Mechanisms of microbial carbon sequestration in the ocean – future research directions

Abstract

Abstract. This paper reviews progress on understanding biological carbon sequestration in the ocean with special reference to the microbial formation and transformation of recalcitrant dissolved organic carbon (RDOC), the microbial carbon pump (MCP). We propose that RDOC is a relative concept with a wide continuum of recalcitrance. Most RDOC compounds maintain their levels of recalcitrance only in a specific environmental context (RDOCt). The ocean RDOC pool also contains compounds that may be inaccessible to microbes due to their extremely low concentration (RDOCc). This differentiation allows us to appreciate the linkage between microbial source and RDOC composition on a range of temporal and spatial scales. Analyses of biomarkers and isotopic records show intensive MCP processes in the anoxic Proterozoic oceans when the MCP could have played a significant role in regulating climate. Understanding the dynamics of the MCP in conjunction with the better constrained biological pump (BP) over geological timescales could help to predict future climate trends. Integration of the MCP and the BP will require new research approaches and opportunities. Major goals include understanding the interactions between particulate organic carbon (POC) and RDOC that contribute to sequestration efficiency, and the concurrent determination of the chemical composition of organic carbon, microbial community composition and enzymatic activity. Molecular biomarkers and isotopic tracers should be employed to link water column processes to sediment records, as well as to link present-day observations to paleo-evolution. Ecosystem models need to be developed based on empirical relationships derived from bioassay experiments and field investigations in order to predict the dynamics of carbon cycling along the stability continuum of POC and RDOC under potential global change scenarios. We propose that inorganic nutrient input to coastal waters may reduce the capacity for carbon sequestration as RDOC. The nutrient regime enabling maximum carbon storage from combined POC flux and RDOC formation should therefore be sought.

Microbial Processing of Sediment‐Derived Dissolved Organic Matter: Implications for Its Subsequent Biogeochemical Cycling in Overlying Seawater

“…With the FT‐ICR‐MS analysis, the overall properties of the residual SDOM indicate that the SDOM was microbially modified to a higher unsaturated state and likely containing more aromatic rings in molecules, these properties of residue SDOM are similar to the properties of deep‐sea RDOM (Catalá et al, ; Flerus et al, ; Hertkorn et al, ; Lechtenfeld et al, ; Li et al, ; Medeiros et al, ). The chemical changes of DOM properties (Table ) combined with the succession of microbial community (Figure b) observed in our study might reflect the generation of RDOM via the microbial carbon pump (Jiao et al, ), which proposes that the microbial successive processing on labile DOM generates long‐lived RDOM in the water column (Jiao et al, ; Jiao et al, ). Our SDOM incubation experiment suggests that the microbial mediated RDOM may contribute to the autochthonous recalcitrant carbon in coastal environments (Asmala et al, ), which could be subjected to further cycling through biotic and abiotic processes in the ocean.…”

Contribution of structural recalcitrance to the formation of the deep oceanic dissolved organic carbon reservoir

“…Another explanation for the long‐term persistence of RDOC is the structural recalcitrance of the molecules in a specific environmental context, which corresponds to the previously proposed concept of biologically inert RDOC (RDOC t ) (Jiao et al ., ). This idea is supported by the evidence that the chemical composition of the DOC in the deep ocean, where the RDOC dominates, is different from that of decomposable DOC (Jannasch, ; Lancelot et al ., ; Kaiser and Benner, ; Jiao et al ., ; Benner and Amon, ; Walker et al ., ).…”

Modeling radiocarbon constraints on the dilution of dissolved organic carbon in the deep ocean

“…A DOC pool that persists over millennial timescales in the deep ocean is central to the concept of the “microbial carbon pump” (MCP) [ Jiao et al , , ]. Similar to the solubility, soft‐tissue, and carbonate pumps that maintain a vertical gradient of DIC and alkalinity in the ocean against erosion by ocean circulation [ Volk and Hoffert , ], the MCP is posited to maintain a gradient of DOC from short‐lived, highly reactive DOC in the surface ocean to longer‐lived, less reactive DOC in the deep ocean [ Legendre et al , ].…”

“…Dilution and recalcitrant molecular structures are not mutually exclusive hypotheses. Jiao et al [] recognize that DOC in the deep ocean may be composed of both a fraction that is intrinsically recalcitrant and a fraction that is bioavailable but cannot be degraded due to its low concentration. Jiao et al [] alternatively suggest that bacterial growth observed in Arrieta et al [] was supported by a smaller bioavailable DOC fraction that was either present in the original DOC sample or was produced by processes such as viral lysis, chemolithoautotrophic activities, or grazing during the experiment rather than a release from dilution by enrichments.…”