Current knowledge on the fish gut microbiota has largely been obtained from experiments on laboratory-reared animals. Here, the crucian carp (Carassius auratus) with a mean weight of 159.9 ± 11.4 g (mean ± SD) were collected from their natural habitats (i.e., Wuhu lake and Poyang lake, China), and the gut microbiota were analysed by using the next-generation sequencing of 16S rRNA gene. We obtained more than 430,000 high-quality reads, which constituted more than 1200 operational taxonomy units (OTUs), revealing extremely diverse microbes in the fish gut. Proteobacteria, Fusobacteria, Bacteroidetes and Firmicutes were detected as the prominent phyla (each > 1% of total abundance) within the gut microbiota, regardless of the host habitat or the gut segment (i.e., foregut vs. hindgut). Although the microbes in the hindgut were more diverse (OTU number, Shannon and Chao1; One-way Anova, p > 0.05) than in the foregut, the host habitat had a significant role in shaping the community structures (MRPP, ANOSIM, PERMANOVA, p < 0.01). Interestingly, we also detected a set of common OTUs, whereby genera Aeromonas and Cetobacterium might comprise the core gut microbiota of crucian carp.

Humic acid (HA), prevalent in natural water bodies, has the potential to form carcinogenic disinfection byproducts (DBPs), highlighting the necessity for efficient removal approaches. This study aimed to develop a stable and high-performance catalyst to enhance HA removal efficiency. An optimal cerium-manganese-modified material (CM-AA) was synthesized via impregnation onto activated alumina (AA). Its innovation lies in the first-time utilization of Mn-Ce synergistic effects, combined with ozone (O₃) and hydrogen peroxide (H₂O₂) to construct a ternary catalytic system. Preparation parameters were optimized through orthogonal experiments, and the material was characterized using SEM, EDS, and XPS techniques. The results demonstrated that Mn and Ce were highly dispersed on the surface of AA, providing abundant active sites. CM-AA exhibited excellent recyclability: after 12 cycles of use, the HA removal rate remained above 90%, which was superior to most reported single-metal catalysts. The CM-AA/O₃/H₂O₂ system outperformed binary combinations, achieving a HA removal rate of 95.5% under optimal conditions (10 mg/L HA, 10 mg/L H₂O₂, 4 mg/L CM-AA, 20 min O₃ exposure). The adsorption of HA by CM-AA followed pseudo-second-order kinetics and the Freundlich model, indicating a predominantly physical adsorption mechanism. This study presents a low-cost and efficient bimetallic catalyst, along with a multi-component synergy strategy to enhance oxidation performance. The system shows promising application prospects in drinking water treatment and the purification of HA-polluted wastewater, though its applicability in real water matrices and large-scale application feasibility require further investigation.