Dissecting, and revealing the controls on, the group-specific CO2 fixation budget of the Atlantic Ocean

海洋在决定世界气候方面起着重要作用。在某种程度上，这是由于非常小的单细胞光合生物，即浮游植物产生氧气和消耗二氧化碳（CO2）。浮游植物生物量主要由三个主要类群组成：原绿球藻属和聚球藻属，以及由小于5微米的细胞组成的真核生物。然而，人们对浮游植物“黑箱”内部发生的事情知之甚少，特别是关于碳生物量的分布和特定群体的初级生产，这些信息对于理解这些群体在全球碳循环中的作用至关重要。最近，我们的团队优化了放射性示踪剂培养-流式细胞术分选技术的利用，以揭示北大西洋几个站点的群体特定二氧化碳固定率。这一数据不仅揭示了不同地点间群体特定二氧化碳固定率的差异，还重申了真核生物组分在初级产量估算中的重要性（根据地点的不同，贡献为25-50%），尽管在数量上它们远远超过原核生物组分。在这里，我们建议扩展这项工作，以便首次揭示在流域尺度上，以及在地表水和深层叶绿素最大值（DCM）下的群体特定的二氧化碳固定率。我们将沿着大西洋经向样带（横贯英国和福克兰群岛之间的大西洋）连续几年开展这项工作，以便i)获得大西洋特定群体的完整二氧化碳固定预算，ii)可以评估年际变化。此外，我们将通过对分类种群的荧光原位杂交和对原核属（聚球藻和原绿球藻）的谱系特异性寡核苷酸探针或对光合作用的picoeukaryote （PPE）部分的类别特异性探针来研究不同分类谱系对picoplankton类群率的精确贡献。因此，该项目将提供主要“参与者”的基本信息和二氧化碳就地固定的路线，这是一个支撑海洋碳循环的过程。此外，我们将通过船上的瓶子实验，在添加营养物质或改变辐照度后，研究群体特异性碳固定率的环境控制。这将使我们了解环境扰动如何控制特定群体的二氧化碳固定潜力。我们将把后一项工作与功能基因组学（转录组学）方法结合起来，专门针对PPE部分，为控制这一群体的潜在调节因子提供补充的分子评估。这是基于这样一种想法，即转录谱分析将“让生物体告知我们这些生物体正在响应的关键环境参数”。总的来说，这项工作将使我们对海洋二氧化碳固定的途径和控制的理解取得重大进展，这些信息对于预测海洋碳循环至关重要。

The oceans play a major role in determining world climate. In part this is due to the production of oxygen and the consumption of carbon dioxide (CO2) by very small, single celled photosynthetic organisms, the picophytoplankton. Picophytoplankton biomass is dominated by three main groups: the prokaryotic genera Prochlorococcus and Synechococcus, and eukaryotes comprising cells <5 micrometres in size. However, little is known of what goes on inside the picophytoplankton 'black box' particularly with respect to the distribution of carbon biomass and group-specific primary production, information which is fundamental to understanding the roles of these groups in the global C cycle. Very recently our team has optimised utilisation of radiotracer incubation-flow cytometric sorting technology, to reveal group-specific CO2 fixation rates at several stations in the North Atlantic. As well as revealing variability in group-specific CO2 fixation rates between sites this data reiterates the importance of the eukaryotic fraction in primary production estimates (contribution 25-50% dependent on location) even though numerically they are vastly outnumbered by their prokaryotic counterparts. Here, we propose to extend this work so that for the first time we can reveal group-specific CO2 fixation rates at the basin scale, as well as in both surface waters and at the deep chlorophyll maximum (DCM). We will perform this work along an Atlantic Meridional Transect, which traverses the Atlantic Ocean between the UK and the Falkland Islands, and in consecutive years, so that i) a complete group-specific CO2 fixation budget of the Atlantic Ocean is attained and ii) inter-annual variability can be assessed. Moreover, we will examine the precise contribution of different taxonomic lineages to the picoplankton group rates using fluorescent in situ hybridisation of sorted populations and lineage-specific oligonucleotide probes for the prokaryotic genera (Synechococcus and Prochlorococcus) or class-specific probes for the photosynthetic picoeukaryote (PPE) fraction. Hence, this project will provide fundamental information of the major 'players' and routes of CO2 fixation in situ, a process that underpins marine C cycling. Furthermore, we will investigate environmental control of group-specific C fixation rates using on-board bottle experiments following either nutrient addition or shifts in irradiance. This will allow us to understand how environmental perturbation controls the CO2 fixation potential of specific groups. We will couple this latter work with a functional genomics (transcriptomics) approach specifically targeted at the PPE fraction to provide a complementary molecular assessment of the potential regulatory factors controlling this group. This is based on the idea that transcriptional profiling will 'let the organism inform us of the key environmental parameters that these organisms are responding to'. Taken together this work will make major inroads in our understanding of the routes and controls of marine CO2 fixation, information which is essential for a predictive understanding of marine C cycling.