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

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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.