Making and breaking DMS by salt marsh microbes - populations and pathways, revealed by stable isotope probing and molecular techniques

有一种令人回味的气体，叫做二甲基硫化物，简称DMS，我们大多数人都闻到过，因为它是海边气味的一个组成部分。但它远比这重要得多。海洋微生物每年产生约3亿吨，其中约10%逃逸到大气中。这不仅让人想起在海边的日子，而且DMS在空气中被化学修饰成化合物，导致海洋上空形成云，影响天气和气候。而且，当下雨时，这些化合物会回到地球，这是基本元素硫全球循环的重要一步。还有一件事。即使是少量的DMS，也会吸引不同的海洋动物——鱼、企鹅和小甲壳类动物都尽可能快地游、飞或划向DMS。原因是他们知道哪里有DMS哪里就有食物。这是因为DMS是一种生化过程的副产品，当不同的微生物吞噬另一种含硫分子时，这种分子有一个可笑的长名字——二甲基磺丙酸盐。这种DMSP是由海洋中的微小浮游生物、海藻和生活在海边的极少数陆地植物大量产生的。在东英吉利大学，我们发现了微生物是如何制造DMS的，在沃里克大学，我们研究了其他海洋微生物如何进一步转化这种气体。我们使用分子生物学、基因克隆和DNA测序来识别一系列微生物中让它们进行这些反应的基因。对于这两个过程，我们发现一些非常意想不到的生物可以制造或分解DMS，它们可以以完全不同的令人惊讶的方式完成这一过程。大多数研究都是在实验室培养的纯化菌株上进行的。这让我们能够识别基因和它们各自的功能，但它并没有告诉我们哪些是最重要的途径，哪些微生物是自然环境中的关键角色。这是因为绝大多数生活在自然界的细菌从未被培养过。幸运的是，一些最新的技术让我们能够研究这种“困难”的微生物。穆雷尔教授发明了一个巧妙的方法，就是用一种与正常基质化学成分相同但实际上更重的基质来喂养自然种群的微生物。所以，在我们的例子中，我们将使用DMS和DMSP的形式，其中碳原子的原子量是13，而不是更传统的12。当微生物消化如此重的分子时，重碳会被整合到包括DNA在内的分子中。通过从轻形式中纯化这种重DNA，并在基因中寻找特征序列，可以识别使用DMS或DMSP的微生物和真菌，并推断出它们这样做的机制。我们将在北诺福克盐沼的泥浆上做这些实验。这些是米草属植物的家园，米草属植物是少数几种制造DMSP的陆地植物之一。这种植物也很重要，因为它已经被人类传播到世界各地，现在是世界上许多沿海地区的严重害虫，杀死了许多本地物种。毫不奇怪，米草属植物的根周围有很多DMSP，其中充满了消耗或产生DMS的细菌和真菌。因此，我们将对这些微生物进行普查，其中一些可能是科学上的新发现。我们的发现应该与DMS和DMSP的其他热点有关，比如珊瑚和海洋中浮游生物的大量繁殖。尽管微生物数量很少，但它们对我们环境的影响比我们大多数人意识到的要大。鉴于DMS气体的环境后果，了解影响其产生和破坏的细菌和真菌类型以及涉及的各种潜在途径中的哪一种非常重要。这可能有助于我们模拟气候变化等环境变化如何改变这些过程的平衡。

There is an evocative gas, called dimethyl sulfide - DMS for short - which most of us have smelled, since it is a component of the smell of the seaside. But it is far more important than that. Around 300 million tons are made each year by marine microbes, around 10% of which escapes into the atmosphere. Not only does this bring back memories of days by the sea, but DMS is chemically modified in the air to compounds that cause clouds to form over the oceans, affecting weather and climate. And, when it rains, these compounds come back to earth in a major step in the global circulation of the essential element sulfur. And one more thing. Even in tiny amounts, DMS attracts different marine animals - fish, penguins and tiny crustaceans all swim, fly or paddle towards it as fast as they can. The reason is that they know that where there is DMS there is food. This is because DMS is a by-product of biochemical processes that occur when different microbes devour another sulfur-containing molecule, with a ridiculously long name - dimethylsulfoniopropionate. This DMSP is made in prodigious amounts by tiny plankton organisms in the oceans, by seaweeds and by a very few land plants that live by the sea. At UEA, we discovered how microbes make the DMS and in Warwick, the ways in which other marine microbes can further transform this gas are studied. We use molecular biology, gene cloning and DNA sequencing to identify the genes in a whole range of microbes that let them undertake these reactions. For both processes, we found that some very unexpected organisms can make or can break down DMS and that they can do this in completely different and surprising ways. Most of these studies are on purified strains that we grow in the lab. This lets us identify the genes and their individual functions, but it does not tell us which are the most important pathways and which of the microbes are the key players in natural environments. This is because the great majority of bacteria that live 'out here' in the natural world have never been cultured. Luckily, some very recent techniques let us study such 'difficult' microbes. One neat trick, invented by Professor Murrell, is to feed natural populations of microbes with a version of the substrate that is chemically identical to the normal one but which is, literally, heavier. So, in our case, we will use forms of DMS and DMSP in which the carbon atoms have an atomic weight of 13, not the more conventional 12. When a microbe digests such a heavy molecule, the heavy carbon is incorporated into its molecules, including DNA. By purifying this heavy DNA from the light form and by looking for signature sequences in the genes, the microorganisms and fungi that used the DMS or the DMSP can be identified and the mechanisms by which they do so can be inferred. We will do these experiments on mud from the salt marshes of North Norfolk. These are home to the grass Spartina, one of the few land plants that makes DMSP. This plant is also important because it is has been spread by human hand across the world and is now a serious pest on many coasts all over the world, killing off many native species. Not surprisingly, there is a lot of DMSP around Spartina roots, which teem with bacteria and fungi that consume or make DMS. We will therefore conduct a census of these microbes, some of which may be new to science. Our findings should relate to other hotspots for DMS and DMSP, such as corals and the massive blooms of plankton in the oceans. Although very small, the sheer numbers of microbes mean that they affect our environment more than most of us realise. Given the environmental consequences of the DMS gas, it is important to know which types of bacteria and fungi that affect its production and destruction and which of the various potential pathways are involved. This may help us model how environmental changes such as climate change alter the balance of these processes.