Microbial Carbon Pump ---A New Mechanism for Long-Term Carbon Storage in the Global Ocean (Invited)

Marine dissolved organic matter (DOM) reservoir, containing carbon equivalent to the total carbon inventory of atmospheric CO2, is an important issue in understanding the role of the ocean in climate change. The known biological mechanism for oceanic carbon sequestration is the biological pump, which depends on vertical transportation of carbon either through particulate organic matter (POM) sedimentation or DOM export by mixing and downwelling. Both the POM and the DOM are subject to microbial mineralization and most of the organic carbon will be returned to dissolved inorganic carbon within a few decades. Only a small fraction of the POM escapes mineralization and reaches the sediment where organic carbon can be buried and stored for thousands and even millions of years. The efficiency of the biological pump is currently the basic measure of the ocean’s ability to store biologically fixed carbon. However, the production and fate of the large pool of recalcitrant DOM with an averaged turnover time of 4000-6000 thousands of years in the water column has not been adequately considered to date. Marine microbes essentially monopolize the utilization of DOM. Although their diverse adaptive strategies for using newly fixed carbon are well known, major gaps exist in our knowledge on how they interact with the large pool of DOM that appears to be recalcitrant. This is an important problem, as DOM molecules that are not degraded for extended periods of time constitute carbon storage in the ocean. A newly proposed concept - the “microbial carbon pump (MCP)” (NATURE REVIEWS Microbiology 2010.8:593-599) (also see diagram below) provides a formalized focus on the significance of microbial processes in carbon storage in the recalcitrant DOM reservoir, and a framework for testing hypotheses on the sources and sinks of DOM and the underlying biogeochemical mechanisms. The MCP, through concessive processing of DOM, transforms some organic carbon from the reactive DOM pools to a recalcitrant carbon reservoir, pumping organic carbon from low concentrations of labile DOM to high concentrations of recalcitrant DOM, building up a huge reservoir for carbon storage over time. Meanwhile the MCP transfers more carbon relative to nitrogen and phosphorus from the reactive organic matter pool into recalcitrant organic matter pool. Compared with the solubility pump, an abiotic mechanism for carbon storage in the ocean which has ocean acidification impacts on marine organisms and biogeochemical cycles, the MCP-driven recalcitrant DOM carbon storage does not appreciably alter the buffering capacity of seawater and has no known negative impact on marine organisms. Furthermore, in the ocean warming scenario, the partitioning of biogenic carbon flow will change, with the flow to POM diminishing and that to DOM increasing, and thus the role of the MCP in carbon storage will most likely enhanced. A working group joined by 26 scientists from 12 countries has been formed under the Scientific Committee for Oceanic Research (SCOR-WG134) to address this multi-faceted biogeochemical issue related to carbon cycling in the ocean and global climate changes.