Role of dimethyl sulphide (DMS) in pelagic tritrophic interactions

海洋的生物量只占陆地生物量的1.5%。然而，它们提供的年总产量与陆地相似，海洋中有机物的周转时间比陆地生态系统快1000倍。这突出表明，浮游动物的放牧是不成比例的重要，食草动物之间的竞争是高度激烈的。浮游植物进化出了保护自己免受食草动物侵害的机制，这并不奇怪。这些防御包括形态防御，如抗放牧壳，例如在“装甲”鞭毛藻中，以及化学防御，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化学防御在捕食者和猎物之间的营养相互作用中的作用。然而，众所周知，陆地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），以吸引捕食者的敌人。这反过来又减少了食草动物的数量，使植物免于过度放牧的压力。令人惊讶的是，这种信息化学介导的三营养相互作用尚未在海洋浮游生物中得到记录，我们提出的研究将纠正这一缺点。我们将把活动重点放在一种特定的海洋挥发性物质：二甲基硫化物（DMS）。这种化合物可能是所有海洋微量气体中研究得最好的，因为对DMS研究的兴趣主要集中在它在调节气候方面的作用。我们开始认识到DMS也具有生态重要性，并发现许多生物可以使用DMS的羽状作为方向线索。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。桡足类是微小浮游动物原生生物（单细胞纤毛虫和鞭毛虫）的主要消费者，而这些原生动物是许多小型浮游植物物种的重要食草动物。从生物地球化学角度来看，纤毛虫平均占桡足类动物碳消耗量的30%，约占海洋初级总产量的5%，是年渔业捕捞量（约1亿吨/年活重）的100倍。然而，如果包括微型浮游动物的其他组成部分，特别是鞭毛藻，这些估计可能会高得多。有趣的是，小型浮游动物的放牧会导致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它们的捕食者的“气味”，这可能会使它们的敌人更容易受到桡足类动物的攻击。因此，许多产生dms的浮游植物物种成功竞争并产生足以从太空中看到的藻华（例如球石藻埃米利亚·赫克斯黎）或可能对其他生物有害（例如有毒的鞭毛藻）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。这些数据将使我们能够在营养框架下对放牧诱导的DMS生产进行首次评估。我们还将用新收集的浮游生物进行实地实验，以普利茅斯附近和东北大西洋以颗石藻为主的水域的数据验证我们的实验室结果。我们的数据将为旨在预测DMS不同产量对微型浮游动物对桡足动物放牧的敏感性和桡足动物繁殖力的影响的建模工作提供信息。我们项目的这一部分将通过捆绑博士奖学金来实现。

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.