Role of dimethyl sulphide (DMS) in pelagic tritrophic interactions

海洋仅含有约1.5%的陆地生物量。然而，它们提供了与陆地类似的年总产量，与陆地生态系统相比，海洋中有机物的周转时间快了1000倍。这突显了浮游动物捕食的重要性不成比例，食草动物之间的竞争非常激烈。浮游植物进化出保护自己免受食草动物伤害的机制也就不足为奇了。这些防御包括形态防御，如耐食壳，例如甲壳类，以及影响食草动物选择性的化学防御，如复杂的化学威慑。多年来，我们对化学防御在捕食者和猎物之间的双营养相互作用中的作用积累了很好的理解。然而，众所周知，陆地植物使用另一种狡猾的防御策略，涉及生产挥发性信号化合物(所谓的信息化学物质)，以吸引它们的捕食者的敌人。这反过来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，这种由信息化学介导的三营养相互作用在海洋浮游生物中还没有被记录在案，我们提出的研究将纠正这一缺点。我们将重点关注一种特殊的海洋挥发物：二甲基硫化物(DMS)。这种化合物可能是所有海洋痕量气体中研究最好的，因为人们对DMS研究的兴趣很大程度上涉及到它在调节气候方面的作用。我们开始意识到DMS也具有生态重要性，并发现许多生物可以使用DMS的羽毛作为它们定向的方向提示。例如，一些海鸟使用DMS来定位食物密度高的区域。最近，我们还发现浮游动物桡足类对DMS梯度有反应。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，而微型浮游动物原生动物是许多小型浮游植物物种的重要捕食者。在生物地球化学方面，纤毛虫平均占桡足类消耗的碳的30%，约占海洋初级生产量的5%，按碳计算是年渔业捕获量(约100Mt yr-1活重量)的100倍。然而，如果包括微型浮游动物的其他组成部分，特别是甲藻，这些估计可能会高得多。有趣的是，微型浮游动物的觅食可以导致DMS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捕食者的“气味”，这可能会使他们的敌人更容易受到桡足类动物的攻击。因此，许多产生DMS的浮游植物物种在竞争中获胜也就不足为奇了，它们可以产生足以从太空中看到的藻类水华(例如，浮游球虫Emily huxleyi)，或者可能对其他生物有害(例如有毒的甲藻)。我们的项目将使用实验室实验，我们将量化微型浮游动物和桡足类动物的放牧与浮游植物制造DMS的能力的关系。这些数据将使在三营养框架下对放牧诱导的DMS生产的第一次评估成为可能。我们还将对新采集的浮游生物进行实地实验，以验证我们的实验室结果，并使用普利茅斯附近和东北大西洋以球藻为主体的水域的数据。我们的数据将为建模工作提供信息，这些建模工作旨在预测DMS的不同生产对微型浮游动物对桡足类放牧的敏感性和桡足类的繁殖力的影响。我们项目的这一部分将通过并列的博士生资格来实现。

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.