Fish carbonates - their nature and fate within the marine inorganic carbon cycle

这一建议是基于我们对海洋无机碳循环的理解发生了根本性的重要变化。在此之前，我们最近发现，鱼类排泄的碳酸钙对碳排放做出了重大贡献，但我们目前的估计差异超过10倍（占全球海洋碳酸盐的3%至45%）。背景：随着人类燃烧更多的化石燃料，大气中“温室气体”二氧化碳（CO2）的浓度上升，导致气候变化。大气中的二氧化碳与溶解在海洋中的二氧化碳处于平衡状态，这被称为海洋-大气碳循环。无论海洋中的二氧化碳发生了什么变化，最终都会对大气中的二氧化碳产生影响，从而影响全球气候。当二氧化碳在海水中溶解时，它会形成碳酸氢盐离子。海洋-大气碳循环的一个重要部分是这种碳酸氢盐与海水钙的反应，产生白色碳酸钙的固体沉淀物（石灰石中发现的矿物质）。这些沉淀物非常密集，以白色晶体的连续“雨”形式沉入海底。它们形成和沉降（或再溶解）的速度在海洋碳循环中很重要。大部分碳酸钙是由海洋生物产生的，海洋生物促进这种反应，形成一个坚硬的保护“壳”。参与这种“生物成因”钙化的最著名的生物是珊瑚。然而，那些被认为生产最多的实际上是生活在开阔海洋中的微小浮游植物，称为球石藻。这些和其他微生物的致密骨架通常被认为是海洋碳酸盐的唯一重要来源。科学家们在深海陷阱中收集样本，测量这种碳酸盐“降雨”，用于碳循环的计算机模型。我们最近发现，海洋鱼类也会产生大量的碳酸钙沉淀，但目的完全不同。它们通过大量饮用海水，并积极促进海水钙与自身代谢产生的碳酸氢盐离子的反应，在肠道内产生。然后鱼类将沉淀的碳酸钙排泄到周围的海水中，在那里它可能与所有已知的（浮游生物）来源混合在一起。事实上，一些热带鱼每年会排泄相当于其自身干重的碳酸钙！我们保守估计，鱼类的贡献可能高达全球碳酸盐总产量的45%。这一新发现表明，鱼类也有助于海洋碳循环，但模拟这一循环的科学家以前从未考虑过这一点。事实上，鱼类碳酸盐（比传统来源的碳酸盐更容易溶解）的不同寻常的化学性质可能解释了一个困扰海洋学家几十年的现象——海洋上层“表面上不溶的”碳酸盐的快速溶解。我们的研究是一个多学科项目，首次旨在精确模拟海洋鱼类在不同环境条件下产生多少碳酸钙，并确定其在我们海洋中的命运。这也将有助于预测海洋鱼类的碳酸盐排泄将如何受到未来环境变化的影响，比如温度和二氧化碳。我们预测，在未来，鱼类在这方面将变得更加重要，而海洋浮游生物将变得不那么重要。因此，精确理解这种鱼类对全球海洋碳循环的贡献既是一个新颖的，也是一个对环境重要的话题。

This proposal is based on a fundamentally important and previously unexpected change to our understanding of the marine inorganic carbon cycle. It follows our recent revelation that calcium carbonates excreted by fish make a significant contribution but our current estimates vary over more than a 10-fold range (3 to 45 % of global marine carbonate). BACKGROUND: As humans burn more fossil fuels, atmospheric concentrations of the 'greenhouse gas' carbon dioxide (CO2) rise contributing to climate change. Atmospheric CO2 is in balance with CO2 dissolved in the oceans, in something referred to as the marine-atmospheric carbon cycle. Whatever happens to CO2 in the oceans, will ultimately have an effect on CO2 in the atmosphere, and hence can influence global climate. When CO2 dissolves in seawater, it forms bicarbonate ions. An important part of the marine-atmospheric carbon cycle is the reaction of this bicarbonate with seawater calcium to produce a solid precipitate of white calcium carbonate (the mineral found in limestone). These precipitates are very dense, and sink to the ocean bottom in a continuous 'rain' of white crystals. The rate at which they form and sink (or re-dissolve) is important in the marine carbon cycle. The majority of calcium carbonate is generated by marine life that promotes this reaction to make a hard protective 'shell'. The most famous organisms involved in this 'biogenic' calcification are corals. However, those thought to produce the most are actually microscopic phytoplankton called coccolithophores that live in the open ocean. Dense skeletons of these and other microscopic organisms are normally considered to be the only important source of marine carbonates. Scientists collect samples in deep ocean traps, to measure this carbonate 'rainfall' for use in computer models of the carbon cycle. We have recently discovered that marine fish also produce substantial amounts of precipitated calcium carbonate, but for a very different purpose. They produce it in their intestines, by drinking large volumes of seawater and actively promoting the reaction of seawater calcium with bicarbonate ions that are produced by their own metabolism. Fish then excrete the precipitated calcium carbonate into the surrounding seawater, where it probably mixes with all the better known (planktonic) sources. In fact, some tropical fish will excrete calcium carbonate equivalent to its own dry body weight every year! We have conservatively estimated that the contribution of fish may be up to 45% of the total global carbonate production. This novel discovery suggests that fish also contribute to the marine carbon cycle, but scientists who model this cycle have never previously taken this into account. Indeed, the unusual chemistry of fish carbonates (which are more soluble than carbonate from more traditional sources), may explain a phenomenon that has puzzled oceanographers for decades - the rapid dissolution of 'apparently insoluble' carbonates in the upper layers of the ocean. Our research is a multi-disciplinary project that for the first time aims to precisely model how much calcium carbonate is produced by marine fish under different environmental conditions and determine its fate within in our oceans. This will also help with predictions about how carbonate excretion by marine fish will be affected by future environmental changes, such as temperature and CO2. We predict that fish will become even more important in this regard in the future, whereas marine plankton will become less important. Thus a precise understanding of this fish contribution to the global marine carbon cycle is both a novel and environmentally important topic.