Food dyes, or color additives, are often added into foods, cosmetics and beverages during processing to improve the sensory attributes of the final products. However, the toxicity of tartrazine (TZ), one of the most common azo-dyes, is still unclear, and needs to be ascertained by further study. Hence, in the present study, we aimed to evaluate the effects of TZ consumption on health by using a teleost, crucian carp (Carassius auratus) as the experimental fish. TZ consumption (1.4, 5.5 and 10 mg/kg bwt/day) could cause severe histopathological and cellular alterations in intestine and liver. The height of intestinal villus, thickness of intestinal muscle, and microvilli density were also affected. With the increasing of TZ concentrations, the activities of antioxidant enzymes (CAT, SOD and GSH-Px), exhibited a decreasing trend, while the contents of MDA elevated. Upregulations of pro-inflammatory cytokines (il6 and tnfα), anti-inflammatory cytokines (il8, and il10) and other immune related genes (complement component 3 (c3), lysozymes (lyz), β-defensin 3 (defb3)), were observed after TZ uptake. In addition, TZ consumption also affected the community structure of the microbiota in the intestine of crucian carp. The amount of some probiotic bacteria (Roseomonas, Rhodococcus and Bacillus) and the bacteria (Bacteroides and Clostridium), producing short chain fatty acids, were significantly reduced, and some pathogenetic microorganisms (e.g. Bdellovibrio and Shewanella) were significantly increased after TZ uptake. In summary, the data in the present study indicate that TZ consumption, even at a low concentration, may lead to adverse effects on fish health. Therefore, in aquaculture, it is necessary to be informed about the hazardous effects of TZ, and more attentions should be focused on using natural substitutes.

Assessing the responses of lysozyme activity and antioxidant defences to hypoxia is important to understand the adaptation and tolerance strategies of fish under hypoxia. This study investigated the effects of different dissolved oxygen (DO) levels on Carassius auratus, a natural triploid fish, from Qihe River. The content of malondialdehyde (MDA) and the activities of lysozyme and antioxidant enzymes were measured in the kidney and spleen after hypoxic exposure. At the DO concentrations of 1 and 2 mg L⁻¹, the activities of antioxidant enzymes and lysozyme significantly decreased and the content of MDA significantly increased (P < 0.01). This result suggests that hypoxia decreased antioxidant enzyme and lysozyme activities and caused MDA accumulation in a concentration-dependent manner. The DO level of 4 mg L⁻¹ increased the activities of superoxide dismutase and catalase in the kidney and the activities of glutathione peroxidase and catalase in the spleen (P < 0.05). This result implies that slight hypoxic stress can enhance antioxidant defences to alleviate the damage of oxidative stress. The reduced activities of antioxidant enzymes and the accumulation of MDA at the DO level of ≤2 mg L⁻¹ implied the decrease in antioxidative ability and the occurrence of oxidative stress. The decrement in lysozyme activity indicated that the antibacterial ability was weakened to some degree. Therefore, hypoxic stress at DO levels ≤2 mg L⁻¹ should be removed by aeration to avoid the oxidative damage resulting from the reduced antioxidative ability and prevent the outbreak of diseases caused by weakened antibacterial effects.