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

海洋在决定世界气候方面发挥着重要作用。在一定程度上，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微浮游生物。海洋蓝藻是原氯球藻属和聚球藻属的近缘属，是光合作用微微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)的固存过程，被称为生物C泵，是我们在应对气候变化方面拥有的最大形式的自然资本。虽然这些蓝藻不断生长和分裂，但控制它们生长速度的最重要因素之一是养分的可获得性，从而控制通过光合作用固定的二氧化碳数量。海洋地区对磷(P)、氮(N)和铁等基本营养素的供应差异很大。在磷水平较低的海洋地区，例如北大西洋和地中海，微囊藻通过改变细胞组成来保存磷，它们通过改变脂肪组成来做到这一点。膜脂构成了所有细胞的结构基础，是细胞和外部环境之间的屏障。磷脂是蓝藻细胞膜的主要成分，但在磷耗竭的条件下，这些含磷的脂类被不含磷的硫代磷脂所取代。这种自然改造过程的生理和生态后果尚不清楚。换句话说，我们不知道这种改造如何影响二氧化碳的固定速率，也不知道这如何影响这些生物运输(获取)其他营养物质的能力，进而影响这些生物的元素组成和它们释放有机C的速率。这一点很重要，因为海洋蓝藻不仅是全球二氧化碳固定的关键贡献者，而且由于全球变暖导致海洋涡旋的扩大，它们的丰度预计在未来几年将会增加。因此，了解海洋蓝藻初级生产量在气候变暖的情况下是下降、增加还是保持不变，以及导致这种变化的机制，对于预测未来海洋生态系统的功能变化至关重要。因此，在本提案中，我们将确定在当前和高二氧化碳水平下，磷枯竭生长过程中的脂质重塑如何影响海洋蓝藻固定二氧化碳的能力，获得关键的宏微营养物质，从而改变其元素组成。这不仅影响到初级生产力的准确估计，而且影响到作为食草动物猎物的这些细胞的营养质量(因此能量转移到更高的营养水平)，反之，当细胞下沉时从水柱中移走的细胞的元素组成--从而C、N和P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，其后果是什么。所有获得的数据将用于改进目前的生态系统模型公式，描述营养限制对初级生产的影响。考虑到脂类重塑对初级生产力的影响的新公式将被落实到欧洲区域海洋生态系统模型中，从而大大改进了对海洋初级生产力的模拟。因此，该提案将提供直接估计，并为理解脂类重塑在控制海洋初级生产力方面的作用提供机制基础。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的控制点，并最终增强它们的预测能力。

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.