Microbes such as bacteria in the ocean, which mediate the microbial carbon pump (MCP), was reported to substantially influence the carbon cycle of the Earth system. These tiny unicellular organisms, probably smaller than 1 micrometer (1/150 of a single hair tip), are playing a disproportionately big role in a process so-call 'carbon sequestration'. In this process, they take up labile organic carbon produced by phytoplankton, transfer into the recalcitrant form that can resist from degradation for thousands of years, and eventually remove carbon dioxide from the atmosphere. They act functionally as a "pump" in sucking anthropogenic greenhouse gases into the ocean interior as a butter and thus regulating the global climate. Therefore, the MCP is considered an "invisible hand" behind the vast oceanic dissolved carbon reservoir.

However, the MCP was yet to be fully understood. In particular, this important process has not been quantified using a comprehensive physical-biological coupled model of a regional ocean. A recent paper published in SCIENCE CHINA Earth Sciences, attempted to address this issue. A multidisciplinary group from Xiamen University, University of Delaware, and Fuzhou University conducted the study with the support from '973' project - 'Processes and mechanisms of carbon sequestration by microbial carbon pump in the ocean'.

In this research, a state-of-the-art regional ocean model was deployed, which was capable to simulate the physical and biological dynamics in the ocean. The MCP processes were explicitly simulated, and, henceforth, a variety of MCP properties in time and space can be better depicted and understood in the South China Sea. Averagely in the South China Sea, the carbon sequestration rate of MCP was estimated to be about 1/6 of another better-understood biological carbon sequestration process, namely the biological pump.

Moreover, one of the many benefits of such a modeling system is that the future state of the ocean can be projected quantitatively given the condition of a changing environment. In this case, the model was simulated with the scenario that sea surface temperature increased for two- and four-degree centigrade warmer than the current state. It is found that the importance of MCP in sequestrating oceanic carbon under global warming may be further highlighted because the smaller planktonic organisms favorable for MCP is less vulnerable to the reduced nutrient supply in the projection. In the four-degree warming scenario, the rate of carbon sequestration by MCP compared with that from biological pump may raise several (~2%) percent, which can be remarkable considering the delicate balance of the microbial food web and carbon cycle and the extremely long age of the recalcitrant carbon it produces. Since the oceanic dissolved organic carbon pool largely formed through MCP contents as much carbon as the atmosphere, the changes in MCP might significantly modify the balance between these two reservoirs. The results of this study even imply the potential of geoengineering with the concept of MCP. Surely, before any engineering on the earth system, furthermore studies on this big player in the carbon cycle should be carried out.

The first author, Dr. Wenfang Lu notes that this study is, as commented in the peer-review, the 'first' and a 'timely' attempt to simulate the MCP process in the China Sea. Although the simulation is somewhat in an early stage, the important message we can learn is the controlling environmental factors of the spatial and temporal distribution of MCP. It provides a groundwork upon which future studies can build.

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This work was primarily supported by the National Basic Research Program (Grant No. 2013CB955704), and the National Program on Global Change and Air-Sea Interaction (Grant No. GASI-03-01-02-05). It was also partially supported by the SOA Global Change and Air-Sea Interaction Project (Grant No. GASI-IPOVAI-01-04), the National Natural Science Foundation of China (Grant Nos. 41630963, 41476007 & 41476005).