REMineralisation of organic carbon by marine bActerIoplanktoN (REMAIN) - reducing the known unknown

浮游植物光合作用中的二氧化碳吸收和细菌、浮游动物和浮游植物呼吸过程中产生的二氧化碳之间的平衡，会影响海洋中可以储存多少碳，从而影响大气中有多少碳会影响气候。然而，尽管它起着至关重要的作用，但我们对组成浮游生物群体(如细菌)的呼吸的了解严重有限，因为我们没有一种方法可以区分一个群体的呼吸与群落的其他群体的呼吸。我们求助于测量只包含通过过滤器的细胞的亚样本的呼吸。就像我们呼吸时吸入氧气一样，浮游生物在呼吸时也吸入氧气，因此测量呼吸作用的标准方法是水中氧气的减少。不幸的是，低呼吸速率意味着氧气的测量必须经过数小时，而过滤对浮游生物食物网的破坏可能会导致重大错误。最近开发出一种更灵敏的方法(减少四唑盐INT)，该方法可以在几分钟内产生结果，并且不涉及破坏浮游生物食物网，这是向前迈出的一大步，揭示了以前未知的变异性。出乎意料的是，结果还表明，可归因于细菌大小类别的呼吸比例一直很低，即使在细菌是最多浮游生物的社区也是如此。这对我们理解不同浮游生物类群产生的二氧化碳的数量有着深远的影响，并提出了两个新的问题--如果不是细菌大小类，哪个大小类对浮游生物呼吸作用贡献最大；如果不是浮游生物的类型，什么影响呼吸作用的可变性。包括我们在内的海洋科学家已经兴奋地开始使用新的INT技术，但它还没有在所有浮游生物群落中进行彻底的测试。事实上，最近的数据表明，这种方法有时会低估呼吸作用，因为并非所有浮游生物都能吸收INT，有时又高估了呼吸作用，因为与呼吸作用无关的化合物会影响INT。因此，虽然这种方法可能会对我们的理解产生重大影响，从而预测海洋中的二氧化碳循环，但在完成对该方法的全面测试之前，这些新的有趣的结果无法得到证实。这就是我们要做的。我们召集了一个国际专家团队，利用独特的采样机会-大西洋子午线样带-进行实验室和实地工作的创新结合，这使得我们可以研究大西洋的五个不同的浮游生物生态系统。我们将首先开发一种新的荧光方法来跟踪INT在具有代表性的浮游生物中的摄取，以量化不同的细胞如何摄取它。然后，我们将测量添加了浮游生物呼吸化学抑制剂的海水样本和实验室培养物的INT减少，以及自然产生的有机化合物浓度的增加，以确定这些有机化合物在多大程度上导致对呼吸的高估。这些结果将被用来改进和验证INT方法。最后，我们将参加研究巡航，以确定浮游生物群落的大小类别的浮游生物呼吸，同时识别这些大小类别的浮游生物和有机化合物的浓度。该项目的主要成果--将浮游生物呼吸按浮游生物大小分类--对海洋科学家有益，他们的目标是预测气候变化将如何影响浮游生物产生二氧化碳，政策制定者对海洋中可以储存多少碳感兴趣，还有潜在的商业公司对开发用于医疗或水质应用的荧光探针感兴趣。

The balance between the uptake of CO2 during phytoplankton photosynthesis and the production of CO2 during bacterial, zooplankton and phytoplankton respiration influences how much carbon can be stored in the ocean and hence how much remains in the atmosphere to affect climate. Yet, despite its crucial role, our knowledge of the respiration of component plankton groups such as bacteria, is severely limited because we do not have a method which can differentiate the respiration of one group from that of the rest of the community. We resort to measuring the respiration of a subsample which contains only cells which have passed through a filter. Just as we take in oxygen when we breathe, plankton take in oxygen when they respire and so the standard way to measure respiration is as the decrease in oxygen in the water. Unfortunately the low rates of respiration mean that measurements of oxygen have to be made over many hours and the disruption of the plankton foodweb by the filtration can lead to major errors.The recent development of a much more sensitive method (reduction of the tetrazolium salt INT) which can produce results in minutes and does not involve disruption of the plankton foodweb, is a major step forward and has revealed previously unknown variability. Unexpectedly, the results also suggest that the proportion of respiration attributable to the bacterial size class is consistently low, even in communities where bacteria are the most numerous plankton. This has profound implications for our understanding of the amount of CO2 produced by different plankton groups, and poses two new questions - which size class contributes most to plankton respiration if not the bacterial size class, and what influences the variability in respiration if not the type of plankton present. Marine scientists including ourselves have excitedly started to use the new INT technique, but it has not been thoroughly tested for all plankton communities. In fact, recent data suggest that the method can sometimes underestimate respiration because not all plankton can take up INT and sometimes overestimate respiration because compounds not associated with respiration can affect the INT. Thus while this method could potentially enable a critical improvement in our understanding and thus prediction of CO2 cycling in the ocean, these new intriguing results cannot be confirmed until a comprehensive test of the method has been completed. This is what we will do.We have brought together an international team of experts to undertake an innovative combination of laboratory and field work, taking advantage of a unique sampling opportunity - the Atlantic Meridional Transect - which allows the study of five different plankton ecosystems in the Atlantic Ocean. We will first develop a novel fluorescent method of tracking the uptake of INT into a representative range of plankton to quantify how different cells take it up. Then we will measure the INT reduction of both seawater samples and laboratory cultures to which have been added chemical inhibitors of plankton respiration and increasing concentrations of naturally occurring organic compounds, to determine to what extent these organic compounds lead to an overestimate of respiration. These results will be used to improve and validate the INT method. Finally we will participate in the research cruise to determine plankton respiration in size classes of the plankton community alongside identification of the plankton in these size classes and the concentration of organic compounds. The main deliverable of the project - apportionment of plankton respiration to plankton size classes - is of benefit to marine scientists who aim to predict how a changing climate will affect plankton production of CO2, policy makers interested in how much carbon can be stored in the ocean, and potentially commercial companies interested in the development of the fluorescent probe for medical or water quality applications.