11:00 AM - 11:15 AM
[AOS13-07] Heme B as a tool to constrain stable iron isotopic composition of biological iron
Keywords:Biogeochemistry, Iron cycle, Stable iron isotope, Heme
Iron is an essential element required by organisms in catalyzing numerous biochemical reactions. It is a key micronutrient that regulates marine biogeochemical cycles, as the availability of dissolved iron limits primary production across up to 40% of the global surface ocean (Browning and Moore, 2023). Dissolved iron in the surface ocean is supplied from multiple sources (e.g., aeolian dusts, sediment porewaters, rivers, hydrothermal vents), contributions of which have been estimated based on stable iron isotopic composition (δ56Fe) of dissolved iron (Fitzsimmons and Conway, 2023). However, direct evidence of the iron source supporting the primary production is lacking, due to the difficulty in selectively analyzing δ56Fe of biological iron in environmental samples such as suspended particulate matter. To this end, we have been developing a novel method of compound-specific stable iron isotope analysis of heme B. Heme B is a Fe(II)-chelating tetrapyrrole molecule that catalyzes various biochemical reactions in almost all life forms. As it constitutes a major fraction of the Fe pool in organisms, its δ56Fe is expected to reflect that of biological iron in environments.
The method consists of sample extraction, heme B isolation by high-performance liquid chromatography (Isaji et al., 2020), heme-Fe extraction by UV and acid digestion, heme-Fe isolation by ion-exchange chromatography, and isotopic measurements by multiple-collector inductively coupled plasma mass spectrometry. The procedural blank is less than 5% of the analyte, and the analytical precision assessed with authentic heme B standard material is better than 0.1‰ (2σ) at amount of heme B down to ~2 nmol.
The high sensitivity of the method allows its application across diverse biological and environmental samples, which is expected to offer novel perspectives into biology and biogeochemistry of iron. For example, concentration of heme B is typically on the order of picomoles per liter in marine suspended particulate matter (SPM) in the photic zone (e.g., Gledhill et al., 2013; Honey et al., 2013; Isaji et al., 2020). Measurement of δ56Fe of heme B in marine SPM thus requires sampling of hundreds to thousands liters of seawater, which can be achieved by in-situ filtration or by using a plankton net. Concentration of heme B ranges widely in deep-sea sediments from 0.01 to 10 nmol g−1 dry weight (Isaji et al., 2020; Isaji et al., in prep.). Therefore, several grams of sample is sufficient in the case of sediments with high heme B concentration, which is typically observed in depositional environments where marine organic matter is well-preserved. Initial results of δ56Fe measurements of environmental samples (SPM, sediments, fish tissues) will be reported in the talk.
The method consists of sample extraction, heme B isolation by high-performance liquid chromatography (Isaji et al., 2020), heme-Fe extraction by UV and acid digestion, heme-Fe isolation by ion-exchange chromatography, and isotopic measurements by multiple-collector inductively coupled plasma mass spectrometry. The procedural blank is less than 5% of the analyte, and the analytical precision assessed with authentic heme B standard material is better than 0.1‰ (2σ) at amount of heme B down to ~2 nmol.
The high sensitivity of the method allows its application across diverse biological and environmental samples, which is expected to offer novel perspectives into biology and biogeochemistry of iron. For example, concentration of heme B is typically on the order of picomoles per liter in marine suspended particulate matter (SPM) in the photic zone (e.g., Gledhill et al., 2013; Honey et al., 2013; Isaji et al., 2020). Measurement of δ56Fe of heme B in marine SPM thus requires sampling of hundreds to thousands liters of seawater, which can be achieved by in-situ filtration or by using a plankton net. Concentration of heme B ranges widely in deep-sea sediments from 0.01 to 10 nmol g−1 dry weight (Isaji et al., 2020; Isaji et al., in prep.). Therefore, several grams of sample is sufficient in the case of sediments with high heme B concentration, which is typically observed in depositional environments where marine organic matter is well-preserved. Initial results of δ56Fe measurements of environmental samples (SPM, sediments, fish tissues) will be reported in the talk.