to scaled concentrations of the following antibacterials: enrofloxacin (EFX), ciprofloxacin(CPX), sulfaguanidine (SGD), sulfadiazine (SDZ), sulfamethazine (SMZ), sulfaquinoxaline (SQO), sulfaclozine (SCZ), sulfamerazine (SMA), sulfadimetoxine (SDM) and trimethoprim (TMP). Furthermore, the toxicity of 4 mixtures was evaluated (TMP+SDZ, TMP+SDM, TMP+SQO and EFX+CPX). The calculated EC50s spanned three orders of magnitude, varying from 4.33 mg L-1 (EFX) to 421,1 mg L-1 (SDM) in D.curvirostris and from 5.21 mg L-1 (SGD) to 379.8 mg L-1 (SCZ) in D.magna. D.curvirostris showed to be more sensitive than D. magna to the assayed compounds, with the exception of SMA, SDZ and SDM. With binary mixtures simple additivity was mainly recorded. It was concluded that D.curvirostris is well suitable for the acute immobilisation test, and that its sensitivity to xenobiotics may often be greater than that of D.magna, probably due to its higher surface/volume quotient; besides, being naturally present in the water ponds of northern Italy, D.curvirostris can give a realistic picture of the local environmental impact of xenobiotics. Tests with D.curvirostris were also useful in confirming that, as already observed in previous experiments with D.magna, binary mixtures of antibacterials usually act in accordance to the expectable and reassuring criterion of simple addition.