Biochemical and genetic diversity of a critical step in the sulphur cycle - molecular studies of bacterial dimethyl sulphide production

二甲基硫化物(简称DMS)气体会对动物造成奇怪的影响。像虾一样的小甲壳类动物、一些海鸟甚至海豹都对它感到兴奋，因为它的气味是附近食物的标志。而且，对于普通民众来说，这是海滩边备受喜爱的纪念品，是海边令人回味的气味的一部分。但DMS远比这重要得多。它代表着硫从海洋转移到陆地的主要途径，因此是自然硫循环的关键部分，在自然硫循环中，生物将硫从一种形式转移到另一种形式。你可能不知道，但你体内的许多分子都含有硫，没有硫我们就不能生存。二甲基硫对我们的环境也很重要，因为当它进入空气中时，它会被氧化形成其他化合物，充当海洋上空云形成的“种子”，就像零星的尘埃在水晶花园中引发结晶一样。这可能会影响我们的气候，因为它会减少到达地球表面的光量，甚至可能导致“全球变暗”。此外，它的氧化在很大程度上促成了酸雨现象。海洋中产生了大量的二甲基硫--每年约3亿吨。它是由使用另一种分子的微生物产生的，这种分子的名字长得离谱--二甲基磺酸异丙酸酯(DMSP)--作为食物来源。这种DMSP是由海洋中的海藻和浮游生物组成的，帮助它们在海上生活的压力下生存下来。当它们死亡时，DMSP被释放出来，然后被一些细菌和真菌吃掉，其中一些细菌和真菌产生DMS作为副产品。其他细菌则更优雅--它们以一种非常不同的方式吞噬DMSP，不会产生这种微生物的硫磺打嗝。鉴于DMS的重要性，令人惊讶的是，今天我们对其产生的遗传基础知之甚少。然而，东安格利亚大学的科学家最近通过识别与制造DMS有关的细菌基因，取得了一项重要突破。在这样做的过程中，他们发现了至少三种完全不同的细菌产生这种气体的方式。不仅如此，在自然环境中，这些基因可以在非常不同的海洋细菌之间转移。更值得注意的是，它们从一些细菌转移到了完全不同的生命形式，如真菌。而且，这些酶中的一些，以及含有它们的有机体，是从未被怀疑参与这一重要过程的类型。佐治亚大学的其他研究人员已经在对不产生DMS的途径至关重要的基因方面做了相关工作。利用这些信息和他们自己的发现，UEA小组发现了一种海洋细菌--名为Roseovarus nubhibens--它是一种真正的DMSP贪食者。它有两种，可能有三种不同的方式来产生DMS，而且，如果这还不够，它还可以通过另一种不产生DMS的途径来分解DMSP。UEA小组现在想要研究在这个物种中制造DMS的酶的性质，并看看单个细菌如何在不同的环境下决定它应该使用哪种不同的途径-可用的DMSP的浓度，介质的酸度，细胞密度等等。通过了解这些新发现的系统上的这些功能，我们将更好地了解细菌如何以及为什么选择以一种方式而不是另一种方式来分解DMSP，以及为什么有些细菌产生大量重要的气体，而另一些细菌只产生少量气体。这些知识将为地球上生命最重要的元素之一的自然循环提供伟大的新见解。

The gas dimethyl sulphide (DMS for short) does strange things to animals. Tiny shrimp-like crustaceans, some seabirds and even harbour seals get excited by it, since its aroma is a sign of nearby food. And, for the general populace, it is a much-loved memento of days by the beach, being part of the evocative smell of the seaside. But DMS is much more important than that. It represents the major route for transfer of sulphur from the oceans to land and, as such, is a critical part of the natural sulphur cycle, in which living organisms transfer sulphur from one form to another. You may not know it, but many of the molecules in your body contain sulphur and without it we cannot survive. DMS is also important for our environment because when it gets into the air, it is oxidised to form other compounds that act as 'seeds' for cloud formation over the oceans, just as flecks of dust set off crystallisation in crystal gardens. This may affect our climate, by reducing the amounts of light that reaches Earth's surface, perhaps even causing 'global diming'. Furthermore, its oxidation contributes significantly to the phenomenon of acid rain. DMS is made in huge amounts in the oceans - around 300 million tons each year. It is generated by microbes that use another molecule, with a ridiculously long name - dimethylsulphoniopropionate, (DMSP) - as a food source. This DMSP is made by seaweeds and the masses of plankton in the oceans, helping them to survive the stresses of life at sea. When they die, the DMSP is liberated, and is then eaten by some bacteria and fungi, and some of these produce the DMS as a by-product. Other bacteria are more genteel - they devour DMSP in a very different way, which does not produce this microbial sulphurous burp. Given the importance of DMS, it is quite astonishing that today we know so little about genetic basis of its prodution. However, scientists at the University of East Anglia have recently made an important breakthrough by identifying the bacterial genes that are involved in making the DMS. In doing this, they have uncovered at least three completely different ways in which bacteria can make this gas. Not only that, but in the natural environment, these genes can be transferred among very different marine bacteria. Even more remarkably, they were transferred from some bacteria to completely different life forms, such as fungi. And, some of these enzymes, and the organisms that contain them, are of types that had never been suspected of being involved in this important process. Other researchers, at the University of Georgia, have done related work on the genes that are important for the pathway that does not make DMS. Using that information, and their own findings, the UEA group have found one marine bacterium - called Roseovarius nubinhibens - which is a real glutton for DMSP. It has two, possibly three, different ways to make DMS and, as if that were not enough, it can also break down DMSP by the other pathway that does not make DMS. The UEA group now wants to look at the properties of the enzymes that make DMS in this species and to see how a single bacterium decides which of its different pathways it should use under different circumstances - the concentration of the DMSP that is available, the acidity of the medium, the cell density and so on. By learning about these features on these newly discovered systems, we will be much better placed to understand how and why bacteria choose to break down the DMSP in one way, rather than another, and why some make lots of the important gas, but others only make a little. This knowledge will provide great new insights into the natural cycling of one of the most important elements for life on earth.