Mortality rates in key phytoplankton functional types: the nature of cell death and its biogeochemical consequence

在阳光照射的海洋上层，无数的浮游微生物通过光合作用将太阳的能量转化为活的组织。这些微小的单细胞生物被称为“浮游植物”，它们的光合作用将二氧化碳从大气中吸收到海洋中。它们利用水中的二氧化碳和营养物质来构建它们生长和繁殖所需的细胞成分，同时它们也释放氧气。为了捕捉阳光，浮游植物使用一种叫做色素的分子，比如叶绿素。科学家们已经开发出卫星方法，这样他们就可以从太空中观察叶绿素，看看浮游植物在哪里。研究海洋的科学家们感兴趣的不仅仅是有多少叶绿素。他们想知道有多少二氧化碳通过光合作用被使用或固定到新的组织中：这被称为初级生产。初级生产很重要，因为浮游动物越多，鱼类就能通过吃浮游植物而茁壮成长。科学家测量浮游植物初级生产的速率，然后比较不同地点和时间的速率，以了解不同海洋生态系统的工作方式。因此，初级产量是海洋科学的一个非常基本的衡量标准，因为它描述了在食物网的底部产生了多少能量。用于测量海洋特性（如叶绿素、温度和盐度）的精密地球观测卫星、近海浮标和气象站的发展带来了巨大的进步，但我们仍然无法估计海洋中的竞争和死亡等生物过程，而这些对于确定浮游植物的生长数量很重要。过去人们认为浮游植物可以无限地分裂，也就是说，它们是“功能性不朽的”，数量的减少只是因为被浮游动物吃掉，被病毒感染或从阳光照射的水域沉没。但现在我们知道浮游植物是会死的，它们会变老死亡，或者因为不能生长而死亡。我们不知道这种情况发生的频率，因为很难识别单细胞生物的死亡，而且“一刀切”的规则可能并不适用，因为浮游植物高度多样化——有些浮游植物彼此之间的关系不如人类与树木的关系那么密切，而且在形式、功能和生活史上也有很大的差异。然而，这些必不可少的微生物控制着维持地球上所有其他生命的过程，比如氧气的产生。的确，浮游植物在10亿年前制造了地球的氧气大气层。在过去15年左右的时间里，科学家们揭示了浮游植物的自然死亡对海洋生态系统能量流动的重要性：在某些情况下，超过一半的海面浮游植物可能已经死亡。死细胞不能生长和分裂，但可能仍然含有叶绿素，因此，检测叶绿素似乎并不像我们曾经认为的那样是初级生产的好指标，这表明我们关于能量如何在食物网中流动的想法可能过于简单化了。我们在这里提出的研究旨在更好地了解浮游植物种群是如何在我们广阔的蓝色星球上生长、分裂和死亡的。

In the sunlit upper layers of the sea, countless billions of floating microbes convert the energy of the sun into living tissue through photosynthesis. These tiny one-celled creatures are called 'phytoplankton', and their photosynthesis draws carbon dioxide (CO2) down from the atmosphere and into the ocean. They use the CO2 and nutrients from the water to build the cell components that they need to grow and multiply and whilst doing this they also give off oxygen. To catch the sunlight the phytoplankton use molecules called pigments, such as chlorophyll. Scientists have developed satellite methods so that they can look at chlorophyll from space and see where the phytoplankton are. Scientists who study the oceans are interested in more than just how much chlorophyll there is. They want to know how much CO2 is used, or fixed, into new tissue by photosynthesis: this is known as primary production. Primary production is important because the more there is the more zooplankton and fish can thrive by eating the phytoplankton. Scientists measure the rate of phytoplankton primary production, and then compare rates at different places and times to understand the way different marine ecosystems work. Primary production is therefore a very fundamental measurement for the marine sciences because it describes how much energy is generated at the base of the food web. Developments in the use of sophisticated Earth-observing satellites, offshore buoys and weather stations for measuring ocean properties of the ocean (such as chlorophyll, temperature, and salinity) are bringing great advances, but we still cannot estimate biological processes like competition and mortality in the ocean and these are important in determining how much phytoplankton will grow. It use to be assumed that phytoplankton could divide indefinitely i.e. that they were 'functionally immortal' and that population losses came only from being eaten by zooplankton, infected by viruses or sinking out of the sunlit waters. But now we know that phytoplankton are mortal, and that they will grow old and die, or die because they cannot grow. We do not know how often this happens, because it is difficult to recognise death in unicellular organisms and a 'one size fits all' rule may not apply because phytoplankton are highly diverse - some are less related to each other than humans are to trees, and there is also great variation in form, function and life-history. Nevertheless, these essential microbes control the processes, such as oxygen production, which sustain all other life on Earth. Indeed, the phytoplankton made the Earth's oxygen atmosphere a billion years ago. In the last 15 years or so scientists have revealed how important the natural death of phytoplankton could be for the energy flow of marine ecosystems: in some cases, more than half of the surface-dwelling phytoplankton may be dead. Dead cells cannot grow and divide, but may still contain chlorophyll, so it seems that detecting chlorophyll is not as good an indicator of primary production as we once thought and suggests that our ideas of how energy flows in the food web may be simplistic. The research that we propose here aims to better understand how populations of phytoplankton grow, divide and die in the vast expanses of our blue planet.