The effects of gut-derived Halomonas titanicae on the growth, antioxidant activity, and antimicrobial properties of turbot ( Scophthalmus maximus )

Within the field of sustainable aquaculture, the use of innovative probiotics has gained increasing attention. We previously isolated a marine bacterial strain from the intestine of healthy turbot ( Scophthalmus maximus ) and identified it as Halomonas titanicae HT-Tc3. This study aimed to systematically evaluate the probiotic potential of the bacterium in turbot. Two experimental groups were established: a control group (CK) fed a basal diet, and a treatment group (TC) fed the basal diet supplemented with H. titanicae HT-Tc3. Over a 60-day feeding trial, we assessed the effects of TC supplementation on growth performance, digestive enzyme activity, antioxidant capacity, intestinal histology, and gut microbiota composition. A subsequent challenge test with Aeromonas salmonicida BHAS-1 was conducted to evaluate the long-term protective effects following withdrawal of TC supplementation. The results demonstrated that dietary TC significantly improved growth performance compared with the CK group: on day 60, the final body weight of the TC group was 34.8% higher, and body length was 10.2% higher than those of the CK group (P < 0.05). For antioxidant enzymes, compared with the CK group, the TC group showed 53.4% higher superoxide dismutase (SOD) activity and 82.2% higher alkaline phosphatase (AKP) activity on day 60 (P < 0.05). For digestive enzymes, amylase (AMS), trypsin (TPS), and lipase (LPS) activities in the TC group were increased by 33.3%, 35.0%, and 15.4%, respectively, on day 60, with statistically significant differences in AMS and TPS (P < 0.05), whereas the change in LPS was not significant (P > 0.05). Intestinal morphological parameters, including villus height, villus width, and muscular layer thickness, showed an increasing trend in the TC group compared with the CK group. Furthermore, TC supplementation increased the relative abundance of several potentially beneficial bacterial taxa relative to the CK group. In the challenge assay, the TC group achieved a 16-percentage-point higher survival rate than the CK group (28% vs. 12%) following A. salmonicida infection, demonstrating that the beneficial effects persisted even after cessation of probiotic administration. These findings indicate that H. titanicae HT-Tc3 is a promising novel probiotic candidate for sustainable turbot aquaculture.