Partitioning of C, N and P between particulate and dissolved phases during growth of phytoplankton at different pH.

海洋浮游植物在大气、海洋和海洋沉积物之间的碳(C)、氮(N)和磷(P)等生物重要元素的循环中发挥着核心作用。在短期(几周)内，浮游植物可以繁殖，形成大量新的细胞，这些细胞对表层海洋中的颗粒有机物(POM)做出了贡献。在这样做的过程中，它们从海水中吸收营养物质(N和P)和二氧化碳(CO2)。这种二氧化碳被大气中的二氧化碳所取代，后者溶解在表层海洋中，恢复了长期的海洋-大气平衡。在水华期间，一些细胞被食草动物吃掉，支持海洋食物网，而另一些细胞死亡或粘在一起下沉。到达海洋沉积物的物质构成了“生物碳泵”，它能够在地质时间尺度上掩埋大气中的二氧化碳和其他营养物质。然而，这些并不是同化营养的唯一命运。在浮游植物的生长过程中，有机分子从细胞释放到周围的海水中。这些有机物(溶解有机物/DOM)被细菌利用，细菌将其降解，再生营养物质，并向大气中释放二氧化碳和其他气候活跃(或温室)气体。因此，同化营养物的命运，无论是POM还是DOM，都对海洋食物网的生产力、可从大气中清除的二氧化碳以及从表层海洋向大气释放温室气体具有重要影响。在水华期间，浮游植物对营养物质在POM和DOM之间的分配发生了很大的变化，尽管我们对这一过程的定量理解是有限的。事实上，没有可靠的、定量的数据来描述这种与浮游植物细胞健康相关的划分。没有这些数据，我们就无法开发和改进数学模型，使我们能够调查对海洋生态系统和全球气候变化的影响。这个项目将解决我们在理解上的这一重大缺陷。伴随浮游植物水华期间营养物质消耗的一个重要因素是海水pH值从8.2上升到8.5以上。最终，如果浮游植物的pH值超过它们的耐受性，浮游植物就会停止运作，这可能会影响水华繁殖过程中的物种演替。在模型中，这一方面通常被忽略。我们没有定量或严格的数据来描述营养限制和pH升高的组合，这可能会影响驯化过程中的营养分配，从而影响水华的生产力和生物地球化学影响。该项目将具体探讨pH值变化对海洋浮游植物生长动态的影响。与水华期间海水pH升高的时期不同，有证据表明，随着人类活动产生的人为二氧化碳在表层海洋中溶解，未来几十年海洋将发生酸化(pH值下降)。对浮游植物的生长、营养分配以及它们适应相对酸性环境的能力的影响是完全未知的。对海洋环境的影响及其提供的服务需要紧急调查。该项目由斯旺西大学和普利茅斯海洋实验室联合开展，将使具有代表性的浮游植物的培养在不同的条件下(养分可获得性、pH值)，代表当前和未来(酸化的海洋)情况。描述生物体生长和活动变化的数据将支持数学模型的构建和测试。从那时起，研究结果将被纳入生态系统模型，该模型将检验改进的浮游植物活动描述和英国陆架海海洋酸化对海洋生产力和生物地球化学的影响。

Marine phytoplankton play a central role in the cycling of biologically important elements, such as carbon (C), nitrogen (N) and phosphorous (P) between the atmosphere, ocean and marine sediments. Over short periods (weeks) phytoplankton can proliferate, forming vast blooms of new cells that contribute to the Particulate Organic Matter (POM) in the surface ocean. In so doing, they take up nutrients (N and P) and carbon dioxide (CO2) from seawater. This CO2 is replaced by atmospheric CO2 that dissolves in the surface ocean and restores the long-term ocean-atmosphere balance. During a bloom, some cells are consumed by grazers, supporting marine food webs, while others die or stick together and sink. Material reaching the marine sediment contributes to the 'biological carbon pump' which is capable of burying atmospheric CO2 and other nutrients over geological time scales. However, these are not the only fates for assimilated nutrients. During the growth of phytoplankton, organic molecules are released from the cells to the surrounding seawater. These organics (dissolved organic matter / DOM) are used by bacteria which degrade them, regenerating nutrients and releasing CO2 and other climatically active (or greenhouse) gases to the atmosphere. Consequently, the fate of assimilated nutrients, as either POM or DOM, has important implications for the productivity of marine food webs, for CO2 that may be removed from the atmosphere and for the release of greenhouse gases to the atmosphere from the surface ocean. During blooms, the partitioning of nutrients by phytoplankton between POM and DOM changes substantially although our quantitative understanding of this process is limited. In fact, there are no robust, quantitative data available that describe this partitioning in relation to the health of the phytoplankton cells. Without these data we are unable to develop and refine mathematical models that allow us to investigate the implications for marine ecosystems and for global climate change. This project will address this important shortfall in our understanding. An important factor accompanying the consumption of nutrients during phytoplankton blooms is the increase in seawater pH, from 8.2 to greater than 8.5. Ultimately phytoplankton cease to function if the pH exceeds their tolerance, with implications for species succession during bloom propagation. This aspect is usually ignored in models. We have no quantitative or rigorous data available which describes the combination of nutrient limitation and elevated pH, which is likely to effect nutrient partitioning during the acclimation process, and hence the productivity and biogeochemical impact of the bloom. This project will specifically address the impact of changes in pH upon the growth dynamics of marine phytoplankton. In contrast to periods of elevated seawater pH during blooms, evidence points to an acidification of the oceans (pH falls) during the coming decades as anthropogenic CO2 derived from human activity dissolves in the surface ocean. The impact upon the growth of phytoplankton, nutrient partitioning, and their capacity to acclimate to a relatively acidic environment is completely unknown. The implications for the marine environment and the services that it provides warrants urgent investigation. This project, conducted jointly between Swansea University and Plymouth Marine Laboratory, will see cultures of representative phytoplankton subjected to different conditions (nutrient availability, pH) representing current day and future (acidified oceanic) situations. Data describing changes in growth and activity of the organisms will support the construction and testing of mathematical models. The results will thence be incorporated into ecosystem models that will examine the implications for marine productivity and biogeochemistry of the improved description of phytoplanktonic activity, and of ocean acidification for the UK shelf seas.