** Progress in Earth and Planetary Science is the official journal of the Japan Geoscience Union, published in collaboration with its society members.
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Biogeosciences
202502202502
Distributions of heme B and its implications on Fe biogeochemistry in the eastern Indian Ocean and the western North Pacific Ocean
Isaji Y, Sugie K, Ishikawa NF, Ogawa NO, Matsumoto K, Yoshikawa C, Obata H, Honda MC, Ohkouchi NYuta Isaji, Koji Sugie, Naoto F. Ishikawa, Nanako O. Ogawa, Kazuhiko Matsumoto, Chisato Yoshikawa, Hajime Obata, Makio C. Honda, Naohiko Ohkouchi
Marine iron cycle, Heme B, Biogenic iron, Eastern Indian Ocean, Western North Pacific Ocean
Distribution of heme B and relevant environmental factors in the ocean. The ranges of particulate heme B concentrations and their normalized values are based on the observation in the Eastern Indian Ocean and the Western North Pacific Ocean. The ranges of biogenic Fe (BioFe) concentrations are calculated from heme B concentrations.
Heme B is an iron-coordinated porphyrin cofactor that facilitates essential biochemical reactions. As a major iron component in almost all life forms, the abundance of heme B in the ocean provides novel insights into iron biogeochemistry. In this study, we investigated the distribution of heme B in suspended particulate material collected from the surface mixed layer of the eastern Indian Ocean and the western North Pacific Ocean. Within the photic zone of the regions studied, particulate heme B concentrations ranged 1.24–8.39 pmol L−1and were positively correlated with particulate organic carbon and chlorophyll a concentrations, consistent with the biologically ubiquitous nature of heme B. Profiles of heme B normalized to particulate organic carbon (heme B/POC) and chlorophyll a (heme B/chl a) revealed a complex response of the microbial heme B pool to environmental factors. In the eastern Indian Ocean, heme B/POC increased in response to enhanced iron bioavailability. Notably, a sharp increase in heme B/POC, up to 3.04 µmol mol−1 in the Bay of Bengal, was attributed to the alleviation of iron stress due to substantial iron inputs from monsoonal aeolian dust and riverine sources. Conversely, heme B/POC as low as 0.88 µmol mol−1 in the South Indian Ocean was consistent with the previous incubation experiments indicating iron limitation. In the western North Pacific Ocean, relatively low heme B/POC values in both the iron-limited subarctic and nitrogen-limited subtropical regions highlighted the influence of factors beyond iron bioavailability. In the subarctic region, an elevation in dissolved iron concentrations due to seasonal deepening of the surface mixed layer was counterbalanced by greater iron investment in photosynthetic proteins to acclimate to low light intensities. On the other hand, microbial communities in the subtropical western North Pacific Ocean were less likely to have experienced iron stress. However, a reduction in the intracellular abundance of heme B-containing photosynthetic proteins and nitrate reductase under nitrogen-limited conditions may have resulted in heme B/POC values comparable to those observed in the iron-limited regions. Based on our particulate heme B measurements, we estimated the global particulate biogenic iron pool in the surface ocean, which showed consistency with model-simulated estimates. This study highlights the utility of heme B as a valuable parameter for understanding iron biogeochemistry, which is critical for elucidating the links between marine iron and carbon cycles.
