Role of dimethyl sulphide (DMS) in pelagic tritrophic interactions

海洋仅包含陆地生物量的1.5%。然而，海洋生态系统提供的年生产总量与陆地生态系统相似，海洋生态系统中有机物的周转时间比陆地生态系统快1000倍。这突出表明浮游动物的食草作用非常重要，食草动物之间的竞争也很激烈。浮游植物进化出保护自己免受食草动物侵害的机制并不奇怪。这些包括形态防御，如抗放牧的外壳，例如在“装甲”甲藻，和化学防御，如复杂的化学威慑，影响食草动物的选择性。多年来，我们已经积累了很好的理解的作用，化学防御的捕食者和猎物之间的双营养相互作用。然而，众所周知，陆生植物使用另一种狡猾的防御策略，包括产生挥发性信号化合物（所谓的信息化学物质），吸引捕食者的敌人。这反过来又减少了食草动物的数量，并将植物从过度的放牧压力中释放出来。令人惊讶的是，这种信息化学介导的三营养相互作用还没有记录海洋浮游生物和我们提出的研究将纠正这一缺陷。我们的活动将侧重于一种特殊的海洋挥发物：二甲基硫。这种化合物可能是所有海洋痕量气体中研究得最好的，因为对二甲基硫醚研究的兴趣主要集中在其调节气候的作用上。我们开始意识到二甲基硫醚也具有生态重要性，并发现许多生物可以利用二甲基硫醚羽流作为定向线索。例如，一些海鸟利用二甲基硫醚来确定食物密度高的地区。最近，我们还发现浮游桡足类动物对DMS梯度的反应。桡足类是微型浮游动物原生动物（单细胞纤毛虫和鞭毛虫）的主要消费者，这些原生动物是许多小型浮游植物物种的重要食草动物。在生物地球化学方面，纤毛虫平均占桡足类消耗碳的30%，约占海洋初级生产总量的5%，是年捕鱼量（约1亿吨/年活重）的100倍。然而，如果将微型浮游动物的其他组成部分，特别是腰鞭毛虫包括在内，这些估计数可能会高得多。有趣的是，微型浮游动物的摄食可导致二甲硫醚产量的急剧增加，这取决于浮游植物制造这种气体的能力。因此，浮游植物可能会积极影响它们的捕食者的“气味”，这可能使它们的敌人更容易受到桡足类的攻击。因此，许多生产二甲基甲酰胺的浮游植物物种成功地进行竞争并能产生大到从太空都能看到的藻华（例如球石藻Emiliania huxleyi）或对其他生物体有害（例如有毒的甲藻）也就不足为奇了。我们的项目将使用实验室实验，我们将量化的微型浮游动物和桡足类的关系，浮游植物的能力，使二甲硫醚的放牧。这些数据将使第一次评估放牧引起的生产DMS在一个三营养的框架。我们还将用新鲜收集的浮游生物进行实地实验，以验证我们的实验室结果与来自普利茅斯和东北大西洋以颗石藻为主的沃茨的数据。我们的数据将告知建模工作，旨在预测二甲基硫的差异生产的微型浮游动物桡足类的放牧和桡足类的繁殖力的敏感性的影响。我们项目的这一部分将通过一个绑博士生奖学金实现。

The oceans contain only about 1.5% of terrestrial biomass. However, they provide a similar amount of total annual production to that on land and the turnover time for organic matter is 1000-times faster in marine in comparison to terrestrial ecosystems. This highlights that grazing by zooplankton is disproportionally important and competition among grazers is high. It is not surprising that phytoplankton have evolved mechanisms to protect themselves from grazers. These include morphological defences such as grazing-resistant shells, for example in 'armoured' dinoflagellates, and chemical defences such as sophisticated chemical deterrence that influence the selectivity of grazers. Over the years we have accumulated a good understanding of the role of chemical defences in the bitrophic interactions between predators and their prey. However, it is also well known that land plants use another cunning defence strategy that involves the production of volatile signalling compounds (so called infochemicals) that attract the enemy of their predators. This in turn reduces the number of herbivores and releases the plants from excessive grazing pressure. Surprisingly, such infochemical-mediated tritrophic interactions have not been documented for oceanic plankton and our proposed research will rectify this shortcoming. We will focus our activities on one particular marine volatile: dimethyl sulphide (DMS). This compound is probably the best-studied of all marine trace gases, because much interest in DMS research concerns its role in regulating climate. We are starting to appreciate that DMS also has ecological importance and find that many organisms can use plumes of DMS as directional cue for their orientation. For example, some sea birds use DMS to locate areas of high food density. Recently, we also found that zooplankton copepods react to DMS gradients. Copepods are dominant consumers of microzooplankton protists (unicellular ciliates and flagellates) that are important grazers of many small phytoplankton species. In biogeochemical terms ciliates account for, on average, 30 % of the carbon consumed by copepods, representing approximately 5 % of total oceanic primary production and 100 fold the annual fisheries catch (~ 100 Mt yr-1 live weight) in carbon terms. However, these estimates may be considerably higher if other components of the microzooplankton, in particular dinoflagellates, are included. Interestingly, grazing by microzooplankton can result in a dramatic increase of DMS production and this is dependent on the ability of the phytoplankton to make this gas. Hence, phytoplankton may actively influence the 'smelliness' of their predators and this likely makes their enemies more susceptible to copepod attack. It is then not surprising that many of the DMS-producing phytoplankton species are competing successfully and can produce algal blooms that are large enough to be seen from space (for example the coccolithophore Emiliania huxleyi) or can be harmful to other organisms (for example toxic dinoflagellates). Our project will use laboratory experiments where we will quantify grazing of microzooplankton and copepods in relationship to the ability of phytoplankton to make DMS. These data will enable a first assessment of grazing-induced production of DMS in a tritrophic framework. We will also conduct field experiments with freshly collected plankton to verify our laboratory results with data from coccolithophore-dominated waters off Plymouth and in the North-East Atlantic. Our data will inform modelling efforts that aim to predict the effect of differential production of DMS on the susceptibility of microzooplankton to copepod grazing and the fecundity of copepods. This part of our project will be realised through a tied PhD studentship.