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

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.

This proposal provides cutting-edge science to assess the functioning of the Microbial Carbon Pump in relation to changing environmental conditions such as temperature and inorganic nutrient concentration. As such, the proposed research is expected to have its major impact within the academic community (see Academic Beneficiaries section). However, there is an urgent need to inform policymaker's decisions and to heighten the general public's awareness of climate change issues which this project will also address through the mechanisms outlined below.

The main potential impact of this project on policy would be the delivery of a robust, physiologically based, model formulation describing MCP dynamics. This formulation will be embedded in the European Regional Seas Ecosystem Model (ERSEM) significantly improving the capability of this model to simulate the extant ocean carbon cycle and to reliably test future scenarios hypotheses. Indeed, this project will provide the wherewithal to enable an explicit description of one of the largest pools of carbon on Earth, a pool hitherto considered in steady-state and thus ignored. The new version of ERSEM will thus be a valuable tool to provide evidence-based policy advice on marine climate change which is increasingly required by EU governments. The new model will directly benefit the National Partnership for Ocean Prediction (NPOP) and the UK Met Office. Both already use ERSEM to disseminate knowledge and provide consultancy regarding marine ecosystem services, protection, management and insight into climate related issues to both policy makers and the general public.

A number of surveys have shown that public awareness about climate change is increasing. However, although it is widely understood that human-induced climate change is occurring, uncertainty about the impacts and feedbacks remain. For this reason public opinion increasingly looks for science-based answers to climate change questions. The outcomes of our project, highlighting the potential of marine microbes to mitigate the projected increases in atmospheric carbon dioxide, will thus be of broad public interest. High impact scientific publications (Nature, Science, PNAS, GRL etc) will be the natural vehicles to disseminate results. However we will also disseminate the project's findings through less specific literature such as the Marine Climate Change Impacts Partnership (MCCIP) annual report and the PML Annual Review, which is distributed internationally to 800+ recipients. We will also use 'The Conversation' as a platform for science-public dissemination, one that specifically promotes interactions with the public.

As opportunities arise, we will seek to present our research to UK and international audiences. A public facing webpage will be created and hosted by PML and the project's news and updated results will be promoted via social media channels, e.g. PML's Twitter feed and Marine Ripple Twitter feed (hosted by PML). We will work closely with colleagues in the Communications Group at PML who will help us to gain and co-ordinate press coverage throughout the project.