Biophysical and ionic controls of buoyancy in diapausing calanoid copepods

鱿鱼类桡足类动物是世界海洋中的重要动物。它们是海洋浮游动物生物量的最大组成部分，是全球碳循环的主要环节。在北大西洋和北极，类callanoid桡足类是鳕鱼、鲭鱼和鲱鱼等重要商业鱼类的重要食物。许多花萼类桡足动物生命周期的一个关键特征是在很深的地方越冬，处于一种类似冬眠的状态。这增加了它们存活到下一季的机会，因为有时留在表层没有什么别的好处，它们可以避免被捕食。类callanoid桡足类的一个显著特征是它们含有异常高的脂肪（或脂质）。大量的脂质储存既是宝贵的能量储备，也是浮力的主要决定因素。达到中性浮力对桡足类动物在冬季很重要，因为它们必须尽量减少游泳的努力，以节省能量。在准备过冬的同时，必须在不影响桡足类动物获得中性浮力的能力之间寻求平衡。迄今为止，试图模拟这种平衡的最佳科学努力已被证明是不令人满意的。最近，已经确定了两种潜在的浮力控制机制。在一项研究中，萨托里斯和他的同事们发现，与活跃的、生活在水面上的桡足类动物相比，滞育桡足类动物体液（血淋巴）中的离子平衡有所不同。在第二项研究中，参与本提案的科学家表明，在深海的典型压力下，富含omega-3多不饱和脂肪酸（PUFAs）的脂质从液态变为固态。只有当PUFAs占脂质储存的50%以上时，这种效果才会发生，巧合的是，这种脂质储存通常存在于深度滞育的桡足类动物中，而不存在于那些仍然活跃在表面的桡足类动物中。目前，这两种机制只在南大洋桡足类中被发现，尽管先前在科学文献中被“误解”的证据也表明北半球的物种采用了类似的技术。我们将在北大西洋，北极和邻近的海湖的许多地点进行调查，以确定三种类calanoid桡足动物的脂质组成和血淋巴离子浓度。调查考虑到了环境的影响，特别是桡足类的微浮游食物的种类和可得性。这将决定是否存在对脂质储存中-3脂肪酸水平的有效调节。这种主动调节在冬季结束时可能特别重要，因为它是控制上升回表层的时间和速度的一种手段。我们的抽样策略，在研究过程中产生的新的分析技术和数据集的应用将允许这些问题得到解决。其次，利用统计技术，我们将重新考虑迄今为止模拟越冬深度的努力，并通过纳入额外的数据和机制寻求改进。例如，在由液态变为固态的过程中，脂质所占体积会减小，其对压力增加的响应也会发生变化。离子平衡的影响也将被考虑，主要是它如何帮助桡足动物在面对任何物理干扰时保持理论上的中性浮力深度。这项研究计划是基于我们最近的发现，即脂类的生物物理特性是控制海洋中生命分布的主要因素。这一发现为脂质在海洋生物中的作用提供了一个令人兴奋的新视角，为研究开辟了一个全新的方向，对我们对整个海洋食物网的理解具有深远的意义。

Non-technical summary Calanoid copepods are key players in World's oceans. They are the largest constituent of oceanic zooplankton biomass and are a major link within global carbon cycles. In the North Atlantic and Arctic, calanoid copepods are a vital food for commercially important fish species such as cod, mackerel and herring. A key feature of many calanoid copepod life-cycles is a phase of overwintering at great depth, in a state analogous to hibernation. This increases their chances of surviving to the next season through avoiding predation at times when there is little else to be gained by remaining within the surface layers. A notable feature of calanoid copepods is that they contain exceptionally high amounts of fat (or lipid). The large lipid store is both a valuable energy reserve and a major determinant of buoyancy. The attainment of neutral buoyancy is important to copepods over winter since they must minimise swimming effort in order to save energy. A balance must be sought between provisioning for the winter without disturbing the ability of the copepod to achieve neutral buoyancy. The best scientific efforts at trying to simulate this balance have so far proved to be unsatisfactory. Recently, two potential additional mechanisms of buoyancy control have been identified. In one study, Sartoris and colleagues found that diapausing copepods contained a different balance of ions in their bodily fluids (haemolymph) compared to active, surface dwelling copepods. In a second study, scientists involved in the present proposal showed that lipids rich in omega-3 polyunsaturated fatty acids (PUFAs) changed from liquid to solid state when under pressures typical of the deep sea. The effect only happened when PUFAs comprised more than 50% of the lipid store which, coincidentally, was commonly found in deep diapausing copepods, but not in those still active at the surface. At present, both of the mechanisms have only been identified in Southern Ocean copepods, although previously 'misinterpreted' evidence in the scientific literature also suggests that northern hemisphere species employ similar techniques. We will carry out surveys across a number of locations in the North Atlantic, Arctic and adjacent sea-lochs to determine lipid composition and haemolymph-ion concentrations in three calanoid copepod species. The surveys take into account environmental influences, particularly the type and availability of the microplanktonic food of copepods. This will determine whether there is any active regulation of the levels of omega-3 fatty acids in the lipid stores. Such active regulation may be of particular importance towards the end of winter as a means of controlling the timing and rate of ascent back into the surface layers. Our sampling strategy, application of novel analytical techniques and datasets generated during the research will allow these questions to be addressed. Secondly, using statistical techniques we will reconsider efforts made so far to simulate overwintering depth and seek improvements through including additional data and mechanisms. For instance, in changing from a liquid to solid state, the volume occupied by a lipid will be decreased and its response to increasing pressure will change. The effects of ionic balance will also be considered, mainly in how it may assist copepods maintain their theoretical neutral buoyancy depth in the face of any physical disturbance. This research proposal is based on our recent discovery, that the biophysical properties of lipids are a major factor controlling the distribution of life in the oceans. This finding gives an exciting new perspective on the role of lipids in marine organisms, opening up a fundamentally new direction for research, with profound implications for our understanding of the entire ocean food web.