REMineralisation of organic carbon by marine bActerIoplanktoN (REMAIN) - reducing the known unknown

The balance between the uptake of CO2 during phytoplankton photosynthesis and the production of CO2 during bacterial, zooplankton and phytoplankton respiration influences how much carbon can be stored in the ocean and hence how much remains in the atmosphere to affect climate. Yet, despite its crucial role, our knowledge of the respiration of component plankton groups such as bacteria, is severely limited because we do not have a method which can differentiate the respiration of one group from that of the rest of the community. We resort to measuring the respiration of a subsample which contains only cells which have passed through a filter. Just as we take in oxygen when we breathe, plankton take in oxygen when they respire and so the standard way to measure respiration is as the decrease in oxygen in the water. Unfortunately the low rates of respiration mean that measurements of oxygen have to be made over many hours and the disruption of the plankton foodweb by the filtration can lead to major errors.

The recent development of a much more sensitive method (reduction of the tetrazolium salt INT) which can produce results in minutes and does not involve disruption of the plankton foodweb, is a major step forward and has revealed previously unknown variability. Unexpectedly, the results also suggest that the proportion of respiration attributable to the bacterial size class is consistently low, even in communities where bacteria are the most numerous plankton. This has profound implications for our understanding of the amount of CO2 produced by different plankton groups, and poses two new questions - which size class contributes most to plankton respiration if not the bacterial size class, and what influences the variability in respiration if not the type of plankton present.

Marine scientists including ourselves have excitedly started to use the new INT technique, but it has not been thoroughly tested for all plankton communities. In fact, recent data suggest that the method can sometimes underestimate respiration because not all plankton can take up INT and sometimes overestimate respiration because compounds not associated with respiration can affect the INT. Thus while this method could potentially enable a critical improvement in our understanding and thus prediction of CO2 cycling in the ocean, these new intriguing results cannot be confirmed until a comprehensive test of the method has been completed. This is what we will do.

We have brought together an international team of experts to undertake an innovative combination of laboratory and field work, taking advantage of a unique sampling opportunity - the Atlantic Meridional Transect - which allows the study of five different plankton ecosystems in the Atlantic Ocean. We will first develop a novel fluorescent method of tracking the uptake of INT into a representative range of plankton to quantify how different cells take it up. Then we will measure the INT reduction of both seawater samples and laboratory cultures to which have been added chemical inhibitors of plankton respiration and increasing concentrations of naturally occurring organic compounds, to determine to what extent these organic compounds lead to an overestimate of respiration. These results will be used to improve and validate the INT method. Finally we will participate in the research cruise to determine plankton respiration in size classes of the plankton community alongside identification of the plankton in these size classes and the concentration of organic compounds.

The main deliverable of the project - apportionment of plankton respiration to plankton size classes - is of benefit to marine scientists who aim to predict how a changing climate will affect plankton production of CO2, policy makers interested in how much carbon can be stored in the ocean, and potentially commercial companies interested in the development of the fluorescent probe for medical or water quality applications.

Here we describe the non-academic communities who will benefit from the results of REMAIN and how they will benefit. In the Pathways to Impact section we detail the activities we will undertake in REMAIN in order to achieve this impact.

Policy makers

The lack of a validated dataset of bacterioplankton respiration restricts our ability to predict how a changing climate (increasing temperature, changing inorganic and organic nutrient concentrations) will affect the capacity of the ocean to mediate climate. This proposal aims to deliver lasting impact by providing increased understanding of the factors influencing the magnitude and variability of respiration attributable to the cells identified within size classes of the plankton community and thus carbon flow through the microbial foodweb now and in a future climate.

Our research will therefore benefit policy forming bodies such as National and International Governmental Environment and Climate Change Departments (e.g. UK Governmental bodies including DBEI and DEFRA and their international equivalents) and non-governmental organisations (NGOs), such as SCOR (Scientific Committee on Ocean Research), WCRP (World Climate Research Programme), IOC/UNESCO (Intergovernmental Oceanographic Commission / United Nations Educational, Scientific and Cultural Organization) and Future Earth with a particular interest in global carbon cycling and prediction of environmental change.

Governmental departments and regulatory agencies such as Cefas (Centre for Environment, Fisheries and Aquaculture Science), which assess the status of coastal marine ecosystems in the context of the Marine Strategy Framework Directive (MSFD) will benefit from REMAIN, as one of the key needs for implementing MSFD is better understanding of the functioning of marine ecosystems.

Global Carbon Budget modellers including those at the Tyndall Centre for Climate Change Research (http://www.tyndall.ac.uk/) who contribute to the IPCC assessments, will benefit from an improved understanding of the contribution of bacterioplankton to the production of CO2, and how this may be different in different ocean regions irrespective of the phytoplankton biomass or productivity.

Commercial companies

Commercial companies, particularly those involved in the use of bacterial enzymes, water quality and the functioning of antibiotics may benefit from REMAIN. Our development and testing of a new fluorescent probe to visualise intra-cellular aerobic respiratory activity may lead to a new method to differentiate active and dormant bacteria relevant to a range of physiological, medical and water quality applications.

General Public

REMAIN will be of interest to the general public, especially those who use the marine environment for sport and leisure, but also those who visit aquaria and museums or have a particular interest in the marine ecosystem or the microscopic structure of cells. Marine ecology is also of interest to school children, educators and the science media as a visual tangible example to use in learning and teaching the fundamentals of how the natural world works. The public will benefit from our research in terms of increased interest, enthusiasm and understanding of the marine environment, particularly of the importance of the smallest organisms in the sea.

Researcher development

REMAIN will be of benefit to the Researcher Co-I and PDRAs employed on the project, and the under- and postgraduate students whom the PI and Co-Is teach. The Researcher Co-I and PDRAs will benefit from extending their international scientific network and skillset, and broadening their understanding and training in flexibility and problem solving during fieldwork, thereby developing both their scientific and transferable skills. Under- and postgraduate students will benefit in terms of connecting with current research on marine plankton activity.