Understanding and modelling the Microbial Carbon Pump under changing nutrient concentrations and temperature

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Seawater is a complicated soup of chemicals including dissolved organic material (DOM), such as sugars, fats and amino acids all containing carbon. In fact, there is roughly the same amount of carbon within marine DOM as there is CO2 in the atmosphere. So how did this carbon become DOM, and what controls its production and fate? Atmospheric CO2, dissolves in seawater where small single celled organisms called phytoplankton incorporate it into organic molecules essential for their growth. Some of these organic molecules leak from healthy cells, while more are released when cells die, or are eaten, creating an oceanic pool of DOM. Many people are familiar with the concept that phytoplankton support marine food webs and that dead cells and detritus generated by different biological processes sink to the seafloor to be buried in sediments. This process effectively transports carbon, originally present as atmospheric CO2, to the seafloor; this is termed the 'Biological Carbon Pump' (BCP). A separate process, which scientists have only recently become aware of, provides another way of removing and storing atmospheric CO2. The key role in this process is played by even smaller organisms which are numerically the most abundant life form in the oceans: the bacteria. Bacteria quickly act upon the DOM released from phytoplankton and the activities of their associated food web, scavenging parts they can most readily use for growth. Progressively, over weeks and months, sequential scavenging of components of DOM gradually transforms the chemical nature of the remaining material so that the residual molecules contain little else worth taking. These molecules, commonly defined as 'refractory-DOM', are biologically worthless, and are left to travel the Earth's Oceanic currents. The process described here is called the 'Microbial Carbon Pump' (MCP) and is thought to have slowly accumulated and stored a staggering amount of refractory-DOM over the past millennia, estimated to be 624 gigatonnes. This incredible reservoir of carbon is currently thought to be stable, with abiotic removal processes (e.g. photo degradation) balancing its production. However, recent studies suggest that that the projected decrease in surface ocean inorganic nutrient availability due to climate change could modify MCP activity, increasing refractory-DOM production with respect to its consumption. This implies that marine bacteria have the potential to mitigate the anthropogenic increase in atmospheric CO2 by shunting more carbon into refractory-DOM. This hypothesis, if verified, will radically change the way we think of the capacity of the biosphere to modulate climate, suggesting a previously overlooked climate-active role for marine bacteria.The only way we have to understand if this mechanism is significant is to use numerical models and run them under changing environmental conditions. However, to date, no ocean or Earth system models account for MCP dynamics.In this project, we will conduct laboratory experiments to provide the required level of physiological information and understanding needed to enable us to develop the first model describing the MCP and its relationship with nutrient concentration and temperature. This outcome will be the first critical step toward the simulation of the MCP in present and future oceans. To achieve this ambitious goal, the project will bring together a multidisciplinary team of internationally recognised scientists, from chemical analysts to system biology and ecosystem modellers. The project team will be boosted by the partnership with Prof N. Jiao (Xiamen University, China) who first proposed the MCP concept in a seminal paper 7 years ago.