Abstract:

Cadmium (Cd) is a heavy metal and a globally common and highly toxic environmental pollutant that can threaten aquatic health through oxidative stress. Melatonin (MT), a known antioxidant and free radical scavenger, has been shown to be effective in mitigating heavy metal-induced intestinal damage. However, only a few studies have been conducted on the use of MT to ameliorate Cd-induced oxidative damage and improve microbial diversity in the intestinal tissues of aquatic organisms. In this study, adult zebrafish (Danio rerio) were used as experimental subjects for a 30-day exposure experiment using a semi-static exposure method, which was divided into four groups: control group, Cd group, MT group, and Cd+MT group (mixture of the two substances). Intestinal pathology slides were used to determine intestinal hydrogen peroxide (H₂O₂), malondialdehyde (MDA) content, superoxide dismutase (SOD), and glutathione peroxidase (GSH-PX) activities. Intestinal flora Alpha and Beta diversity, intestinal flora composition, relative abundance of dominant flora, and biomarkers of each group were analyzed by high-throughput sequencing of intestinal flora 16SrDNA. The results showed that the intestinal wall of zebrafish in the Cd group was thinned, the intestinal villi appeared to be partially abraded and detached, the tips of the villi were exposed, the epithelial gap of the intestinal villi was enlarged, the number of vacuolated cells was increased, the cells were extensively lysed, the cytoplasmic damage was dispersed, and the phenomenon of relaxation of connective tissues occurred simultaneously, whereas the intestinal tissues of the zebrafish that were co-exposed to Cd and MT were partly restored to the integrity of the intestinal tissues, and the phenomenon of intestinal damage was significantly reduced. Cd led to a significant increase in zebrafish intestinal MDA and H₂O₂ content and a significant decrease in GSH-PX and SOD activities. However, co-treatment decreased H₂O₂ and MDA content and elevated GSH-PX and SOD activities. 16SrDNA sequencing revealed that Cd caused changes in the diversity of zebrafish intestinal flora, the Shannon index and Simpsoneven index increased, and the number of dominant flora decreased, while the addition of MT alleviated the effect of Cd on intestinal flora diversity and partially restored dysbiosis to a certain extent. The current study showed that melatonin could reduce Cd-induced oxidative damage and protect the diversity of intestinal flora in adult zebrafish caused by Cd, which provides a theoretical basis for the development of products against Cd poisoning.

Key words: cadmium, melatonin, zebrafish, oxidative damage, intestinal microbiota

摘要：

重金属镉（Cd）是一种全球常见的剧毒环境污染物，其可通过氧化应激威胁水生生物健康。褪黑素（Melatonin，MT）是一种已知的抗氧化剂和自由基清除剂，已被证明可有效缓解重金属引起的肠道损伤。然而，MT改善Cd诱导的水生生物肠道组织氧化损伤及微生物多样性的研究较少。该研究以成年斑马鱼（Danio rerio）为实验对象，采用半静态暴露方式，进行30 d的暴露实验，实验分为4组：Con组、Cd组、MT组、Cd+MT组（两种物质混合液），进行肠道病理切片观察，测定肠道过氧化氢（H₂O₂）和丙二醛（MDA）含量、超氧化物歧化酶（SOD）和谷胱甘肽过氧化物酶（GSH-PX）活性，通过肠道菌群16SrDNA高通量测序分析肠道菌群Alpha、Beta多样性、肠道菌群组成、优势菌群相对丰度和各组的生物标志物。结果显示：Cd组的斑马鱼肠壁变薄、肠绒毛出现了部分磨损和脱落，绒毛尖端暴露，肠绒毛上皮间隙增大，空泡化细胞增多，细胞广泛裂解，细胞质损伤分散，同时出现了结缔组织松弛现象，而共同暴露于Cd和MT的斑马鱼肠道组织结构部分已经恢复完整，肠损伤现象明显减少；Cd导致斑马鱼肠道MDA和H₂O₂含量显著上升，GSH-PX和SOD活性显著下降，而共同处理降低了H₂O₂和MDA含量、升高了GSH-PX和SOD活性；16SrDNA测序发现Cd导致斑马鱼肠道菌群多样性改变，Shannon指数及Simpsoneven指数升高，优势菌群数目减少，而添加MT一定程度缓解Cd对肠道菌群多样性的影响及菌群失调得到部分恢复。研究表明，褪黑素能减轻镉导致的成年斑马鱼肠道氧化损伤及保护肠道菌群多样性，为开发防治镉中毒的产品提供了理论依据。

关键词: 镉, 褪黑素, 斑马鱼, 氧化损伤, 肠道菌群