Abstract
Dark carbon fixation (DCF) by chemoautotrophic microorganisms can sustain food webs in the seafloor by local production of organic matter independent of photosynthesis. The process has received considerable attention in deep sea systems, such as hydrothermal vents, but the regulation, depth distribution, and global importance of coastal sedimentary DCF have not been systematically investigated. Here we surveyed eight coastal sediments by means of stable isotope probing (13C‐DIC) combined with bacterial biomarkers (phospholipid‐derived fatty acids) and compiled additional rates from literature into a global database. DCF rates in coastal sediments range from 0.07 to 36.30 mmol C m−2 day−1, and there is a linear relation between DCF and water depth. The CO2 fixation ratio (DCF/CO2 respired) also shows a trend with water depth, decreasing from 0.09 in nearshore environments to 0.04 in continental shelf sediments. Five types of depth distributions of chemoautotrophic activity are identified based on the mode of pore water transport (advective, bioturbated, and diffusive) and the dominant pathway of microbial sulfur oxidation. Extrapolated to the global coastal ocean, we estimate a DCF rate of 0.04 to 0.06 Pg C year−1, which is less than previous estimates based on indirect measurements (0.15 Pg C year−1), but remains substantially higher than the global DCF rate at deep sea hydrothermal vents (0.001–0.002 Pg C year−1).
Highlights
Chemoautotrophic microorganisms obtain their metabolic energy by the oxidation of various reduced inorganic substrates, such as ammonium, nitrite, ferrous iron, and sulfide, and they use this energy to synthesize organic molecules from dissolved inorganic carbon (DIC), a process here forth referred to as dark carbon fixation (DCF)
The Dark carbon fixation (DCF) rates in the top centimeter (0–1 cm) of the sediment was compared to the DOU, assuming that reoxidation in the surface is fueled by O2 diffusion from the sediment‐ water interface, while the depth‐integrated DCF rate was compared to the TOU, assuming that nonlocal injection of O2 at depth can stimulate deep chemoautotrophic carbon fixation
DCF varies broadly in coastal sediments related to mineralization rates, water depth, pore water transport mechanisms, and microbial sulfur oxidation pathways
Summary
Chemoautotrophic microorganisms obtain their metabolic energy by the oxidation of various reduced inorganic substrates, such as ammonium, nitrite, ferrous iron, and sulfide, and they use this energy to synthesize organic molecules from dissolved inorganic carbon (DIC), a process here forth referred to as dark carbon fixation (DCF). Chemoautotrophic microbes typically thrive in redox gradient systems, that is, transitions between reduced and oxidized environments, such as the chemocline of water bodies or the oxic‐anoxic interface in marine sediments (Jørgensen, 1982; Labrenz et al, 2005). In terms of seafloor environments, deep sea hydrothermal vents form the most conspicuous ecosystems, as both symbiotic and free‐living chemoautotrophic microorganisms are the main primary producers, which obtain their energy from the oxidation of sulfide and other reduced compounds that are enriched in the vent fluids (Nakagawa & Takai, 2008). The total sulfide production by sulfate reduction in coastal marine sediments is roughly one order of magnitude higher than the sulfide output from hydrothermal vents (Howarth, 1984). Coastal sediments have a far greater potential for chemoautotrophy‐based primary production than deep sea ecosystems. The prevalence and magnitude of DCF in coastal sediments remains poorly documented
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