Biophysical and ionic controls of buoyancy in diapausing calanoid copepods

非技术概述甲壳类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，也是全球碳循环中的一个主要环节。在北大西洋和北极，甲鱼类桡足类是具有重要商业价值的鱼类物种的重要食物，如鳕鱼、鲱鱼和鲱鱼。许多甲壳类桡足类生活周期的一个关键特征是在类似冬眠的状态下，在很深的地方越冬。这增加了它们存活到下一个季节的机会，因为它们在留在表层没有什么其他东西可以获得的时候避免了捕食。甲壳类桡足类动物的一个显著特征是它们含有异常高的脂肪(或脂肪)。巨大的脂肪储备既是宝贵的能量储备，也是浮力的主要决定因素。达到中性浮力对于桡足类在冬天很重要，因为它们必须最大限度地减少游泳努力，以节省能量。必须寻求平衡，既要为冬季提供食物，又不能干扰桡足类动物实现中性浮力的能力。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浮力控制机制。在一项研究中，Sartoris和他的同事发现，滞育的桡足类动物的体液(血淋巴)中的离子平衡与活跃的、表面栖息的桡足类动物不同。在第二项研究中，参与这项研究的科学家们发现，在深海典型的压力下，富含omega-3多不饱和脂肪酸(PUFAs)的脂肪会从液态转变为固态。这种影响只有当多不饱和脂肪酸占脂肪储存的50%以上时才会发生，巧合的是，这种脂肪储存在深层滞育的桡足类中很常见，但在那些仍然活跃在水面的不饱和脂肪酸中却没有。目前，这两种机制都只在南大洋的桡足类中被发现，尽管之前科学文献中被误解的证据也表明，北半球的物种使用了类似的技术。我们将在北大西洋、北极和邻近海域的多个地点进行调查，以确定三个甲壳类桡足动物物种的脂肪组成和血淋巴离子浓度。调查考虑了环境影响，特别是桡足类微型浮游食物的类型和可获得性。这将决定脂肪储存中的omega-3脂肪酸水平是否有任何积极的调节。这种积极的调节在冬季接近尾声时可能特别重要，因为它是控制返回表层的时间和速度的一种手段。我们的抽样策略、新的分析技术的应用和研究期间产生的数据集将使这些问题得到解决。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，在从液体变为固体的过程中，脂质所占的体积会减少，其对压力增加的反应也会改变。离子平衡的影响也将被考虑，主要是在面对任何物理干扰时，它如何帮助桡足类保持其理论上的中性浮力深度。这项研究建议是基于我们最近的发现，即脂类的生物物理性质是控制海洋中生命分布的主要因素。这一发现为类脂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.