Histological and physiological alterations in juvenile turbot (Psetta maxima, L.) exposed to sublethal concentrations of ozone-produced oxidants in ozonated seawater

Ozone is a powerful oxidizing agent and is widely used for disinfection and improvement of water quality in aquaculture plants. However, the generation of ozone-produced oxidants (OPO) during ozonation of seawater may lead to toxicity impacts. To determine adverse effects of continuous, sublethal OPO concentrations, juvenile turbot (Psetta maxima, L.) were exposed to three different OPO concentrations (0.06, 0.10 and 0.15 mg/l) for up to 21 days. Fish were sampled after 1, 7 and 21 days of exposure to cover short-term, intermediate and long-term OPO effects. Gills were analyzed for morphological alterations, hemoglobin and hematocrit were quantified to assess loss in gill functionality. Plasma cortisol was measured as physiologic stress marker. Gill histology revealed significant histopathological alterations with increasing OPO concentration and prolonged times of exposure. However, hemoglobin concentrations were only elevated during short-term exposure at the highest OPO concentration. Hematocrit values did not show any differences between OPO-exposed specimens and the control group. At 0.15 mg/l, plasma cortisol was elevated after 24 h. The results demonstrate that sublethal OPO concentrations of 0.10 and 0.15 mg/l cause histological and physiological alterations in juvenile turbot, characterising ozone-produced oxidants as substantial stressor in recirculation systems. At an OPO concentration of 0.06 mg/l, just slight alterations in the tested parameters were observed suggesting concentrations of ≤ 0.06 mg/l as acceptable OPO levels for the rearing of juvenile turbot.