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. 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.
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