Fish gut carbonates and the control of ocean alkalinity

目前，全球海洋吸收了约30%的人为二氧化碳排放。调节海洋-大气CO2交换的碳循环，以及影响海洋吸收能力的溶解碳和碱度的相关垂直分布，取决于几个过程。这些被描述为一系列相互作用的“泵”：物理/化学溶解度泵;生物“软组织”泵;和碳酸钙泵。了解这三个泵，它们如何相互作用，以及它们的大气CO2反馈对于准确预测海洋碳循环和全球气候未来将如何变化尤为重要。碳酸钙是一种由一系列海洋生物产生的白色白垩质矿物。重要的是，当它溶解时，它会增加海水的碱度，这可以将海水中的CO2浓度降低到低于大气中的CO2水平，并从大气中“吸收”人为CO2。因此，确切地知道它在哪里溶解（离海洋表面有多近）是理解碳酸钙泵在调节海洋化学和大气CO2中所起作用的关键。碳酸钙泵的操作不仅取决于生产率，还取决于海洋生物产生的碳酸盐矿物的类型，它们下沉的速度以及这些碳酸盐矿物溶解的速度。大多数海洋碳循环模型都假设碳酸盐的产生主要是由浮游生物和颗石藻（微型藻类）控制的。然而，我们现在知道，海洋硬骨鱼（硬骨鱼）排泄出大量的碳酸盐。我们现在也知道，这种碳酸盐在矿物学上是多种多样的，这取决于鱼类的种类，它在鱼的肠道中不断产生，并作为废物排出。这一过程对海洋碳酸钙泵的潜在意义在PI Wilson（Science，2009）领导的初始建模工作中得到了认可，该工作保守地认为鱼类可能占全球海洋碳酸钙总产量的至少3-15%，实际上高达45%。自从第一次建模以来，这一过程背后的科学已经取得了巨大的进步。作为一个群体（以及通过其他人的工作），我们现在知道，鱼类产生了各种各样的碳酸盐矿物类型，现有的知识表明它们应该以非常不同的速度溶解。因此，第一次建模工作中关于鱼类产生均匀和相对可溶的碳酸盐类型的假设不再有效。虽然我们已经可以解决一些知识差距，但几乎没有或没有关于占全球鱼类生物量约94%的鱼类的数据-包括几乎没有关于仅占鱼类生物量至少60%的中层鱼类的数据。每日垂直迁移的巨大的生物量被假设为驱动一个新的“向上的碱度泵”，这可能会提供一个重要的抵消碱性的向下运输驱动的其他既定的过程。此外，我们现在有充分的证据表明，鱼类的生产率随着代谢率的变化而变化（在全球重要的活跃海洋上层鱼类中代谢率最高），重要的是还取决于摄食和饮食（特别是饮食中的钙含量）。因此，早期模型中所有鱼类都以相同的速率产生碳酸盐的必要假设再一次不再现实，无法用于建模。除了这些问题之外，我们也几乎没有关于这些碳酸盐沉入海洋或溶解的速度的数据。因此，该项目的目的是提供有关鱼类碳酸盐产量，矿物学，溶解度和下沉率的新经验数据，以告知第一个空间和矿物学解决的全球产量估计，从而使我们能够参数化模型，评估鱼类对海洋碳循环的贡献，无论是在目前的条件下，还是未来的气候变化情景。

The global oceans currently absorb ~30% of anthropogenic CO2 emissions. The carbon cycle that regulates this ocean-atmosphere CO2 exchange, and the associated vertical distribution of dissolved carbon and alkalinity that influences the ocean's absorption capacity, depends on several processes. These are described as a series of interacting "pumps": a physical/chemical solubility pump; a biological 'soft tissue' pump; and a calcium carbonate pump. Understanding these three pumps, how they interact, and their atmospheric CO2 feedbacks is especially critical for accurate predictions of how the marine carbon cycle and global climate will change in the future. Calcium carbonate is a white, chalky mineral produced by a range of marine organisms. Importantly, when it dissolves it increases the alkalinity of seawater, which can reduce the seawater CO2 concentration below atmospheric CO2 levels and 'suck' anthropogenic CO2 from the atmosphere. Knowing exactly where it dissolves (how near the ocean surface) is therefore key to understanding the role this calcium carbonate pump plays in regulating ocean chemistry and atmospheric CO2. The operation of the calcium carbonate pump not only depends on the production rate but also the types of carbonate minerals that are produced by marine organisms, the rate at which they sink, and how rapidly these carbonate minerals then dissolve. Most ocean carbon cycle models make the assumption that carbonate production is dominated by the plankton and coccolithophores (microscopic algae). However, we now know that very large amounts of carbonate are excreted by marine bony fish (teleosts). This carbonate, which we now also know is mineralogically diverse depending on the fish species, is continuously produced in the intestines of fish and excreted as waste. The potential significance of this process to the marine CaCO3 pump was recognised in an initial modelling exercise led by PI Wilson (Science, 2009) which conservatively suggested that fish may account for at least 3-15% of total marine CaCO3 production globally, and realistically as much as 45%. Since that first modelling exercise the science behind this process has advanced hugely. As a group (and through the work of others) we now know that fish produce a hugely diverse range of carbonate mineral types, which existing knowledge would suggest should dissolve at very different rates. As a result, the assumptions in the first modelling efforts that fish produce uniform and relatively soluble carbonate types are no longer valid. Whilst we can already address some of the knowledge gaps, there is little or no data for fish from families that comprise ~94% of global fish biomass - including almost no data for mesopelagic fish that alone account for at least 60% of fish biomass. The daily vertical migration of their immense biomass is hypothesised to drive a novel "upward alkalinity pump", which may provide an important offset to the downward transport of alkalinity driven by other established processes. Also, we now have good evidence to show that production rates by fish vary with metabolic rate (which is greatest in the globally significant active epipelagic fishes), and importantly also depending upon feeding and diet (especially the calcium content of the diet). Thus, again, necessary assumptions in early models that all fish produce carbonate at the same rate are no longer realistic to use for modelling. Over and above these issues we also have little to no data on the rates at which these carbonates sink in the oceans or dissolve. The aim of this project is therefore to deliver new empirical data on fish carbonate production, mineralogies, solubilities and sinking rates to inform the first spatially- and mineralogically-resolved global production estimates, thus enabling us to parameterise models assessing fish contributions to the marine carbon cycle both under present day conditions, and for climate change scenarios in the future.