T he efficiency of a nanotube TiO2-based photo-electrocatalysis (PEC) on a recirculating aquaculture system (RAS) and its impact on the growth and health of fish reared at different densities and stages of development is the aim of the Fish-PhotoCAT project, (PRIMA2019). Monitoring the histopathology of organs in fish allows to assess the impact of stressors (pollution, toxins, or changes in water quality) on organ systems, providing insights into how environmental changes can affect fish health . This study aimed to evaluate rainbow trout (Oncorhynchus mykiss ) fry health status when reared in a PEC system at low densities. R ainbow trout fry (7.5 g) was reared at 5 kg/m 3 for 21 days in 500 L tanks, in independent RAS . All six tanks were equipped with the standard water filtration setup: in three of the tanks, this constituted the only filtration system (CTR) and in the other three, a PEC system was installed. Water physical and chemical parameters were monitored throughout the trial. At the end of the trial , fish were weighed and measured for total length; the gills and liver were sampled for histological analyses, as the alterations of the structure of these tissues can reflect the water quality . Gene expression analyses of three cytokines (IL-1β, IL-10, and TNF-α), HSP70, and Claudin-30 were performed on the gills, and the expression of enzymes involved in the oxidative stress response was evaluated in the liver , as it is the primary organ involved in detoxification and metabolism of toxic compounds arising from environmental stress . Authorization code: OPBA_20_2020.
The mean concentration of nitrates was significantly higher in the CTR (122.211 mg/L vs. 108.510 mg/L; p<0.001), probably because of the parallel ammonia oxidation to molecular nitrogen performed by the PEC. On the other hand, no significant differences were found regarding NH3 and nitrite concentrations between the experimental groups . All groups exhibited similar weight, length, specific growth rates, and conditions factors. Gills morphology was not affected by the PEC system , as demonstrated by the histologic and histometric results (Fig. 1A and B, respectively) . The cytokines, HSP70, and Claudin-30 expressions showed no differences between groups , confirming the histological observations. No differences were found in the l iver morphology between experimental groups (Fig.1C) . No melano-macrophage centers were present: in rainbow trout, in the liver, these generally begin to appear within the first few weeks of life but only become clearly visible and structurally defined during the juvenile stages, typically within a few months of age. The expression of enzymes involved in the oxidative stress response in the liver showed no differences between groups. No TiO2 nanotubes were detected in these tissues confirming the safety of this innovative PEC system. These are encouraging results that indicate a beneficial effect of the TiO2 -based PEC reactor in a RAS, even if further investigation is needed.