9:45 AM - 10:00 AM
[MTT42-04] Development of iron stable isotope measurement for biological tissue and its application to the marine organisms
Keywords:iron stable isotope, metabolism, marine food web
Iron (Fe) is one of the essential elements in the biota and its biogeochemical cycle strongly controls the primary productivity in the ocean. Despite complexity of Fe cycle in surface ocean, application of Fe stable isotope through marine ecosystem has been limited. Here we show the variation of δ56Fe of marine organisms collected from Northwest Pacific Ocean. The samples (n=32) include fishes (4 species), cephalopod (1 species), cetaceans (3 species), and zooplankton (mainly copepods) inhabiting around Japan. The process of Fe stable isotope fluctuation was discussed by focusing on (1) trophic level, (2) habitat, and (3) metabolic process.
The δ56Fe was analyzed using multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS, Thermo Fisher Scientific, Germany). The pretreatment was established by improving the anion-exchange column separation for removing matrixes. The accuracy of the analytical method was evaluated using 4 types of biological certified reference materials (CRM; DORM-4, DOLT-5, BCR-414, and ERM-CE464). The difference between measured and certified values was <0.06 ‰ which is smaller than the precision of duplicate measurements (0.09‰). We obtained new δ56Fe value for BCR-414 and DOLT-5 which showed -0.09 ± 0.07 ‰ and -2.32 ± 0.07 ‰, respectively.
The measured δ56Fe of marine organisms were as following, zooplankton: -0.03 ± 0.08 ‰ (n = 1), fish muscle: -2.64 ‰ to -0.71 ‰ (n = 19, average: -1.65 ‰), cetacean muscle: −2.37 ‰ to −1.74 ‰ (n = 9, average: −2.15 ‰). Most biological samples had lower δ56Fe than the reported value of surface seawater (+0.3 to +0.7‰, Conway and John, 2014). The δ56Fe values increased in order of cetacean < fish < zooplankton, suggesting that light isotopes tend to enrich with increase of trophic level. This is consistent with the sequential change of isotope value through the food chain as observed by other elements such as nitrogen and calcium although directions of isotope shift are variable. Despite general decreasing trend of δ56Fe with trophic level, Japanese sardines (Sardinops melanostictus), an important prey for migratory fish, showed significantly lower values than the higher predator fishes. This indicated that sequential depletion of heavy isotope is not only the predominant fractionation process, thereby species specific fractionation process controlled by physiological characteristics should be examined.
Regional differences were assessed by comparing δ56Fe of Japanese sardines (off Kochi and off Miyagi) and skipjack tuna (Katsuwonus pelamis, offshore Kamchatka, off Sanriku, off Guam, and East China Sea) of which data of multiple regions were available. No significant regional difference was observed, although the number of samples from each region was limited. The range of δ56Fe of all skipjack tuna (-1.46 ‰ to -0.71 ‰) had clearly higher than that of sardines (-2.64 ‰ to -1.73 ‰), suggesting that the difference of species control δ56Fe greater than that of region.
According to the pharmacokinetic property of Fe, potential isotope fractionation processes are classified into absorption, metabolism (change in chemical form), and excretion. The higher δ56Fe in the liver than in muscle was plausibly derived from large isotope fractionation between heme Fe (predominat Fe form in red blood cell and muscle) and non-heme Fe (mainly stored in liver) in the body. Hence, the ratio of Fe burden between heme Fe and non-hem Fe pools possibly control tissue specific isotope value. Intestinal absorption is another important fractionation process. Rate of absorption can be a controlling factor of δ56Fe of whole body due to the preferential absorption of light isotope. Although precise estimation of absorption efficiency of each species is difficult, mass balance calculation using fisheries ecological data suggested absorption efficiency can be a most important controlling factor. Our data provided here can be useful benchmark of Fe isotope systematics through marine ecosystem.
The δ56Fe was analyzed using multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS, Thermo Fisher Scientific, Germany). The pretreatment was established by improving the anion-exchange column separation for removing matrixes. The accuracy of the analytical method was evaluated using 4 types of biological certified reference materials (CRM; DORM-4, DOLT-5, BCR-414, and ERM-CE464). The difference between measured and certified values was <0.06 ‰ which is smaller than the precision of duplicate measurements (0.09‰). We obtained new δ56Fe value for BCR-414 and DOLT-5 which showed -0.09 ± 0.07 ‰ and -2.32 ± 0.07 ‰, respectively.
The measured δ56Fe of marine organisms were as following, zooplankton: -0.03 ± 0.08 ‰ (n = 1), fish muscle: -2.64 ‰ to -0.71 ‰ (n = 19, average: -1.65 ‰), cetacean muscle: −2.37 ‰ to −1.74 ‰ (n = 9, average: −2.15 ‰). Most biological samples had lower δ56Fe than the reported value of surface seawater (+0.3 to +0.7‰, Conway and John, 2014). The δ56Fe values increased in order of cetacean < fish < zooplankton, suggesting that light isotopes tend to enrich with increase of trophic level. This is consistent with the sequential change of isotope value through the food chain as observed by other elements such as nitrogen and calcium although directions of isotope shift are variable. Despite general decreasing trend of δ56Fe with trophic level, Japanese sardines (Sardinops melanostictus), an important prey for migratory fish, showed significantly lower values than the higher predator fishes. This indicated that sequential depletion of heavy isotope is not only the predominant fractionation process, thereby species specific fractionation process controlled by physiological characteristics should be examined.
Regional differences were assessed by comparing δ56Fe of Japanese sardines (off Kochi and off Miyagi) and skipjack tuna (Katsuwonus pelamis, offshore Kamchatka, off Sanriku, off Guam, and East China Sea) of which data of multiple regions were available. No significant regional difference was observed, although the number of samples from each region was limited. The range of δ56Fe of all skipjack tuna (-1.46 ‰ to -0.71 ‰) had clearly higher than that of sardines (-2.64 ‰ to -1.73 ‰), suggesting that the difference of species control δ56Fe greater than that of region.
According to the pharmacokinetic property of Fe, potential isotope fractionation processes are classified into absorption, metabolism (change in chemical form), and excretion. The higher δ56Fe in the liver than in muscle was plausibly derived from large isotope fractionation between heme Fe (predominat Fe form in red blood cell and muscle) and non-heme Fe (mainly stored in liver) in the body. Hence, the ratio of Fe burden between heme Fe and non-hem Fe pools possibly control tissue specific isotope value. Intestinal absorption is another important fractionation process. Rate of absorption can be a controlling factor of δ56Fe of whole body due to the preferential absorption of light isotope. Although precise estimation of absorption efficiency of each species is difficult, mass balance calculation using fisheries ecological data suggested absorption efficiency can be a most important controlling factor. Our data provided here can be useful benchmark of Fe isotope systematics through marine ecosystem.