Limited iron isotope variation among tissues of a marine fish: A case study of wild chub mackerel (Scomber japonicus)

Abstract

Iron homeostasis in marine organisms operates under chronically low iron bioavailability, which may shape the strategies of iron uptake and storage in fish. Stable iron isotope ratios (δ56Fe) have emerged as tracers of iron storage and uptake in terrestrial mammals, yet the physiological drivers of isotope fractionation in marine organisms remain poorly understood. Here, we investigatedδ56Fe variation and iron speciation across eight tissues of wild chub mackerel (Scomber japonicus), along with pool size estimation of key Fe species, including ferritin-bound (ferric) and heme-bound (mainly ferrous) Fe, using Fe K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy. In all the specimens, the liverδ56Fe values were higher than the average value of all tissues, with an apparent isotopic shift between ferritin- and heme-bound Fe (Δ56Fe) in the liver averaging2.72 %±3.03 % (2 S.D.). In contrast to the liver, no enrichment of heavy Fe isotope was observed in the ovary and red muscle despite their high ferritin-Fe contribution, suggesting a high interconversion rate between ferritin- and heme-bound Fe pools in these tissues. The overall range ofδ56Fe variation among tissues was smaller than that reported in mammals. Our results suggested that muscularδ56Fe in marine teleost is primarily governed by source signatures and intestinal uptake efficiency, while tissue heterogeneity due to heavy Fe storage by ferritin exerts only a minor influence. These findings highlight the potential ofδ56Fe as a proxy for intestinal iron acquisition in fish and provide new geochemical perspectives on iron cycling through marine food webs.