The microbial carbon pump concept: Potential biogeochemical significance in the globally changing ocean

Highlights

• The biological carbon pump sequesters carbon (C) by vertical transfer to deep ocean.

• The microbial carbon pump (MCP) sequesters C by producing DOC with lifetime >100 years.

• We define the magnitude of the MCP as the rate of production of DOC_>100.

• Our novel estimate of the magnitude of the MCP in the World Ocean is 0.2 Pg C year^−1.

• Of 7 predicted climate-related ocean changes, 3 could enhance C sequestration by MCP.

Abstract

Three vertical ocean carbon pumps have been known for almost three decades to sequester atmospheric carbon in the deep-water and sediment reservoirs, i.e. the solubility pump, the carbonate pump, and the soft-tissue (also known as organic, or biological) carbon pump (BCP). These three pumps maintain the vertical gradient in total dissolved inorganic carbon between the surface and deep waters. The more recently proposed microbial carbon pump (MCP) would maintain a gradient between short- and long-lived dissolved organic carbon (DOC; average lifetimes of <100 and >100 years, respectively). Long-lived DOC is an additional proposed reservoir of sequestered carbon in the ocean. This review: examines critically aspects of the vertical ocean carbon pumps and the MCP, in particular their physical dimensions and their potential roles in carbon sequestration; normalises the dimensions of the MCP to allow direct comparisons with the three vertical ocean carbon pumps; compares the MCP and vertical ocean carbon pumps; organises in a coherent framework the information available in the literature on refractory DOC; explores the potential effects of the globally changing ocean on the MCP; and identifies the assumptions that generally underlie the MCP studies, as bases for future research. The study: proposes definitions of terms, expressions and concepts related to the four ocean carbon pumps (i.e. three vertical pumps and MCP); defines the magnitude for the MCP as the rate of production of DOC with an average lifetime of >100 years and provides its first estimate for the World Ocean, i.e. 0.2 Pg C year⁻¹; and introduces an operational “first-time-sequestration” criterion that prevents organic carbon fluxes from being assigned to both the BCP and the MCP. In our review of the potential effects of predicted climate-related changes in the ocean environment on the MCP, we found that three of the seven predicted changes could potentially enhance carbon sequestration by the MCP, and three could diminish it.

Introduction

In the World Ocean, there is a strong vertical gradient in the concentration of dissolved CO₂, and there are several different forms that make up total dissolved inorganic carbon (C_T):

$$C_{\text{T}}={\text{CO}}_2(\text{dissolved})+{\text{H}}_2{\text{CO}}_3+{\text{HCO}}_3^{-}+{\text{CO}}_3^{2-}$$

where H₂CO₃, HCO₃⁻ and CO₃²⁻ are carbonic acid, and bicarbonate and carbonate ions, respectively. In the literature, C_T is also called dissolved inorganic carbon (DIC) and total CO₂ (TCO₂ or ∑CO₂). The global mean C_T in deep waters below 1200 m is higher than in the surface mixed layer, i.e. 2284 and 2012 μmol kg⁻¹, respectively (Volk and Hoffert, 1985, and Fig. 1). This requires that physical, chemical and biological processes counteract the continuous erosion by diffusive ocean mixing (on timescale of ∼1000 years) of the vertical differences in the concentration of C_T. Volk and Hoffert (1985) referred to the processes that maintain the C_T gradient in the World Ocean as the “ocean carbon pumps”, and these pumps have an important effect on the air–sea CO₂ fluxes on century timescales (IPCC, 2013, Section 6.3.2.5.5).

In their seminal paper, Volk and Hoffert (1985) defined three vertical ocean carbon pumps, corresponding to different forms of carbon in the marine environment: the solubility pump for C_T, the carbonate pump for particulate inorganic carbon (CaCO₃), and the soft-tissue pump (also called organic carbon pump, but more generally biological carbon pump) for particulate organic carbon (POC) although this pump can also include dissolved organic carbon (DOC). The expression “biological carbon pump” (BCP) refers to either the organic component of the ocean carbon pump only, or both the organic and CaCO₃ components. Volk and Hoffert (1985) and Passow and Carlson (2012) used biological carbon pump for the sum of the two components. In this review, BCP refers only to the organic component, consistent with the Glossary in IPCC (2013, Annex III).

Recently, another ocean carbon pump, called the microbial carbon pump (MCP), was proposed by Jiao and collaborators (Jiao et al., 2010, Jiao et al., 2011, Jiao and Azam, 2011). The MCP concept was developed within the context of marine microbiology with proposed links with marine biogeochemistry. A key assumption of the MCP is that the production mechanisms of long-lived DOC in the ocean are largely microbial. In contrast, the BCP concept was mostly developed, studied and modelled within the context of marine food webs and ocean biogeochemistry. The BCP has been central to studies that explore the present and future responses of biogeochemical fluxes to the globally changing ocean, which are critical to help inform governmental policy decisions (e.g. Passow and Carlson, 2012). If the oceanographic community is to consider the MCP along with the other ocean carbon pumps in assessments of the biologically mediated biogeochemical fluxes of carbon in the changing ocean, it is important that the MCP concept be expressed in the same units as the three other ocean carbon pumps (i.e. normalised).

Although well accepted now, the three vertical ocean carbon pumps were, at their origin, conceptual constructions that were based on a few observations and aimed at providing frameworks for the development and testing of hypotheses and models. This was clear in the seminal paper of Volk and Hoffert (1985), e.g. “distinguishing the strengths and efficiencies of the pumps may help formulate questions about models and possible ocean changes in the future”. Similarly in the recent paper of Jiao et al. (2010), the authors proposed “the microbial carbon pump as a conceptual framework to address the role of microbial generation of RDOM and relevant carbon storage”. The vertical ocean carbon pumps concept was proposed almost 30 years ago whereas the MCP concept is quite recent, hence the first now rests on extensive quantitative evidence (e.g. Sarmiento and Gruber, 2006) and has produced well accepted numerical models (e.g. Kwon et al., 2009), whereas the second (e.g. DeVries et al., 2014) is still largely hypothetical.

In the literature, expressions such as ocean carbon pump (biological, carbonate, organic, soft tissue, or solubility), carbon export and carbon sequestration are often used by authors differently, hence potentially creating confusion. Here, we propose and define a coherent set of expressions for use by the research community. The acronyms and symbols of quantities, their definitions and areal dimensions, and the corresponding units are summarised in Table 1. For easy reference, the definitions proposed in the text are assembled in a logical framework in Table 2, and schematised in Fig. 2. Our definitions are largely consistent with those of Passow and Carlson (2012). For the different fractions of dissolved organic carbon, we use the definitions proposed by Hansell (2013). Because our study examines ocean carbon pumps, we will refer below to DOC and POC instead of dissolved and particulate organic matter (DOM and POM, respectively).

A recent paper by Jiao et al. (2014) presented a general overview of the field and addressed future research directions for studies on the MCP based primarily on microbiological considerations. The objectives of the present in depth review are sequentially to: (1) critically examine key aspects of the vertical ocean carbon pumps, in particular their physical dimensions and their roles in carbon sequestration; (2) critically examine key aspects of the MCP, and normalise its dimensions to allow direct comparisons with the three vertical ocean carbon pumps; (3) compare the MCP with the vertical ocean carbon pumps; (4) organise in a coherent framework the information available on refractory dissolved organic carbon; (5) explore the potential effects of the globally changing ocean on the MCP; and (6) identify the assumptions that generally underlie the MCP studies, as bases for future research.

In the following sections, we briefly describe and compare the functioning of the three vertical ocean carbon pumps, and examine carbon sequestration in the ocean, the DOC fractions and the microbial carbon pump. We then compare the microbial and the vertical ocean carbon pumps, review reports of refractory dissolved organic carbon processes in the ocean, and examine potential effects of climate change on the microbial carbon pump. We conclude by reviewing the main assumptions found in the major publications on the MCP.