Iron nutrition and host microbiota regulation in turbot (Scophthalmus maximus L.): The health risk of excess surpasses that of deficiency

This research studied the effects of iron (Fe) on physiology and microbiota (intestine and surrounding water) of turbot ( Scophthalmus maximus L.). Three purified diets were designed: Fe0, Fe350, and Fe1750, with 0, 350, and 1750 mg/kg Fe supplementation from FeSO 4 , respectively. A total of 225 84-d-old fish (initial body weight 8.19 ± 0.08 g) were evenly distributed into nine tanks (25 fish per tank, 150 L seawater) for a 60-d feeding trial. The results showed that Fe deficiency (Fe0) or excess (Fe1750) had no obvious effects on growth performance ( P > 0.05), but significantly affected overall mineral composition as revealed by principal component analysis ( P < 0.001) and the expression of Fe transport related genes ( P < 0.05). Turbot showed high Fe tolerance through ion transport regulation: under Fe deficiency, gene expression of transferrin/transferrin receptor, divalent metal-ion transporter 2b, and ferroportin were significantly up-regulated, whereas under excess, they were significantly down-regulated ( P < 0.05). Iron excess also increased the risk of oxidative stress and liver damage, as indicated by the elevated activities of antioxidant enzymes ( P < 0.05) and the increased levels of alanine transaminase ( P = 0.002), aspartate transaminase ( P = 0.022), and the reduced gene expression of liver forkhead box protein P3b and perilipin 2 ( P < 0.05). Compared with Fe excess, Fe deficiency exerted a milder effect on hematological parameters, with moderate changes in plasma biochemical indices. Furthermore, the retention was negatively correlated with Fe addition level ( r = −0.787, P < 0.001). Dietary Fe supplementation showed strong negative correlations with the contents of Zn (in intestine, liver, and gill), Mn (in muscle, intestine, and liver), Cu (in blood and intestine), Ca (in liver and gill), and Mg and P (in gill) ( r < −0.5, P < 0.05). The analysis of microbiota revealed that the microbial co-occurrence network was most stable in the Fe350 group, with higher abundances or key microecological niches by microbes related to Fe metabolism and host health (e.g., Delftia acidovorans , Pseudomonas fluorescens , and Lactobacillus helsingborgensis ). Excessive Fe also enriched the pathogens Photobacterium damselae and Vibrio campbellii in turbot. These results indicate that both Fe deficiency and excess can cause imbalances in physiological health, Fe and microbiota homeostasis, with excess Fe showing greater risks.