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，这是以前意想不到的。在此之前，我们最近发现，鱼类排泄的碳酸钙做出了重大贡献，但我们目前的估计相差超过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，从而导致气候变化。在所谓的海洋-大气碳循环中，大气中的二氧化碳与海洋中溶解的二氧化碳处于平衡状态。无论海洋中的二氧化碳发生了什么，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。当二氧化碳溶解在海水中时，它会形成重碳酸盐离子。海洋-大气碳循环的一个重要部分是这种重碳酸盐与海水中的钙反应，生成白色碳酸钙(石灰岩中的矿物)的固体沉淀物。这些沉淀物密度很大，在不断的白色晶体“雨”中沉入海底。它们形成和下沉(或重新溶解)的速度在海洋碳循环中很重要。大多数碳酸钙是由海洋生物产生的，海洋生物促进了这种反应，形成了坚硬的保护壳。参与这种生物钙化的最著名的生物是珊瑚。然而，那些被认为产生最多的实际上是生活在开阔海洋中的微型浮游植物，称为球石生物体。这些微生物和其他微生物的致密骨骼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，以测量碳酸盐“降雨量”，以用于碳循环的计算机模型。我们最近发现，海鱼也会产生大量的沉淀碳酸钙，但目的非常不同。它们通过饮用大量海水并积极促进海水钙与自身新陈代谢产生的碳酸氢根离子的反应，在肠道中产生碳酸氢盐。然后，鱼将沉淀的碳酸钙排入周围的海水中，在那里它可能与所有更知名的(浮游)来源混合在一起。事实上，一些热带鱼每年会排出相当于自身干体重的碳酸钙！我们保守地估计，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，鱼类也对海洋碳循环做出了贡献，但对这一循环进行建模的科学家以前从未考虑过这一点。事实上，鱼的碳酸盐(比更传统来源的碳酸盐更易溶)的不同寻常的化学成分，可能解释了一个困扰海洋学家数十年的现象--海洋上层的“明显不溶”的碳酸盐迅速溶解。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量将如何受到未来环境变化的影响，例如温度和二氧化碳。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，准确了解这种鱼类对全球海洋碳循环的贡献既是一个新颖的、对环境具有重要意义的话题。

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.