A novel pathway for the production of the climate cooling gas dimethyl sulfide - how important is the mddA gene to global DMS emissions?

海边的气味实际上是由微生物产生的一种名为二甲基硫化物(DMS)的气体化合物造成的。这种气体很重要，因为它是一种非常丰富的有机硫化合物，从海洋环境中释放到空气中。全球每年大约生产3亿吨二甲基硫，主要由细菌产生。此外，DMS氧化产生的化学产物有助于在海洋上空形成云层，在一定程度上影响到达地球表面的阳光，从而影响气候。反过来，这些产品以雨水的形式返回地球，代表着全球硫循环的关键组成部分。有趣的是，DMS对包括海鸟、甲壳类动物和海洋哺乳动物在内的许多生物都是一种有效的化学诱导剂，因为它们将DMS与食物联系在一起。目前人们普遍认为DMS的产生主要是由于微生物降解海洋浮游植物、海藻和少数耐盐植物产生的渗透剂二甲基磺基丙酸酯(DMSP)。我们和Ron Kiene的初步工作促使我们质疑是否只有这些过程才能产生DMS。在我们的初步数据中，我们有：1.发现了一种微生物途径，依赖于甲硫醇的DMS产生(MDD)途径，该途径产生DMS，但不涉及DMSP。2.展示了假单胞菌是如何通过一种名为mddA.3的基因制造DMS的。研究表明，该基因广泛存在于大豆固氮共生菌--日本慢根瘤菌、引起结核病的结核分枝杆菌和一些蓝藻中。4.表明MDD途径在咸水和淡水沉积物中都是活跃的，mddA基因在生活在海洋沉积物中的细菌中含量丰富。研究表明，其他细菌还有其他未被发现的从甲硫醇中制造DMS的方法。我们希望调查这种新的DMS产生途径对这种气候变化气体的全球生产有多重要。为了回答这个问题，我们将对不同的海洋和淡水环境进行采样，并调查MDD途径在这些环境中有多活跃，以及这一产生DMS的新途径是如何受到调控的。我们已经知道，这条MDD途径可能在我们的大多数样本地点都很活跃，包括来自盐沼、淡水湖、泥炭沼泽和海水的泥浆。同样重要的是知道哪些微生物负责这一过程(由MDD介导)以及它们为什么产生DMS。我们将使用一套强大的微生物生态学技术，结合基因工具来鉴定通过这一新的MDD途径产生DMS的微生物和关键基因。我们将确定：a)生活在缺氧和缺氧泥浆样品中以及海水中的微生物；b)当我们通过MDD途径增加DMS产量时，这些微生物群落如何变化；c)在这些不同的环境中，哪些形式的mddA基因(以及该基因编码的酶)负责DMS的高产量。为了了解环境中的细菌是如何以及为什么MDD活性的，我们将详细研究几个模型细菌，其中一些已经从我们的样本站点分离出来。这将涉及识别和突变编码MDD途径的基因，以确定它们使用该途径的原因。这将在具有特定基因“mddA”的细菌上完成，但在那些没有特定基因的细菌上也是如此，这将使我们能够识别新的MDD基因。考虑到气候活性气体DMS的环境后果，重要的是了解哪些类型的微生物影响其生产，以及涉及到各种潜在途径中的哪些。这将有助于我们在未来模拟环境变化如何影响这些气候过程的平衡。

The "smell of the seaside" is actually caused by a gaseous compound called dimethyl sulfide (DMS) that is produced by microbes. This gas is important because it is a very abundant organic sulfur compound which is released to the air from the marine environment. Globally, approximately 300 million tons of DMS per annum is produced, mainly by bacteria. Also, chemical products arising from DMS oxidation help form clouds over the oceans, to an extent that affects the sunlight reaching the Earth's surface, with effects on climate. In turn, these products are delivered back to Earth as rain, representing a key component of the global sulfur cycle. Interestingly, DMS is a potent chemo-attractant for many organisms including seabirds, crustaceans and marine mammals that all move towards DMS because they associate DMS with food.Currently it is widely accepted that DMS is mainly produced as a result of microbes degrading the osmolyte dimethylsulfoniopropionate (DMSP), which is produced by phytoplankton in the oceans, by seaweeds and by a few salt-tolerant plants. Our preliminary work and that of Ron Kiene, has prompted us to question whether it is solely these processes that produce DMS.In our preliminary data we have:1. Found a microbial pathway, the methanethiol-dependent DMS production (Mdd) pathway, that produces DMS but which does not involve DMSP. 2. Shown how the bacterium "Pseudomonas deceptionensis" makes DMS via a gene called mddA.3. Shown that this gene is found in a wide range of bacteria such as Bradyrhizobium japonicum, a nitrogen-fixing symbiont of soybeans, Mycobacterium tuberculosis, the causative agent of tuberculosis and some cyanobacteria. 4. Shown that the Mdd pathway is active in both salty and freshwater sediments and that the mddA gene is abundant in bacteria living in marine sediments.5. Shown that other bacteria have other undiscovered ways of making DMS from methanethiol.We wish to investigate how important this novel DMS production pathway is for the global production of this climate changing gas. To answer this question, we will sample various marine and freshwater environments and investigate how active the Mdd pathway is in these environments and how this novel pathway for the production of DMS is regulated. We already know that this Mdd pathway is probably active in most of our sample sites, which include mud from a saltmarsh, a freshwater lake, a peat bog and seawater. It is equally important to know which microbes are responsible for the process (mediated by Mdd) and why they produce DMS. We will use a powerful suite of microbial ecology techniques, combined with genetic tools to identify the microbes and the key genes involved in producing DMS via this new Mdd pathway.We will identify: a) the microbes living in both the oxic and anoxic mud samples and in seawater; b) how these microbial communities change when we enrich for increased DMS production via the Mdd pathway and c) which forms of the mddA gene (and the enzyme encoded by this gene) are responsible for high DMS production in these varied environments. To understand how and why bacteria in the environment are Mdd active, we will study in detail a few model bacteria, some of which have been isolated from our sample sites. This will involve identifying and mutating the genes encoding the Mdd pathway to ascertain why they use it. This will be done with bacteria that have a specific gene "mddA", but, also on those that do not, which will allow us to identify new mdd genes. Given the environmental consequences of the climate-active gas DMS, it is important to know which types of microbes affect its production and which of the various potential pathways are involved. This will help us in the future to model how changes in the environment impact on the balance of these climate processes.