[B-BG01] Earth and Planetary Science Frontiers for Life and Global Environment

[BBG01-P03] Constraints on the source microorganisms of heme B in deep-sea sediments

*Yuta Isaji¹, Nanako O. Ogawa¹, Hisami Suga¹, Naohiko Ohkouchi¹ (1.Japan Agency for Marine-Earth Science and Technology)

Keywords:heme, sediments, biomarker

Heme B is a Fe(II)-chelating tetrapyrrole molecule that catalyzes various biochemical reactions in almost all life forms. Despite its central roles in driving the biogeochemical cycle, only few studies have investigated the distribution and dynamics of heme B in natural environments. We have recently developed a highly-sensitive analytical method to quantify heme B from various environmental samples down to 1 femtomole (Isaji et al., 2020, Anal. Chem.). Our new method enabled analysis of heme B in deep-sea sediments for the first time, revealing that heme B is present (> 0.01 nmol g⁻¹ dry sediment weight) throughout the sediment cores at least as old as 200 kyrs. Here, we aim to constrain the biological source of sedimentary heme B, which may originate either from microorganisms living in the water column (mainly phototrophic), or from subseafloor microorganisms living in the sediments (non-phototrophic). Analysis of cultured microorganisms revealed that heme B/TOC ratios of non-phototrophic prokaryotes (0–3.8 µmol mol⁻¹) can be as high as those of phytoplankton (0.2–5.6 µmol mol⁻¹). Thus, whether the subseafloor microorganisms are a significant source of sedimentary heme B depends on the cellular abundance and reproduction rate of the subseafloor microorganisms. Assuming the cellular abundance of Quaternary sediments (10⁶–10⁷ cells g⁻¹: Morono et al., 2011; Bradley et al., 2020) and the cell division rate (once per 1000 years: Jørgensen, 2011), we can roughly estimate that less than 10% of the sedimentary heme B may be derived from subseafloor microorganisms. This consideration is in line with parallel shifts in carbon and nitrogen isotopic compositions of chloropigments and heme B throughout the sediment core recovered from Namibian margin, which argues against active de novo synthesis of isotopically-distinct heme B in the sediments. This study provides important basic insights in answering the following questions: (1) does sedimentary heme B reflect the abundance and isotopic signals of that produced in the past ocean surface? (2) can heme B be used as a “biomarker” of living life, particularly under extreme environments?