Microbial Physiology Governs the Oceanic Distribution of Dissolved Organic Carbon in a Scenario of Equal Degradability

Andrea Mentges,Thorsten Dittmar,Christoph Feenders,Curtis Deutsch,Sinikka T Lennartz,Bernd Blasius

doi:10.3389/fmars.2020.549784

Abstract

Dissolved organic carbon (DOC) forms one of the largest active organic carbon reservoirs on Earth and reaches average radiocarbon ages of several thousand years. Many previous large scale DOC models assume different lability classes (labile to refractory) with prescribed, globally constant decay rates. In contrast, we assume that all DOC compounds are equally degradable by a heterotrophic microbial community. Based on this central assumption, we simulate DOC concentrations using a simple biogeochemical box model. Parameterized correctly, the simple model of neutral DOC uptake produced a recalcitrant carbon pool of 33mmolC/m3, throughout the entire virtual ocean. The spatial distribution of DOC in the model was independent of the distribution of DOC sources from primary production and particle degradation. Instead, DOC concentrations were primarily driven by spatial gradients in microbial physiology, e.g. mortality rate or growth efficiency. Applying such a gradient, we find DOC concentrations of ~70mmolC/m3 at the surface and ~35mmolC/m3 in the deep ocean. Introducing model variations such as seasonally-varying supply rates or temperature-dependent DOC uptake did not significantly alter model results. DOC spatial patterns are thus not necessarily shaped by the co-cycling of separate reactivity fractions, but can also arise from gradients in physiological parameters determining DOC uptake. We conclude that neutral DOC uptake can lead to realistic large-scale patterns of DOC concentration in the ocean.

Highlights

Dissolved organic carbon (DOC) forms one of the largest active organic carbon reservoirs on Earth, holding more than 600 Pg of carbon (Hansell et al, 2009)
We modeled the oceanic distribution of DOC and microbial biomass using a simple spatially-resolved biogeochemical box model
The central assumption of the model is neutral DOC uptake, i.e., that all DOC compounds are degradable by a heterotrophic microbial population

Summary

Introduction

Dissolved organic carbon (DOC) forms one of the largest active organic carbon reservoirs on Earth, holding more than 600 Pg of carbon (Hansell et al, 2009). A Box Model of Neutral DOC Uptake compounds resist degradation and accumulate in the ocean (Williams and Druffel, 1987) These recalcitrant DOC compounds can reach radiocarbon ages of more than 10,000 years (Follett et al, 2014), which indicates that they persist over several ocean mixing cycles of ∼300–1,400 years each (Khatiwala et al, 2012). It is assumed, that the stable DOC compounds have specific structural features that make them intrinsically resistant to microbial uptake This notion is supported by the observation that the radiocarbon age of DOC correlates with the structural features of DOC compounds, such as size and polarity (Loh et al, 2004; Benner and Amon, 2015; Walker et al, 2016). It was demonstrated that concentration-limited uptake can explain the amount and longevity of DOC in the ocean (Mentges et al, 2019)

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