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

海洋在决定世界气候方面起着重要作用。在某种程度上，这是由于非常小的单细胞生物产生氧气和消耗二氧化碳（CO2），这些生物被称为光合作用的浮游生物。原绿球藻属和聚球藻属的海洋蓝藻是光合浮游生物的原核成分，是地球上最丰富的两种光养生物！通过将大气中的二氧化碳固定到生物质中，这些生物充当了这种关键温室气体的汇。这种碳(C)封存的过程，被称为生物碳泵，是我们在对抗气候变化的斗争中所拥有的最大形式的自然资本。当这些蓝藻不断生长和分裂时，控制它们生长速度的最重要因素之一，即通过光合作用固定的二氧化碳量，是营养物质的可用性。海洋区域在这些必需营养物质如磷(P)、氮(N)和铁的供应方面差异很大。在磷含量较低的海洋地区，如北大西洋和地中海，壁青杆菌通过重塑其脂质组成来修改其细胞成分以保存磷。膜脂构成了所有细胞的结构基础，是细胞与外界环境之间的屏障。磷脂是蓝藻细胞膜的主要成分，但在P耗竭条件下，这些含P的脂质被不含P的亚脂质所取代。这种自然重塑过程的生理和生态后果尚不清楚。换句话说,我们不知道这个改造如何影响利率的二氧化碳固定或运输这如何影响这些生物的能力(收购)其他营养素,反过来影响这些生物的元素组成,它们释放的速度有机c。这是很重要的,因为不仅是海洋蓝藻关键贡献者全球二氧化碳固定但他们丰富预计将增加在未来几年内由于海洋环流的全球扩张变暖。因此，了解它们的初级产量在气候变暖的情况下是下降、增加还是保持不变，以及造成这种变化的机制，对于预测海洋生态系统功能的未来变化至关重要。因此，在本提案中，我们将确定在当前和升高的CO2水平下，在P消耗生长过程中的脂质重塑如何影响海洋蓝藻固定CO2的能力，从而改变其元素组成，获取关键的宏量和微量营养素。这不仅影响了对初级产量的准确估计，也影响了这些作为食草动物猎物的细胞的营养质量（从而影响了能量向更高营养水平的转移），相反，当细胞下沉时，从水柱中移除的细胞的元素组成——从而影响了碳、氮和磷的输出。我们还将确定限制N是否也会触发脂质重塑反应，如果是，其后果。获得的所有数据将用于改进当前描述营养限制对初级生产影响的生态系统模型公式。考虑到脂质重塑对初级生产的影响的新配方将在欧洲区域海洋生态系统模型（ERSEM）中实施，提供对海洋初级生产的实质性改进的模拟。总的来说，该提案将因此为理解脂质重塑在控制海洋初级生产中的作用提供直接估计和机制基础。数据和概念随后将用于ERSEM，以完善海洋光合作用和随后的碳循环控制点，并最终提高其预测能力。

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.