Laterally Transported Particles From Margins Serve as a Major Carbon and Energy Source for Dark Ocean Ecosystems

Abstract

Deep ocean microorganisms consume particulate organic matter that is produced in the surface ocean and exported to deeper depths. Such consumption not only enriches inorganic carbon in the deep ocean but also transforms organic carbon into recalcitrant forms, creating an alternative type of carbon sequestration. However, estimates of deep microbial carbon demand substantially exceed the available particulate organic carbon exported from the euphotic zone, resulting in an unbalanced dark ocean carbon budget. Here, we combined field-based microbial activity parameters, integrated multiyear particle export flux data, sinking particle fluxes measured by sediment traps, and optical data from Biogeochemical-Argo floats to quantify the main sources of organic carbon to the dark ocean. Laterally transported particles (including sinking and suspended particles) serve as a major energy source, which directly provide organic carbon and enhance new organic carbon production by dark carbon fixation, reconciling the mismatch in the regional carbon budget. Plain Language Summary Particulate organic matter, produced by phytoplankton in the upper ocean, can sink through the water column and act as a source of organic matter to the deep ocean. These particles are decomposed to carbon dioxide by microorganisms, resulting in dissolved inorganic carbon and organic carbon resistant to decomposition in the deeper ocean. This process controls the biological sequestration of CO 2 by the oceans. However, there is an imbalance between the low amount of organic carbon exported from the photic zone and the high microbial demand for carbon in the dark ocean. We attempted to explain how the deep ocean carbon and energy supply can meet the microbial metabolic demand. Four main organic carbon sources were measured and quantified in the South China Sea: particles that come from the photic zone, particles that move laterally through the ocean, dark carbon fixation, and dissolved organic carbon. We found that laterally transported particles from the surrounding margins provide a direct source of organic carbon and also allow for much new organic carbon production through dark carbon fixation. These particles, which provide a major energy source to dark ocean ecosystems, help resolve the mismatch in the regional carbon budget.

Lagrangian Observation of 234Th and Its Application in Constraining the Sinking of Particulate Organic Carbon on the Slope of the Northeastern South China Sea

“…A recent study reveals that a large amount of POC is required to support the consumption of deep microbes in the SCS interior (Shen et al, 2020). Our results provide direct evidence for the cross-transport of POC.…”

“…In this study, the northeastern SCS was selected to investigate the influence of horizontal advection on the estimation of 234 Th flux (Figure 1). Over the past decade, increasing studies report the widely observed cross-shelf dispersion of particulate matter (Shih et al, 2019;Shen et al, 2020;Yang et al, 2021b;Ge et al, 2022) driven by the dynamic physical processes (Li et al, 2018;Jia et al, 2019). Thus, the northeastern SCS could be an ideal site for attesting to the influence of physical processes on the application of the 234 Th/ 238 U method.…”

Temperature‐Regulated Turnover of Chromophoric Dissolved Organic Matter in Global Dark Marginal Basins

“…Strong downward ventilation in the Med and Japan Seas resulted in the warmest and coldest dark ocean temperatures, respectively. Importantly, ventilation also regulates the downward and lateral supply of labile carbon substrates (Lazzari et al., 2012; Ramondenc et al., 2016; Shen, Jiao, et al., 2020). For example, strong ventilation supplied a large DOC flux (9.2–13.8 g C m −2 yr −1 ) to the deep East Med Sea, although POC export (0.5 g C m −2 yr −1 ) was quite low from this area (Table ).…”

Plankton respiration in the Atacama Trench region: Implications for particulate organic carbon flux into the hadal realm

“…Sources of carbon other than the gravitational settling flux and the suspended particles originated from the disaggregation of the sinking material will additionally contribute a proportion of the subsurface respiration by injecting organic matter to depth (Boyd et al 2019 ). For instance, vertically migrating zooplankton may actively transport 5.9 mmol C m −2 d −1 to the mesopelagic (Tutasi and Escribano 2020 ) which, together with the downward mixing and diffusion of dissolved organic carbon and the lateral inputs of POC advected from the coast (Shen et al 2020 ), will help to reconcile the unbalanced carbon cycle. Unfortunately, quantifying the relative contribution of all these physical and biological processes that make up the biological pump is outside the scope of this research.…”