The microbial carbon pump (MCP) is a biological process in the ocean where microorganisms, primarily bacteria and archaea, transform dissolved organic carbon (DOC) into refractory dissolved organic carbon (RDOC), which is resistant to further microbial degradation. This process effectively sequesters carbon in the deep ocean for centuries to millennia, contributing significantly to long-term carbon storage and climate regulation.Jiao, N. and Azam, F., 2011. "Microbial carbon pump and its significance for carbon sequestration in the ocean" Microbial carbon pump in the ocean, 10, pp.43–5.
Microbes metabolize labile (easily degradable) DOC from phytoplankton or other sources, producing RDOC as a byproduct. RDOC is chemically stable and persists in the ocean, resisting breakdown. This contrasts with the biological carbon pump, which sequesters carbon via sinking particulate organic matter (e.g., dead organisms or fecal pellets). The microbial carbon pump locks away carbon in the form of RDOC, which can remain in the deep ocean for thousands of years, reducing atmospheric carbon dioxide levels over long timescales. Estimates suggest RDOC accounts for a significant portion of the ocean's ~700 billion tons of dissolved organic carbon, making the microbial carbon pump a critical component of the global carbon cycle.
Diverse microbial communities, including bacteria like Prochlorococcus and Pelagibacter, drive the microbial carbon pump by transforming organic matter through metabolic processes. Environmental factors (e.g., nutrient availability, temperature) influence the efficiency of RDOC production. The microbial carbon pump helps regulate Earth's climate by storing carbon that would otherwise contribute to atmospheric carbon dioxide. It's particularly relevant in the context of climate change, as changes in ocean conditions (e.g., warming, acidification) could affect microbial activity and RDOC production.
Background theory of formation In past traditional frameworks for ocean carbon sequestration the biological carbon pump (BCP) describes the mechanism in which carbon dioxide is converted to organic carbon by marine primary producers in surface waters and is subsequently transferred to depth as particulate and/or dissolved organic carbon (POC/DOC). While a small fraction of the organic carbon taken in by these microorganisms is transported to the deep ocean and ultimately buried in marine sediments, the model fails to account for the large portion of DOC that is remineralized back into carbon dioxide and stays present throughout the water column.
Although the BCP process represents a realistic path for short-term carbon exportation to the deep, it does not account for the persistence of this substantial reservoir of DOC that constitutes approximately 95% of the total organic carbon in the ocean. This represents the discrepancy in the carbon pump models used; that despite the high efficiency of heterotrophic microorganisms in consuming organic substrates, a large fraction of DOC still remains in the ocean for millennia. Because traditional models are based on the physical export of carbon moving from the surface to the depths, they are insufficient to explain the long-term stability and accumulation of this carbon pool present in the ocean.
Early conceptual developments Before the introduction of the microbial carbon pump (MCP) theory for carbon sequestration, there were several other mechanisms used to describe organic matter cycling and transformation in the ocean. The microbial loop emphasized the role of bacteria and archaea in recycling dissolved organic matter (DOM) within the surface ocean, thereby retaining carbon and nutrients to the euphotic zone. Viral shunting described a process by which viral lysis of microbial cells converts particulate organic matter (POM) into dissolved forms (DOM), which prevents nutrient transfer to larger organisms and instead recycles carbon within the microbial communities. These processes improve understanding of short-term carbon cycling, but they do not fully resolve the mechanisms of the underlying long-term DOC persistence. Any organic carbon in the water column is hypothesized to eventually be taken up by surrounding organisms and vertically transported to the sediments through sinking particles.
To provide a clear understanding of the MCP mechanisms, DOM subcategories were made based on bioavailability and turnover times. Labile DOM (LDOM) is rapidly utilized (minutes to days) by surrounding microorganisms and is typically found in surface waters in the form of smaller simple molecules, while semi-labile DOM (SLDOM) persists for months to years before degradation, as it is made up of complex, nitrogen-rich substances. The turnover time is defined by how easily the carbon molecule is either taken up or broken down by surrounding organisms, which indicates why the speed of incorporation by an organism directly relates to the chemical complexity of the substance. The presence of recalcitrant DOM (RDOM) refers to highly complex carbon compounds that are distributed throughout the entire water column and are resistant to microbial utilization allowing long-term persistence with residence times ranging from centuries to millennia. The origin and maintenance of this recalcitrant pool remained an unresolved question in ocean carbon cycling that could not be concluded with the microbial loop and other biochemical carbon pumps, which only observed the direct movement of the carbon and not its chemical makeup.
The concept of the MCP (microbial carbon pump) was introduced to address this gap. It was proposed that microorganisms in the ocean actively transform labile organic carbon into recalcitrant forms (RDOM) that are able to remain in the water persistently over long timescales. This completely changes the way that ocean carbon sequestration is viewed. The biological carbon pump focuses solely on the physical movement of sinking particulate matter, while the MCP evaluates the organic carbon's biochemical makeup that allows it to be retained in the ocean without vertical export. Looking at it this way, the MCP is not referring to a single pathway for carbon storage, but to a whole network of biological processes that spans the whole water column on numerous different scales. These systems work together in tandem to move atmospheric carbon into the ocean sediments below. At the molecular level, microorganisms transform simple compounds such as amino acids and sugars into complex and structurally diverse molecules that resist further degradation. At the cellular level, membrane transport systems, including ATP binding cassette (ABC) transporters, regulate the uptake and release of the organic substrates that will influence the DOC composition in surrounding waters. Looking at the community/ecosystem levels, interactions among the microbial populations govern the production, transformation and persistence of organic matter, ultimately determining its fate in the ocean.
The conclusion of these proposed mechanisms were reached from evaluations of marine bacteria that were capable of producing RDOM directly from labile substrates. In subsequent studies, the microbial metabolites were noted to directly relate to the complex carbon molecules associated with long-term carbon cycling in the ocean, reinforcing the role of microbial processes in shaping the composition and continuance of DOC. It is together that these developments established the microbial carbon pump as a biochemical explanation for the accumulation of recalcitrant carbon in the ocean.
Evidence and distinction of MCP from BCP The major distinction between the MCP and other proposed pumps (biological carbon pump (BCP), carbonate counter pump (CCP), solubility carbon pump (SCP), etc.), is that it is built on an entirely different conceptual understanding in both its mechanism and criteria for carbon burial. In these carbon pumps, carbon sequestration is determined by the physical presence of the carbon moving from the surface waters to be deposited in sedimentation. In particular the BCP is a combination of the photosynthetic process of carbon fixation in the shallow waters and the exportation of this POM to the deep through simple gravitational sinking. The efficiency of this process can be directly based on the efficiency of what carbon molecules from the surface are able to be transported below the depth of rapid remineralization. Using this logic, carbon sequestration can only occur in lower depths of the ocean and in sediments leaving the whole pump largely ineffectual even though the uptake of carbon from microorganism is highly utilized. In contrast to this, the MCP does not rely of sinking rates of particles but rather emphasizes the chemical transformation of the organic matter into more resistant carbon molecules that will persistently exist throughout the water column. Therefore this pump is more defined by its heterotrophic microbial processing instead of particle export. To then determine overall efficiency, the study of the molecular makeup and reactivity of the carbon compounds are highly important compared to their vertical location in the water column. This mechanism challenges the original assumptions of the BCP, that depth is not indicative of carbon storage and biological productivity does not necessarily correspond to regions of efficient long-term sequestration. Many core understandings about carbon cycling in the ocean and how it can be studied were changed with the introduction of this carbon pump theory. While highly productive systems are still considered to enhance vertical carbon export through the BCP, areas such as oligotrophic environments can also substantially contribute to carbon sequestration through the gradual accumulation of RDOM. By decoupling carbon storage from nutrient-driven primary production, our overall understanding of elemental cycling, including carbon-to-nitrogen and carbon-to-phosphorus ratios in DOM pools is altered.