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

浮游植物光合作用期间对二氧化碳的吸收与细菌、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影响着海洋中可以储存多少碳，从而影响到大气中仍有多少碳影响气候。然而，尽管它起着至关重要的作用，我们对浮游生物组成群体（如细菌）的呼吸作用的了解还是非常有限的，因为我们没有一种方法可以将一个群体的呼吸作用与其他群体的呼吸作用区分开来。我们采用测量一个只包含经过过滤器的细胞的子样本的呼吸作用。就像我们呼吸时吸入氧气一样，浮游生物呼吸时也吸入氧气，所以测量呼吸的标准方法是测量水中氧气的减少量。不幸的是，低呼吸速率意味着测量氧气必须在很长时间内进行，过滤对浮游生物食物网的破坏可能导致重大错误。最近开发的一种更灵敏的方法（减少四氮唑盐INT）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，这是向前迈出的重要一步，揭示了以前未知的变异性。出乎意料的是，结果还表明，即使在细菌数量最多的浮游生物群落中，归因于细菌大小类别的呼吸作用比例也一直很低。这对我们理解不同浮游生物群体产生的二氧化碳量具有深远的意义，并提出了两个新问题——如果不是细菌的大小类别，那么哪种大小类别对浮游生物的呼吸作用贡献最大？如果不是浮游生物的类型，那么是什么影响了呼吸作用的可变性？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，因为不是所有的浮游生物都能吸收INT，有时又会高估呼吸作用，因为与呼吸作用无关的化合物会影响INT。因此，虽然这种方法有可能使我们对海洋中二氧化碳循环的理解和预测取得重大进展，但在对该方法进行全面测试之前，这些新的有趣结果还不能得到证实。这就是我们要做的。我们汇集了一个国际专家团队，利用独特的采样机会-大西洋经向样带-对大西洋五种不同的浮游生物生态系统进行研究，进行实验室和实地工作的创新结合。我们将首先开发一种新的荧光方法来跟踪INT的吸收到一个有代表性的浮游生物的范围，以量化不同的细胞如何吸收它。然后，我们将测量添加了浮游生物呼吸化学抑制剂和天然有机化合物浓度增加的海水样本和实验室培养物的INT减少量，以确定这些有机化合物在多大程度上导致了呼吸作用的高估。这些结果将用于改进和验证INT方法。最后，我们将参加研究巡航，以确定浮游生物群落大小类别的浮游生物呼吸，同时确定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——将浮游生物的呼吸作用分配到浮游生物的大小类别——对旨在预测气候变化将如何影响浮游生物产生二氧化碳的海洋科学家、对海洋中可以储存多少碳感兴趣的政策制定者以及对开发用于医疗或水质应用的荧光探针感兴趣的潜在商业公司都有好处。

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.