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 investigated δ⁵⁶Fe 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. The liver δ⁵⁶Fe values were consistently higher than those of other tissues, with an apparent isotopic shift between ferritin- and heme-bound Fe (Δ⁵⁶Fe) in the liver averaging 2.04 ± 0.22 ‰ (2 S.D.). In contrast to the liver, no significant 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 of δ⁵⁶Fe variation among tissues was smaller than that reported in mammals. Our results indicated that muscular δ⁵⁶Fe 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 of δ⁵⁶Fe as a proxy for intestinal iron acquisition in fish and provide new geochemical perspectives on iron cycling through marine food webs.