Elucidating the consequences of picocyanobacterial lipid remodelling for global marine primary production estimates

The oceans play a major role in determining world climate. In part, this is due to the production of oxygen and the consumption of carbon dioxide (CO2) by very small, single celled organisms, which are referred to as the photosynthetic picoplankton. Marine cyanobacteria of the closely-related genera Prochlorococcus and Synechococcus are the prokaryotic components of the photosynthetic picoplankton and are the two most abundant phototrophs on Earth! By fixing CO2 from the atmosphere into biomass these organisms act as a sink for this key greenhouse gas. This process of carbon (C) sequestration, known as the biological C pump, is the greatest form of natural capital we possess in the fight against climate change.

Whilst these cyanobacteria are continually growing and dividing, one of the most important factors controlling the rate at which they grow, and hence the amount of carbon dioxide that is fixed through photosynthesis, is the availability of nutrients. Oceanic regions vary considerably in their supply of these essential nutrients e.g. phosphorus (P), nitrogen (N) and iron. In oceanic regions where the levels of P are low e.g. the North Atlantic Ocean and Mediterranean Sea picocyanobacteria modify their cellular constituents to conserve P. They do this by remodelling their lipid composition. Membrane lipids form the structural basis of all cells, acting as a barrier between the cell and the external environment. Phospholipids are a major component of cyanobacterial cell membranes but under conditions of P depletion these P-containing lipids are replaced with non-P containing sulfolipids. The physiological and ecological consequences of this natural remodelling process are unknown. In other words we do not know how this remodelling affects rates of CO2 fixation or how this affects the ability of these organisms to transport (acquire) other nutrients and in turn affects the elemental composition of these organisms and the rate at which they release organic C.

This is important because not only are marine cyanobacteria critical contributors to global CO2 fixation but their abundance is expected to increase in future years due to expansion of ocean gyres as a result of global warming. Thus, understanding whether their primary production will decline, increase or remain unchanged in the face of climate warming and the mechanisms causing this are ultimately critical to forecasting future changes in the functioning of marine ecosystems.

Hence, in this proposal we will determine how lipid remodelling during P deplete growth under both current and elevated CO2 levels, affects the ability of marine cyanobacteria to fix CO2, acquire key macro- and micro-nutrients thereby modifying their elemental composition. This has consequences not only for accurate primary production estimates but also for the nutritional quality of these cells as prey for grazers (and hence for energy transfer to higher trophic levels) and conversely the elemental composition of cells removed from the water column when cells sink - and thus C, N and P export. We will also determine whether limitation for N also triggers a lipid remodelling response, and if so, its consequences. All of the data obtained will be used to refine current ecosystem model formulations describing the effect of nutrient limitation on primary production. The new formulation that takes into account the effect of lipid remodelling on primary production, will be implemented into the European Regional Seas Ecosystem Model (ERSEM) providing a substantially improved simulation of oceanic primary production.

Overall, the proposal will therefore provide direct estimates, and a mechanistic basis, for understanding the role of lipid remodelling in controlling marine primary production. Data and concepts will subsequently be used in ERSEM to refine control points for marine photosynthesis and subsequent carbon cycling and ultimately enhance their predictive capability.

This project addresses a fundamental question relating to the marine carbon cycle, namely the role of nutrient supply in modulating CO2 fixation in numerically abundant marine cyanobacteria. The work is therefore of utmost relevance to NERC's strategic aims, particularly Biodiversity Science and Climate Change themes. Indeed, a recent Science and Technology Committee report to the House of Commons about investigating the oceans highlighted the importance of "blue skies research" in marine science. It is clear as we move into an era in which environmental sustainability is a key concern, that science that addresses ecosystem sustainability issues is of great interest to the general public and relevant to policy makers, industry, economists and social scientists. Decisions taken by policymakers, for example in the Department of Energy and Climate Change (now within the Department for Business, Energy and Industrial Strategy), are informed by research into microbial ecology as microbial activity has continuous and far-reaching effects on the climate.

An important impact of this project on policy will be the delivery of a more accurate, physiologically-based (and therefore more reliable) version of the European Regional Seas Ecosystem Model (ERSEM). This will be able to properly simulate the extant ocean carbon cycle and to reliably test future scenarios hypotheses. Such models are increasingly required by scientific organizations such as the Intergovernmental Panel for Climate Change (IPCC), which aim to inform policy makers' decisions in relation to marine ecosystem management. ERSEM is already used by the National Centre for Ocean Forecasting (NCOF) and the UK Met Office (PML is in routine communication with these organizations) to underpin knowledge dissemination and provide consultancy regarding marine ecosystem services, protection and management of the marine environment to both policy makers and the general public. All these organizations (and their stakeholders) will therefore benefit from the outcomes of this project. The new model code will be readily available for all the above cited organizations (NCOF, UK Met Office) giving them the possibility to use the refined version of ERSEM to aid dissemination of scientific knowledge and inform policymakers.

Industry is actively looking for scientific breakthroughs that can support innovative mechanisms of carbon sequestration. Thus, whilst the results of this project will primarily provide fundamentally new knowledge of picophytoplankton-nutrient interactions, and particularly the relationship between photosynthesis, CO2 fixation and phosphorus supply, more generally the work will help to explain how nutrients shape the function of a key component of the marine microbial community. In so doing, new insights into mechanisms controlling the CO2 fixation potential of these organisms will be elucidated, which has implications for efficiencies of trophic transfer of carbon and hence of carbon sequestration mechanisms.

A variety of methods will be used to engage with end-users, including a detailed project website, regular updates in social media (such as PML and Warwick's twitter accounts https://twitter.com/PMLGroup; https://twitter.com/WarwickLifeSci), publications in popular magazines (e.g. Planet Earth), and visits and exhibitions at local schools.