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

点击翻译按钮获取中文摘要

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.