Tissue-variation of iron stable isotopes in marine fish coupled with speciation analysis using X-ray absorption fine...

The Fe stable isotope ratio (δ⁵⁶Fe) in tissues is a potential parameter for examining the Fe metabolism in marine fish. Although the effect of ferritin storage has been proposed as a possible cause of heavy isotope (⁵⁶Fe) enrichment in the liver, no speciation and stable isotope ratio coupling data are available. Here, we report the δ⁵⁶Fe values measured by multicollector ICP-MS and the result of Fe K-edge X-ray absorption near-edge structure (XANES) analysis of multiple tissues obtained from a skipjack tuna (Katsuwonus pelamis) and a chub mackerel (Scomber japonicus). Apparent isotopic fractionation between the liver and the muscle samples (Δ⁵⁶Fe_L-M = δ⁵⁶Fe_liver - δ⁵⁶Fe_muscle) from these species was observed (0.85 ‰ and 0.57 ‰, respectively). The dominant Fe species in the muscle was heme Fe (the sum of methemoglobin, oxyhemoglobin, and deoxyhemoglobin), while ferritin was not detected according to the linear combination fitting of the XANES spectra. In the liver, ferritin contribution was ca. 28 %-54 % of the total Fe content. The apparent difference in δ⁵⁶Fe between heme Fe and ferritin was estimated to range from 1.41 ‰ to 1.52 ‰ based on the tissue-specific δ⁵⁶Fe values and the XANES results. These results indicate that the Fe storage as ferritin does not induce the lowering of δ⁵⁶Fe in the muscle, considering the low contribution of the liver Fe to the total Fe content in the body.