Organosulfur cycling in abundant anoxic marine sediments: a case study of saltmarsh sediments

地球的大气、海洋和许多土壤中都有丰富的氧气，这使得多细胞生命的进化成为可能。表层海洋富含氧气，因为光合作用生物(初级生产者)丰富。这些光合作用有机体中的一些可以制造重要的分子，可以释放到大气中。一旦到了那里，它们就会发生反应并形成云，在水蒸气中产生雨水和酸度，因此在气候学和硫磺循环中很重要。其中最广为人知的是二甲基硫化物(DMS)，它是由海洋微生物分解二甲基磺基丙酸酯(DMSP)产生的。据估计，每年有数十亿吨DMSP是由海洋藻类、珊瑚、植物以及我们所显示的海洋细菌产生的。DMSP在海洋生态系统中还有其他关键作用，作为渗透保护剂，为海洋微生物提供营养，与DMS一样，它是许多将其与食物联系在一起的生物的化学诱导剂。海洋环境中含有大量的DMSP和DMS，海边特有的气味来自DMS本身。人们普遍认为，只有表层水域通过光合作用生物产生大量的DMSP和DMS。我们发现异养细菌产生DMSP挑战了这一信念，因为它们不需要光。此外，我们还表明，大量的DMSP(比表层海洋大几个数量级)、DMS和其他有机硫分子存在于无氧的泥浆中，取而代之的是还原铁(称为铁质)和还原硫化物(称为丁辛酸)。这很有趣，也很重要，因为我们不知道这些分子是如何在这些非常不同的环境中生产或消费的，涉及哪些生物体，以及这些分子在生活在那里的微生物群落中扮演着什么角色。鉴于海洋沉积物覆盖了地球表面的70%以上，这一话题具有全球意义。此外，在地球历史上85%的时间里，海洋很可能是没有氧气的，只含有溶解的铁或硫。这些分子在过去的海洋中很重要吗？由于环境条件(包括气候)可能会影响DMSP/DMS的生产，反之亦然，因此了解和预测这些影响是关键。目前对DMSP/DMS产量的估计可能是不准确的，原因是：i)缺乏结合分子、生物地球化学、过程和模型数据的综合研究；以及ii)忽视细菌DMSP产生的输入，特别是来自海洋沉积物的输入。我们将探索的问题是：为什么在富铁的沉积物中有很多DMS，但没有它的相关代谢物，甲硫醇(MESH)，而在富含硫化物的沉积物中则相反？生物体是如何制造这些分子的？为什么？这些分子在泥浆中的细菌群落中扮演着什么角色？这些分子的生产在全球范围内有多重要？我们的项目分为几个工作包。我们将在沃勒姆盐沼进行一项为期一年的详细研究，那里有铁质和正辛质的沉淀池。我们将采集样本，分析我们已确定这些关键模式的沉积物的地球化学和微生物学。我们将使用一系列分子微生物学技术，包括组学工作(微生物群落DNA和RNA)和稳定同位素探测，确定那里有哪些生物体，它们正在做什么。稳定同位素探测使我们能够识别活跃在DMSP循环中的生物体。然后，我们将在实验室分离和培养这些微生物，以了解这些气候重要分子的生产和消费如何随着我们施加的环境变化而变化。最后，我们将对这些变化进行建模并进行外推，以确定这些环境对这些分子的生产和消费有多重要，这将是了解过去和未来的决定性窗口。

There is abundant oxygen in Earth's atmosphere, oceans and many soils, and this has enabled the evolution of multicellular life. The surface ocean is oxygen-rich because photosynthetic organisms (primary producers) are abundant. Some of these photosynthetic organisms make important molecules that can be released to the atmosphere. Once there, they can react and form clouds, generating rain and acidity in water vapour, and thus are important in climatology and sulfur cycling. The most well-known of these is dimethyl sulfide (DMS), which is derived from the action of marine microorganisms catabolising dimethylsulfoniopropionate (DMSP). An estimated several billion tonnes of DMSP is made each year by marine algae, corals, plants, and, as shown by us, marine bacteria. DMSP has other key roles in marine ecosystems, serving as an osmoprotectant, a nutrient for marine microbes, and, like DMS, it is a chemoattractant for many organisms that link it with food. DMSP and DMS are so abundant in marine environments that the characteristic smell associated with the seaside comes from DMS itself.It is widely believed that only surface waters make significant amounts of DMSP and DMS via photosynthetic organisms. Our discovery that heterotrophic bacteria produce DMSP challenges this belief, since they do not require light. Furthermore, we have shown that large quantities of DMSP (orders of magnitude greater than in surface oceans), DMS and other organosulfur molecules exist in mud that is devoid of oxygen, and is instead filled with reduced iron (termed ferruginous) and reduced sulfur (termed euxinic). This was interesting and important, because we don't know how these molecules are produced or consumed in these very different environments, what organisms are involved and what role these molecules play in the microbial communities living there. Given that marine sediments cover over 70 % of Earth's surface, this topic is of global significance. Moreover, for 85% of Earth's history the ocean was likely free of oxygen, and only contained dissolved iron or sulfur. Were these molecules important in these past oceans? What role did they play?As environmental conditions (including climate) likely affect DMSP/DMS production, and vice versa, it is key to understand and predict these effects. Current estimates of DMSP/DMS production are likely inaccurate due to i) a lack of integrated studies combining molecular, biogeochemical, process and modelling data; and ii) ignorance as to the input from bacterial DMSP-production, particularly from marine sediments.Questions we will explore are: Why is there lots of DMS but none of its related metabolite, methanethiol (MeSH), in iron-rich sediments, while in sulfide-rich sediments it is the opposite? How are organisms making these molecules, and why? What role do these molecules play in bacterial communities in the mud? How significant is the production of these molecules on a global scale?Our project is divided into several work packages. We will carry out a detailed, year-long study at Warham saltmarsh, which has ferruginous and euxinic sediment pools in close proximity. We will take samples and analyse the geochemistry and microbiology of sediments where we have identified these key patterns. We will determine what organisms are there, and what they are doing, using a series of molecular microbiology techniques, including 'omics work (on microbial community DNA & RNA) and stable isotope probing, which allows us to identify organisms actively cycling DMSP. We will then isolate and grow these microorganisms in the lab to understand how the production and consumption of these climatologically important molecules varies in response to the environmental changes we impose. Finally, we will model these changes and extrapolate to determine how important these environments are to the production and consumption of these molecules, which will be a definitive window to both the past and future.