CoccolitHophore controls on ocean ALKalinitY (CHALKY)

北大西洋是全球碳循环的关键区域，每年夏天，大面积的地表水都会变成绿松石色。这一现象与一种独特的微藻——球石藻的生长和死亡有关。球石藻用碳酸钙的鳞片（称为球石石）覆盖细胞，这些鳞片在细胞内部产生，并排列在细胞周围的外骨骼中。在某些条件下，例如当营养物质缺乏或病毒感染细胞时，这些球粒就会大量脱落。由于它们独特的光学特性和巨大的数量，它们把水变成了乳白色的绿松石色，可以从太空中探测到。这些蓝绿色的水域（被称为“白色水域”）是数万亿颗石聚集的地方，被认为是颗石藻的大量繁殖。球石藻通过钙化形成球石岩，钙化产生二氧化碳，通过消耗碱度降低海洋的pH值。当海洋表面的球粒岩消失时，它会降低海洋吸收更多二氧化碳的能力。通过这种方式，“白水”被认为会导致海洋碳汇的显著减少。然而，我们现在怀疑这些“白色水域”不是密集的球石团钙化或生长的区域，而是衰老和碎屑物质堆积的区域。人们发现，在“白色水域”之外，球石藻生长得更快，钙化得更多，最近我们发现，它们也被小动物（浮游动物）大量食用，这些动物部分消化了碳酸钙。通过这种方式，球石藻碳酸钙在地表水中的循环利用似乎比以前认为的要多得多，与它们相关的碱度可能保留在表层海洋中。然而，我们很少对这些不同过程（生长、死亡和下沉）的平衡进行耦合测量，以了解球石藻如何控制海洋碱度。这代表了全球海洋碳循环的一个主要不确定性，全球碳预算和地球系统模型正在努力纳入碳酸钙或准确地复制海水碱度的观测结果。“球石团对海洋碱度的控制”（CHALKY）项目旨在通过量化球石团生产和损失过程的平衡及其对碳循环和海气二氧化碳通量的影响来填补这一关键的知识空白。我们将对生态相互作用和对海水化学的影响进行评估，同时改进对球石团的原位和遥感光学检测，使我们能够利用地球观测数据扩大我们对全球海洋的认识，并在历史上使用现有的卫星数据集。CHALKY将首次同时量化球石藻碳酸钙生产（消耗碱度）、病毒裂解（保留碱度）、浮游动物放牧（也保留碱度）和沉入海洋内部的通量（去除碱度）。我们将在从晚春到夏季的过渡期间研究这些过程的平衡，此时原位和卫星数据告诉我们，球石藻是最活跃的。我们将测量这些过程的研究巡航与自主平台和最先进的传感器结合起来，测量海洋化学和原位光学特性。在海水碳酸盐化学和碳循环的背景下，通过量化关键的生长和损失过程，CHALKY将更准确地描述生物如何影响海水吸收二氧化碳的能力，从而使观测结果和模型更紧密地匹配，并将碳酸钙纳入全球碳收支。

Each year in the North Atlantic Ocean, a key region for the global carbon cycle, immense areas of surface water turn turquoise in summer. This phenomenon relates to the growth and death of unique microscopic algae - coccolithophores. Coccolithophores cover their cells with scales of calcium carbonate (called coccoliths), produced internally and arranged into an exoskeleton around the cell. Under certain conditions, for example when nutrients are scarce or viruses infect cells, these coccoliths are shed in huge numbers. Due to their unique optical properties and immense abundance, they turn the water a milky turquoise colour and can be detected from space. These turquoise waters (termed 'white waters') are where coccoliths have accumulated in their trillions and have been considered as coccolithophore blooms.Coccolithophores form coccoliths through calcification, which produces CO2 and reduces the pH of the ocean by consuming alkalinity. When coccoliths are lost from the surface ocean, it reduces the capacity of the ocean to absorb more CO2. In this way, 'white waters' are thought to lead to significant reductions in the ocean's carbon sink. However, we now suspect that these 'white waters' are not areas of intensive coccolithophore calcification or growth, rather they are regions of senescence and an accumulation of detrital material. Coccolithophores have been found to grow faster and calcify more outside of the 'white waters' and more recently we have found that they are also heavily grazed by small animals (zooplankton) who partly digest the calcium carbonate. In this way, coccolithophore calcium carbonate appears to be recycled far more in surface waters than previously thought and the alkalinity they are associated with may be retained in the surface ocean. However, we have few coupled measurements of the balance of these different processes (growth, death and sinking) with which to take an informed view of how coccolithophores control ocean alkalinity. This represents a major uncertainty in the global marine C-cycle, with global C budgets and Earth System Models struggling to incorporate calcium carbonate or accurately replicate observations of seawater alkalinity. The 'coccolithophore controls on ocean alkalinity' (CHALKY) project aims to fill this critical knowledge gap by quantifying the balance of coccolithophore production and loss processes and their impact on C-cycling and air-sea CO2 fluxes. Our assessment of ecological interactions and impacts on seawater chemistry will be carried out while improving in situ and remotely sensed optical detection of coccolithophores to allow us to use Earth Observation data to scale our insights to the global ocean and historically using existing satellite data sets. CHALKY will, for the first time, concurrently quantify coccolithophore calcium carbonate production (consuming alkalinity), viral lysis (retaining alkalinity), zooplankton grazing (also retaining alkalinity) and sinking fluxes into the ocean's interior (removing alkalinity). We will look at the balance of these processes during the transition from late-spring to summer, when in situ and satellite data informs us that coccolithophores are most active. We combine a research cruise measuring these processes with autonomous platforms and state-of-the-art sensors measuring ocean chemistry and in situ optical properties. By quantifying the key growth and loss processes, within the context of seawater carbonate chemistry and C-cycling, CHALKY will inform a more accurate representation of how biology impacts the ability of seawater to absorb CO2, allowing closer matching of observations and models and inclusion of calcium carbonate in global C budgets.