Assessing the bioavailability of iron in southeast Pacific seawater to phytoplankton using iron uptake rates

微小的单细胞植物，被称为浮游植物，需要碳等营养物质才能在海洋表层水中生存和生长。大气中的二氧化碳自然溶解到海洋表面，为浮游植物提供了碳的来源，这些碳可以被利用并转化为有机物；这个过程被称为光合作用，它有助于调节全球气候。除了碳，浮游植物还需要其他必需的营养物质来生长，其中一个关键的营养物质是铁。不幸的是，铁在海水中不能很好地溶解，因此在整个海洋中，铁的溶解浓度通常很低。在某些海洋区域，溶解的铁含量下降到很低的程度，以至于限制了浮游植物的生长，南大洋就是这样一个区域。幸运的是，海水中含有各种各样的溶解有机化合物，其中有我们熟悉的碳水化合物和氨基酸，也有我们知之甚少的化合物。这些不太为人所知的有机化合物（称为配体）与铁结合，有助于保持铁的溶解。这些配体显著影响浮游植物获得铁的数量，使其更具“生物可利用性”。因此，铁的有效性和这些配体的存在都可以影响浮游植物的生长/光合作用，从而对我们的气候产生影响。然而，由于缺乏对天然海水中配体池组成的了解，很难确定铁的生物利用度。评估这一点的一种方法是向天然海水注入放射性铁，它会附着在各种配体上；这个过程被称为放射性标签。然后，可以通过添加到海水混合物中的天然浮游植物培养物对放射性标记铁的吸收速率来评估生物可利用的实际铁量。这个项目的目的是利用从南大洋东南太平洋地区采集的海水来实现这一目标。将获得从三个不同站点和三个不同深度收集的海水的铁吸收率，并提供研究区域铁的“生物可利用性”的代理。这些信息可以用于气候模型，使我们能够更好地了解该地区铁的可用性对浮游植物碳吸收的影响，并预测未来海洋状态的变化如何影响碳循环。

NERC: Isobel Turnbull: NE/S007334/1Tiny single-celled plants, called phytoplankton, need nutrients such as carbon to live and grow in the surface waters of the ocean. Carbon dioxide in the atmosphere naturally dissolves into the surface ocean, providing phytoplankton with a source of carbon which can be used and turned into organic matter; a process known as photosynthesis which helps to regulate global climate. In addition to carbon, phytoplankton also require other essential nutrients to grow and one critical nutrient is iron. Unfortunately, iron does not dissolve well in seawater, and therefore the dissolved concentrations are generally found at low levels throughout the oceans. In certain oceanic regions, the levels of dissolved iron drop so low that they can limit phytoplankton growth, one such region is the Southern Ocean. Fortunately, seawater contains a wide variety of dissolved organic compounds, ones that we are familiar with such as carbohydrates and amino acids, as well as ones that we know little about. These lesser known organic compounds (known as ligands) bond to iron and help keep it dissolved. These ligands significantly impact the amount of iron that can be acquired by phytoplankton, making it more 'bioavailable'. Hence, both iron availability and the presence of these ligands can affect phytoplankton growth/photosynthesis and therefore have an impact on our climate. However, a lack of understanding of what makes up the ligand pool in natural seawater makes it difficult to determine the bioavailability of iron. One way to assess this, is to inject natural seawater with radioactive iron which will attach itself to the various ligands present; a process known as radiolabelling. The amount of actual iron that is bioavailable can then be assessed through the rate of radiolabelled-iron uptake by natural phytoplankton cultures that have been added to the seawater mix. This project aims to do this with seawater taken on an expedition to the southeast Pacific sector of the Southern Ocean. Iron uptake rates will be obtained for seawater collected from three different stations and three different depths, and provide a proxy for how 'bioavailable' the iron in the study region is. This information can then be used in climate models, enabling us to better understand the impact of iron availability in this region on carbon uptake by phytoplankton and predict how future changes to the ocean state may impact the carbon cycle.