What Is Refractory Organic Matter in the Ocean?

Abstract

About 20% of the organic carbon produced in the sunlit surface ocean is transported into the ocean’s interior as dissolved, suspended and sinking particles to be mineralized and sequestered as dissolved inorganic carbon (DIC), sedimentary particulate organic carbon (POC) or “refractory” dissolved organic carbon (rDOC). Recently, the physical and biological mechanisms associated with the particle pumps have been revisited, suggesting that accepted fluxes might be severely underestimated (Boyd et al., 2019; Buesseler et al., 2020). Perhaps even more poorly understood are the mechanisms driving rDOC production and its potential accumulation in the ocean. On the basis of recent conflicting evidence about the relevance of DOC degradation in the deep ocean, we revisit the concept of rDOC in terms of its “refractory” nature in order to understand its role in the global carbon cycle. Here, we address the problem of various definitions and approaches used to characterize rDOC (such as turnover time in relation to the ocean transit time, molecule abundance, chemical composition and structure). We propose that rDOC should be operationally defined. However, we recognize there are multiple ways to operationally define rDOC; thus the main focus for unifying future studies should be to explicitly state how rDOC is being defined and the analytical window used for measuring rDOC, rather than adhering to a single operational definition. We also conclude, based on recent evidence, that the persistence of rDOC is fundamentally dependent on both intrinsic (chemical composition and structure, e.g., molecular properties), and extrinsic properties (amount or external factors, e.g., molecular concentrations, ecosystem properties). Finally, we suggest specific research questions aimed at stimulating research on the nature, dynamics, and role of rDOC in Carbon sequestration now and in future scenarios of climate change.

Fate of dissolved black carbon in the deep Pacific Ocean

“…The removal mechanisms of refractory DOC from the deep water have been thus far unknown 44 , 50 , 51 . The removal process through sorption onto sinking particles may occur not only for DBC but also for bulk DOC, particularly its hydrophobic fraction, suggesting that sorption of hydrophobic DOC onto sinking particles is possibly a factor affecting the DOC concentration in the deep ocean.…”

Controls on turnover of marine dissolved organic matter—testing the null hypothesis of purely concentration‐driven uptake: Comment on Shen and Benner, “Molecular properties are a primary control on the microbial utilization of dissolved organic matter in the ocean”

“…A range of mechanisms have been identified but discussion focuses on the dominant role of the two competing, but not mutually excluding mechanisms of structural recalcitrance and limitation of substrate concentration (see Jiao et al 2011;Arrieta et al 2015;Dittmar 2015;Walker et al 2016;Wang et al 2018). There are still debates on the definitions of what is refractory DOM in the ocean (Baltar et al 2021).…”

Reply to comment: Controls on turnover of marine dissolved organic matter—testing the null hypothesis of purely concentration‐driven uptake

“…Earth's oceans hold a vast quantity of refractory dissolved organic carbon (RDOC) that plays an important role in carbon sequestration and regulation of global climate. Mechanisms behind the long-term persistence of RDOC remain elusive today (Baltar et al 2021). Over the past five decades, there has been growing interest in determining the factors controlling the utilization and accumulation of RDOC in the ocean (Barber 1968;Williams and Druffel 1987;Jannasch 1995;Ogawa et al 2001;Jiao et al 2014;Arrieta et al 2015;Benner and Amon 2015;Walker et al 2016;Benner 2018, 2020;Bercovici et al 2021;Zakem et al 2021).…”