Fish-associated antibiotic resistance genes (ARGs) have attracted increasing attention due to their potential risks to human beings via the food chain. However, data are scarce regarding the antibiotic resistance in fish themselves. Herein, the antibiotic resistance genes (ARGs) were assessed in the gut of four major Chinese freshwater carp (i.e., silver carp, grass carp, bighead carp, and crucian carp) from food retail markets. Results show that the abundances of target ARGs (e.g., tetA, tetO, tetQ, tetW, sulI, sulII, and blaTEM₋₁) and class 1 integrase (intI1) were in the range 9.4 × 10⁻⁶ - 1.6 × 10⁻¹ and 6.7 × 10⁻⁵ - 5.2 × 10⁻² gene copies per 16S rRNA gene, respectively. The sulI, sulII, and tetQ strongly correlated with silver and mercury resistance genes (e.g., silE and merR). The microbial taxa of fish gut could be partly separated among retail markets based on the PCA analysis. About 15.0% of the OTUs in fish gut were shared and 74.5% of the shared OTUs were identified as Acidobacteria, Bacteroidetes, Chloroflexi, Cyanobacteria, Firmicutes, and Proteobacteria. These phyla may constitute the core microbiota in the guts of the four Chinese freshwater carp. The possible ARG hosts were revealed based on the network analysis, and the presence of pathogen-associated resistant genera in fish gut highlights the need to fully understand their potential human health risks.

A combination of previous studies and the present study indicated species-specific debromination of polybromodiphenyl ethers (PBDEs) in teleost fish. Three situations of debromination were found, namely rapid debromination represented by debromination of BDE 99 to BDE 47 observed in common carp, tilapia, crucian carp, and oscar fish; slow debromination represented by debromination of BDE 99 to BDE 49 observed in the abovementioned fish and rainbow trout, salmon, and snakehead; and no or minor debromination observed in catfish. The results of experiments on cofactors, inhibitors, and substrate competitors indicated that activities of outer ring deiodinase of 3, 3′, 5′-triiodothyronine (type I deiodinase), which cannot be inhibited by 6-propyl-2-thiouracil, were responsible for the rapid debromination, and the outer ring deiodinase of thyroxine (type II deiodinase) regulated the slow debromination. The debromination of BDE 99 to BDE 49 was more common, but occurred at a much lower rate (approximately 100 times lower) than the debromination of BDE 99 to BDE 47. This was because the activity of type II deiodinase was nearly two orders of magnitude lower than that of type I deiodinase in the fish species studied. Further studies on debromination of PBDEs and properties of deiodinase in more species are needed to confirm the hypothesis.

The concentrations, distributions, and bioaccumulation of nine organophosphate flame retardants (OPFRs) were investigated in both abiotic and biotic media, comprising river water, sediment, and crucian carp. The highest concentrations were observed in liver (6.22–18.1 ng/g ww), and the levels in muscle (4.23–7.75 ng/g ww) and gonad (3.08–7.70 ng/g ww) were similar. In whole blood, tris(2-butoxyethyl) phosphate (TBOEP; 31.1–256 ng/mL) accounted for 90% of the total OPFR concentration. Distributions of OPFRs differed between biotic and abiotic media, as tris(2-chloroethyl) phosphate (TCEP), tris(1-chloro-2-propyl) phosphate (TCIPP), and TBOEP were dominant in abiotic media, whereas triethyl phosphate (TEP), tri-n-butyl phosphate (TNBP), TCEP, and TBOEP dominated in crucian carp. The TNBP had remarkable accumulation potential among nine OPFRs, which the TNBP concentrations in muscle increased with increased total length and body weight. The higher perfusion rate of TNBP to female eggs were observed rather than to male gonads as the concentrations were higher in males than in females, while the opposite results were observed in gonad. Moreover, the concentration of TNBP in female muscle began to decrease near maximum growth as a sexually dimorphic difference in crucian carp. This is the first study to simultaneously investigate the fate of OPFRs in biotic and abiotic media and to show sex differences.

Organophosphate flame retardants (PFRs) can be rapidly metabolized in the body, and recent studies have shown that the di-alkyl phosphates (DAPs) are important metabolites. The accumulation and distribution of 8 PFRs and their 4 DAPs metabolites were first investigated in whole-body samples and various tissues of three freshwater fish species (topmouth gudgeon, crucian carp and loach) with different feeding habits from locations around Beijing, China. Concentrations of ΣPFRs in whole-body samples across all sampling locations ranged from 264.7 to 1973 ng g−1 lipid weight (lw), while all the paired DAP metabolites were detected in the total range from 35.3 to 510 ng g−1 lw. The calculated log bioconcentration factors (BCFs) of PFRs in whole fish were correlated with their log KOW (P < 0.05). The metabolite/parent ratios (MPRs) of ΣDAPs were calculated and ranged from 0.10 to 1.12 in whole-fish of all species. The MPRs of BBOEP/TBOEP were the highest. With respect to their distribution in different tissues, both the parent PFRs and metabolites were found at relatively higher levels in the liver than in other tissues (muscle, intestine, kidney and ovary), which was markedly different from those observed in avian species in previous studies. The accumulation of PFRs and DAPs in various tissues was not significantly correlated with the lipid content. The highest PFRs level in the liver may be related to the active hepatic accumulation processes. Meanwhile, the MPRs for all 4 pairs were the highest in the kidney relative to the other tissues. To the best of our knowledge, this is the first study of DAPs in the wild animals, and our study may improve the understanding of the accumulation and metabolism of PFRs in the body.

Our previous study in 2011 reported the detection of BPA and PFAAs in 20 species of marine and freshwater fishes. With an emerging evidence to suggest the metabolic-disrupting effects of BPA/PFAAs in animals, the present study was aimed to provide a time-trend analysis to determine the current concentrations of PFAAs and BPA in 20 commercially available Hong Kong species of fishes. Since the manufacture and use of BPA is being prohibited in most nations, the introduction of BPA alternatives has recently been incorporated in the markets. Therefore, the concentrations of BPB, BPF and BPS were determined. In the present study, all freshwater and seawater fish samples showed quantified concentrations [>Limit of Quantification (LOQ<0.5ng/g)] of BPA. BPF was detected in some marine (yellow seafin, bigeye, goldspotted rabbitfish, snubnose pompano, tongue sole, Bleeker's grouper and orange-spotted grouper) and freshwater fishes (mud carp, crucian carp, tilapia, catfish, mandarin fish, grass carp, grey mullet and spotted snakehead). Two of the compounds, BPS and BPB could only be identified in the marine fishes (snubnose pompano, yellow seafin). In PFAA analysis, PFOA, PFDA, PFOS, PFUdA and PFDoA were found in most of the marine and freshwater fishes. PFOS and PFOA were shown to be the two predominant PFAAs in fishes. On the basis of the measured concentrations of bisphenols, BPs (BPA, BPB, BPF, BPS) and PFAAs, the average daily intake for BPs (20.5–31.5ng/kgb.w./day) and PFAAs (1.17–1.83ng/kgb.w./day) were calculated and found to be lower than values of tolerable daily intake (TDI) established in Europe. However, as compared with our previous study in 2011, the present study revealed an approximate 10-fold increase in the concentrations of BPA in the fish samples. Although the hazard ratio of consuming fishes for BPA and PFAA exposure is expected to remain low, possible additive metabolic-disrupting effect of BPA and its analogues as well PFAAs should be taken into consideration for human health risk assessment.

Much more attention has been poured into microplastic pollution in freshwater systems recently. In the present study, the pollution of microplastics (MPs) in surface water and freshwater fish (crucian carp, etc.) were investigated from Gehu Lake, which is the second largest lake in southern Jiangsu after Taihu Lake. The result manifested that the average abundance of MPs was respectively 6.33±2.67 n/L for surface water and 10.7 items per individual for freshwater fish. The distribution of MPs in Gehu Lake varied from place to place, with the highest abundance of MPs was observed in the two estuaries of the eastern part of the lake. It was speculated that topographical factors and human factors were the main factors affecting the abundance and distribution of MPs. Transparent fibers were the main type of MPs in water samples, accounting for 69.70% of all detected particles. Meanwhile, most of the MPs ingested by freshwater fish were fibers, and the main colors were transparent and blue. In addition, the dominant size of the MPs was between 0.1 to 0.5 mm in water and fish samples. Moreover, PES, man-made fiber, and PP were the dominant polymer types in the surface water and fish samples. The results of this investigation can provide basic data for the research and management of MPs in freshwater systems.

The accumulation of ammonia and nitrite in natural water and aquaculture systems would suppresses the immune system of aquatic animal and reduces the fish growth. Nitrifying bacteria have been widely used to reduce the accumulation of ammonia and nitrite in aquaculture systems, but are still ineffective in many cases. An aquaculture model system consisting of red crucian carp, algae, nitrifying bacteria, and pond water from a natural fish culture was established to explore the limitation of algae and light to nitrifying bacteria content and bacterial nitrification in the presence of a predator. The concentrations of nitrifying bacteria and bacterial nitrification in the group containing algae and light were significantly limited, and addition of nitrifying bacteria in algae groups had little effect. In algae-free groups, the concentrations of ammonia and nitrite were decreased by nitrifying bacteria, and the potential ammonia oxidization rate was also increased. Our findings reveal that the combined effects of algae growth and light exposure are responsible for the observed ineffectiveness of nitrifying bacteria in natural aquaculture environments.

Microbial remediation, especially the application of probiotics, has recently gained popularity in improving water quality and maintaining aquatic animal health. The efficacy and mechanism of mixed Bacillus for improvement of water quality and its effects on aquatic microbial community structure remain unknown. To elucidate these issues, we applied two groups of mixed Bacillus (Bacillus megaterium and Bacillus subtilis (A0 + BS) and Bacillus megaterium and Bacillus coagulans (A0 + BC)) to the aquaculture system of Crucian carp. Our results showed that the improvement effect of mixed Bacillus A0 + BS on water quality was better than that of A0 + BC, and the NH₄⁺-N, NO₂⁻-N, NO₃⁻-N, and total phosphorus (TP) concentrations were reduced by 46.3%, 76.3%, 35.6%, and 80.3%, respectively. In addition, both groups of mixed Bacillus increased the diversity of the bacterial community and decreased the diversity of the fungal community. Microbial community analysis showed that mixed Bacillus A0 + BS increased the relative abundance of bacteria related with nitrogen and phosphorus removal, such as Proteobacteria, Actinobacteria, Comamonas, and Stenotrophomonas, but decreased the relative abundance of pathogenic bacteria (Acinetobacter and Pseudomonas) and fungi (Epicoccum and Fusarium). Redundancy analysis showed that NH₄⁺-N, NO₂⁻-N, and TP were the primary environmental factors affecting the microbial community in aquaculture water. PICRUST analysis indicated that all functional pathways in the A0 + BS group were richer than those in other groups. These results indicated that mixed Bacillus A0 + BS addition produced good results in reducing nitrogenous and phosphorus compounds and shaped a favorable microbial community structure to further improve water quality.

Small omnivorous fish often dominate in subtropical shallow lakes, and they may affect the community structure of aquatic organisms on at least two trophic levels. However, in the study of aquatic food webs in subtropical lakes, most ecologists have focused on the effects of large-sized omnivorous species (e.g. common carp), studies of small-sized species being scarce. We conducted a mesocosm experiment with two treatments (fish presence and absence) to examine the effects of a small-sized omnivore, bitterling (Acheilognathus macropterus), on phytoplankton, zooplankton and benthic macroinvertebrates. Our results showed that bitterling presence significantly increased the chlorophyll a concentration and biomass of phytoplankton, which became dominated by cyanobacteria (mainly Aphanizomenon spp.) that accounted for >99% of both total phytoplankton abundance and biomass. Both the abundance and biomass of zooplankton were also higher in the fish-present treatment, but small rotifers became dominant, and the zooplankton:phytoplankton biomass ratio decreased, indicating less grazing on phytoplankton. Moreover, both the abundance and biomass of benthic macroinvertebrates (tubificids) were higher in the bitterling-present treatment than in the controls, which is opposite to the situation found when omni-benthivorous fish (e.g. crucian carp) dominate. Higher biomass of tubificids may, in turn, result in higher sediment nutrient release. Our study suggests that A. macropterus, and maybe also other bitterling species, can alter both pelagic and benthic assemblages via both top-down and bottom-up control effects and lead to more turbid water in eutrophic lakes. Thus, more attention should be paid to these small omnivorous species in the restoration and management of shallow subtropical lakes.