How do eukaryotic phytoplankton produce the most abundant organo-sulphur compound in the world's oceans?

有一种分子叫做二甲基磺胺丙酸酯（DMSP），它的产量非常大——全世界每年有10亿吨——是由无数微小的单细胞海洋浮游生物、许多海藻和生活在海边的一些陆地植物制造的。在其中的一些，大部分的硫和大部分的碳都是以这种分子的形式存在的，但是DMSP的影响甚至比这更广泛。尽管DMSP含量丰富，但对于它的功能还没有明确的共识——也许它可以缓冲细胞免受海水的盐度，也许它可以作为一种防御捕食者攻击的化合物，也许它可以保护细胞免受紫外线的伤害。-也许它是一个分子的万事通。DMSP不仅本身就很有趣，而且它还具有化学遗产，因为它是其他分子的母体化合物，这些分子也非常重要。当产生DMSP的类植物生物将其释放到环境中时，或者通过泄漏，或者更戏剧性的是，当它们被病毒或微观动物裂解时，它被大量的海洋细菌和一些真菌用作DMSP分解代谢过程中的美味食物来源。这代表了全球硫循环的很大一部分，但这还不是全部。DMSP分解产物之一是一种叫做二甲基硫化物（DMS）的气体，它本身就对环境有重要影响。在海洋及其边缘每年产生的3亿吨DMS中，约有10%逃逸到大气中，在那里被氧化成硫酸盐和硫酸。这些分子起着“云凝结核”的作用，导致云的形成，就像尘埃微粒可以使水晶花园中的晶体生长一样。这种现象发生的规模如此之大，以至于它造成了海洋上的大部分云层，并可能影响到达地球的太阳热和光的数量——一种“全球变暗”的形式。DMS也是许多海洋动物——鸟类、海豹和小型甲壳类动物——的信号，因为它是它们浮游生物食物供应的化学特征。对我们来说，作为智人，DMS是海边气味的一部分，尤其是当周围有很多海藻的时候。虽然已经有一些不错的初步研究，其中描述了DMSP合成的一般途径，但没有一种纯化的酶或相应的基因在任何DMSP产生的生物体中被鉴定出来。由于缺乏知识，很难明确地弄清楚DMSP的真正功能是什么。这项研究的重点是一组浮游植物，统称为硅藻，它们产生大量的DMSP。这些微小的植物状生物在世界海洋中大量存在，它们的重量占地球植物总数的四分之一，它们产生的氧气与我们呼吸的氧气的比例大致相同。我们最近在硅藻中发现了四个关键基因，我们坚信这些基因编码的酶参与了DMSP的生产。我们现在计划研究这些基因和它们编码的酶的确切功能，方法是制造不再产生DMSP的突变体，反过来，通过工程菌株过量产生DMSP。这将使我们第一次在任何生物体中，确定地弄清楚这个关键分子的功能。我们还会发现基因是如何表达的；它们是一直处于开启状态，还是对环境中的各种压力（例如）做出反应？这在不同的物种之间有区别吗？这不仅将为DMSP在硅藻中的作用提供新的见解，而且我们的结果应该揭示在其他重要的海洋生物中产生这种关键分子的过程中所涉及的机制。这样的研究只会加强我们评估未来环境变化对这一关键生物合成过程影响的能力。

There is a molecule, called dimethylsulphoniopropionate (DMSP) which is made in very large amounts - a billion tons per year, worldwide - by myriads of tiny single-celled marine plankton, many seaweeds and a few land plants that live near the seaside. In some of these, most of the sulphur and much of the carbon is in the form of this molecule, but the influence of DMSP is even more wide-ranging than that. Despite its abundance, there is no clear agreement as to what is the function of the DMSP - maybe it buffers the cells from the saltiness of the sea, maybe it acts as a defence compound against attack by predators, maybe it protects against ultraviolet light. - maybe it is a molecular jack of all trades.DMSP is not only interesting in its own right, but it also has a chemical legacy, since it is the parent compound for other molecules that are also very important. When the plant-like organisms that make DMSP release it into the environment, either by leakage, or more dramatically when they are lysed by viruses or microscopic animals, it is used as a tasty food source by the teeming marine bacteria and some fungi in a process of DMSP catabolism. This represents a massive part of the global sulphur cycle, but even that is not all. One of the DMSP breakdown products is a gas, called dimethyl sulphide (DMS), which has important environmental impacts in its own right. Of the 300 million tons of DMS made each year in the oceans and their margins, around 10% escapes into the atmosphere, where it gets oxidised to sulphate and sulphuric acid. These molecules act as "Cloud Condensation Nuclei", which cause clouds to form, just as specks of dust can start crystals growing in crystal gardens. This occurs on such a scale that it is responsible for much of the cloud cover over the seas and oceans and may affect the amount of solar heat and light that arrives on Earth - a form of "Global Dimming". The DMS also acts as a signal for many marine animals - birds, seals and tiny crustaceans - since it is a chemical signature for their plankton food supplies. Also to us, as Homo sapiens, DMS is part of the smell of the seaside, especially when there is a lot of seaweed about.Although there have been some nice preliminary studies in which the general pathways for DMSP synthesis have been described, none of the purified enzymes or corresponding genes have been identified in any DMSP-producing organism. This lack of knowledge has made it hard to work out unambiguously what are the true functions of DMSP.This study focuses on one group of phytoplankton, collectively known as diatoms, which produce large amounts of DMSP. These microscopic plant-like organisms are abundant in the world's oceans - all told, they make up a quarter of Earth's plant life by weight and they produce around the same proportion of the oxygen that we breathe. We have very recently identified four key genes in diatoms that we strongly believe encode enzymes that are involved in DMSP production. We now plan to study the exact functions of these genes and the enzymes that they encode by making mutants that no longer make DMSP and, conversely, by engineering strains that over-produce it. This will allow us, for the first time in any organism, to figure out with certainty what are the functions of this key molecule. We will also discover how the genes are expressed; are they switched on all the time, or do they respond to (for example) various stresses in the environment? Does this differ between different species? Not only will this provide new insights on the role(s) of DMSP in diatoms, but our results should shed light on the mechanisms that are involved in this process in other important marine organisms that make this key molecule. Such studies will only strengthen our ability to assess the effects of future environmental changes on this crucial biosynthetic process.