Biophysical and ionic controls of buoyancy in diapausing calanoid copepods

非技术性总结哲水蚤桡足类是世界海洋中的关键角色。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，哲水蚤是鳕鱼、鲭鱼和鲱鱼等重要商业鱼类的重要食物。许多哲水蚤桡足类生命周期的一个关键特征是在很深的地方越冬，处于类似于冬眠的状态。这增加了他们生存到下一个季节的机会，通过避免捕食的时候，没有什么其他可以获得留在表层。 哲水蚤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。在冬季，获得中性浮力对于桡足类动物来说很重要，因为它们必须最大限度地减少游泳的努力以节省能量。必须寻求一个平衡之间的供应冬季不干扰桡足类的能力，以实现中性浮力。到目前为止，试图模拟这种平衡的最好的科学努力被证明是不令人满意的。最近，已经确定了两种潜在的附加浮力控制机制。在一项研究中，Sardian及其同事发现，滞育桡足类与活跃的表面栖息桡足类相比，其体液（血淋巴）中含有不同的离子平衡。在第二项研究中，参与本提案的科学家表明，富含omega-3多不饱和脂肪酸（PUFA）的脂质在深海典型压力下从液态变为固态。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，这种情况通常存在于深滞育桡足类中，但在那些仍然活跃在表面的桡足类中却没有。 目前，这两种机制只在南大洋桡足类中被发现，尽管科学文献中先前“误解”的证据也表明北方物种采用类似的技术。我们将在北大西洋，北极和邻近的海湖泊的一些位置进行调查，以确定脂质成分和血淋巴离子浓度在三个哲水蚤桡足类物种。这些调查考虑到环境影响，特别是桡足类微生物营养食物的类型和可获得性。这将决定是否有任何积极的调节水平的omega-3脂肪酸的脂质商店。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到表层的时间和速度的一种手段。我们的采样策略，新的分析技术和研究过程中产生的数据集的应用将使这些问题得到解决。 第二，利用统计技术，我们将重新考虑迄今为止在模拟越冬深度方面所做的努力，并通过纳入更多的数据和机制来寻求改进。例如，在从液态变为固态时，脂质所占据的体积将减小，并且其对增加的压力的响应将改变。离子平衡的影响也将被考虑，主要是在它如何可能帮助桡足类维持其理论上的中性浮力深度在面对任何物理干扰。 这项研究建议是基于我们最近的发现，即脂质的生物物理特性是控制海洋生命分布的一个主要因素。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，对我们理解整个海洋食物网具有深远的影响。

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.