Evaluation of the health status and histopathology of gills and liver of rainbow trout, Oncorhynchus mykiss, reared ina recirculating aquaculture system with a TiO2-based photo-electrocatalysis filtering technique

The Fish-PhotoCAT project, (PRIMA2019), aims to evaluate the efficiency of a nanotube TiO2-based photo-electrocatalytic (PEC) system on a recirculating aquaculture system (RAS) and its impact on the growth and health of fish reared at different densities and stages of development. This study refers to the low-density trial, where rainbow trout (Oncorhynchus mykiss) weighing 90 g were raised for 30 days at a density of 15 kg/m3 in a RAS. Six 500 L tanks were equipped with the standard water filtration set-up: three tanks of the control groups (CTR), and three of the treated groups (T), where a PEC system was installed together with the classic UV lamp. Water physical and chemical parameters were monitored throughout the trial. At the end of the trial, fish were collected, and gills and liver were sampled for histological analyses as alterations of the structure of these tissues can serve as reliable indicators of the water quality. Furthermore, gene expression analyses of three cytokines (IL-1β, IL-10, and TNF-α) and HSP70 were performed on the gills, and the melanomacrophage centers (MMCs) evaluation was done in the liver, as their number and area are good indicators of environmental conditions. Finally, the presence of TiO2 nanotubes was investigated both in the water by ICP analyses, and in tissues by transmission electron microscopy (TEM). Authorization code: OPBA_20_2020. No significant differences were found regarding NH3 and nitrite concentrations between the experimental groups. The mean concentration of nitrates, however, was significantly higher in the CTR (122.211 mg/L vs. 108.510 mg/L; p<0.001), likely due to the parallel ammonia oxidation to molecular nitrogen performed by the PEC. CTR and T fish exhibited similar specific growth rates and condition factor. No histological differences were found in the structure of the gills between treatments: primary lamellae appeared arranged in double rows and secondary lamellae lined by squamous epithelial cells (Fig. 1A, lamellae, asterisk). The expression of IL-1β, IL-10, TNFα, and HSP70, showed no differences between groups, confirming the histological observations. Liver histological analyses revealed no structural differences between experimental groups (Fig.1B, MMC, arrows). The relative number of MMCs was not statistically different between treatments even if numerically, it was higher in the CTR. The relative area of the MMCs was significantly higher in the CTR (Fig.1C, p<0.01), indicating an activation of the immune response in this group. This difference may be justified by the lower levels of nitrates found in the PEC tanks. Lastly, no TiO2 nanotubes were detected in the water or in the tissues (Fig.1D), implying 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.