Understanding and modelling the Microbial Carbon Pump under changing nutrient concentrations and temperature

海水是一种复杂的化学物质，包括溶解的有机物质(DOM)，如糖、脂肪和氨基酸，都含有碳。事实上，海洋DOM中的碳含量与大气中的二氧化碳含量大致相同。那么，这种碳是如何变成DOM的，是什么控制了它的生产和命运呢？大气中的二氧化碳，溶解在海水中，在那里被称为浮游植物的小型单细胞生物将其结合成对它们的生长至关重要的有机分子。其中一些有机分子从健康细胞中泄漏，而更多的有机分子在细胞死亡或被吃掉时释放，形成了海洋中的DOM池。许多人熟悉浮游植物支持海洋食物网的概念，以及不同生物过程产生的死亡细胞和碎屑沉入海底埋藏在沉积物中。这个过程有效地将碳(最初以大气二氧化碳的形式存在)输送到海底；这被称为“生物碳泵”(BCP)。科学家们最近才意识到的一个单独的过程，提供了另一种去除和储存大气中二氧化碳的方法。在这一过程中发挥关键作用的是更小的生物，它们是海洋中数量最丰富的生命形式：细菌。细菌迅速作用于浮游植物释放的DOM及其相关食物链的活动，觅食它们最容易用于生长的部分。在几周和几个月的时间里，对DOM组分的连续清除逐渐改变了剩余物质的化学性质，使残留的分子几乎没有其他值得带走的东西。这些分子通常被定义为“难熔的DOM”，在生物上毫无价值，只能在地球的洋流中流动。这里描述的过程被称为‘微生物碳泵’(MCP)，人们认为在过去的数千年里，它缓慢地积累和储存了数量惊人的难熔DOM，估计有6240亿吨。这个令人难以置信的碳库目前被认为是稳定的，非生物去除过程(例如，光降解)平衡了它的生产。然而，最近的研究表明，气候变化导致表层海洋无机营养物质有效性的预计下降可能会改变MCP的活性，增加难降解DOM的产量和消费量。这意味着海洋细菌有可能通过将更多的碳转移到难降解的DOM中来缓解大气中二氧化碳的人为增加。如果这一假设得到证实，将从根本上改变我们对生物圈调节气候能力的看法，这表明海洋细菌在气候变化中起到了以前被忽视的作用。我们必须理解这种机制是否重要的唯一方法是使用数值模型，并在不断变化的环境条件下运行它们。然而，到目前为止，还没有海洋或地球系统模型来解释MCP的动态。在这个项目中，我们将进行实验室实验，以提供所需水平的生理信息和理解，使我们能够开发出第一个描述MCP及其与营养物质浓度和温度关系的模型。这一成果将是朝着模拟现在和未来海洋的MCP迈出的关键的第一步。为了实现这一雄心勃勃的目标，该项目将汇集一个由国际公认的科学家组成的多学科团队，从化学分析师到系统生物学和生态系统模型师。项目团队将通过与焦N教授(厦门大学，中国)的合作而得到推动，焦N教授在7年前的一篇开创性论文中首次提出了MCP的概念。

Seawater is a complicated soup of chemicals including dissolved organic material (DOM), such as sugars, fats and amino acids all containing carbon. In fact, there is roughly the same amount of carbon within marine DOM as there is CO2 in the atmosphere. So how did this carbon become DOM, and what controls its production and fate? Atmospheric CO2, dissolves in seawater where small single celled organisms called phytoplankton incorporate it into organic molecules essential for their growth. Some of these organic molecules leak from healthy cells, while more are released when cells die, or are eaten, creating an oceanic pool of DOM. Many people are familiar with the concept that phytoplankton support marine food webs and that dead cells and detritus generated by different biological processes sink to the seafloor to be buried in sediments. This process effectively transports carbon, originally present as atmospheric CO2, to the seafloor; this is termed the 'Biological Carbon Pump' (BCP). A separate process, which scientists have only recently become aware of, provides another way of removing and storing atmospheric CO2. The key role in this process is played by even smaller organisms which are numerically the most abundant life form in the oceans: the bacteria. Bacteria quickly act upon the DOM released from phytoplankton and the activities of their associated food web, scavenging parts they can most readily use for growth. Progressively, over weeks and months, sequential scavenging of components of DOM gradually transforms the chemical nature of the remaining material so that the residual molecules contain little else worth taking. These molecules, commonly defined as 'refractory-DOM', are biologically worthless, and are left to travel the Earth's Oceanic currents. The process described here is called the 'Microbial Carbon Pump' (MCP) and is thought to have slowly accumulated and stored a staggering amount of refractory-DOM over the past millennia, estimated to be 624 gigatonnes. This incredible reservoir of carbon is currently thought to be stable, with abiotic removal processes (e.g. photo degradation) balancing its production. However, recent studies suggest that that the projected decrease in surface ocean inorganic nutrient availability due to climate change could modify MCP activity, increasing refractory-DOM production with respect to its consumption. This implies that marine bacteria have the potential to mitigate the anthropogenic increase in atmospheric CO2 by shunting more carbon into refractory-DOM. This hypothesis, if verified, will radically change the way we think of the capacity of the biosphere to modulate climate, suggesting a previously overlooked climate-active role for marine bacteria.The only way we have to understand if this mechanism is significant is to use numerical models and run them under changing environmental conditions. However, to date, no ocean or Earth system models account for MCP dynamics.In this project, we will conduct laboratory experiments to provide the required level of physiological information and understanding needed to enable us to develop the first model describing the MCP and its relationship with nutrient concentration and temperature. This outcome will be the first critical step toward the simulation of the MCP in present and future oceans. To achieve this ambitious goal, the project will bring together a multidisciplinary team of internationally recognised scientists, from chemical analysts to system biology and ecosystem modellers. The project team will be boosted by the partnership with Prof N. Jiao (Xiamen University, China) who first proposed the MCP concept in a seminal paper 7 years ago.