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

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.

A mechanistic understanding of pathways that liberate the climate-cooling gas dimethyl sulfide (DMS), the biodiversity of microbes carrying out the processes and environmental stimuli that regulate DMS production will provide essential data for future modelling of DMS emissions from both marine and terrestrial environments. As summarised in Academic Beneficiaries, we feel that this work will be of paramount interest to a range of scientists including microbiologists, molecular ecologists, computational biologists, biological modellers and biochemists because of the microbial diversity data that we generate on important and sensitive natural environments. The extensive biological databases and resources that we generate from the natural study sites will be invaluable and will perfectly complement the existing microbiological knowledge of important ecosystems. The environments that we will study are not well-characterised in terms of their microbial diversity and the metagenomic data and data on the distribution and diversity of genes involved in DMS production that will be generated in the project will make a substantial contribution to environmental microbiology. The metabolic pathways we will study and identification of "new genes" will also provide context for the many unidentified genes that are present in the burgeoning number of microbial genomes that are being deposited into databases. We firmly believe that our approach of studying new processes in model organisms and then relating these processes to what is happening in the environment is a good way of reducing the number of "unknown genes" in microbial genomes and metagenome datasets. We do not envisage any immediate commercial or policy outputs from the work although any new enzymes discovered will be assessed for their biotransformation capabilities, particularly with respect to organic sulfur compounds.

Research on biological DMS production is well-represented in recent high impact journal publications and is a well-funded and publicised area of NERC-based research, exemplified in the special report on microbial DMS production in NERC's Planet Earth (Summer 2009). Thus, we are confident that this project will be of interest to a wide scientific audience. We will, of course, continue to disseminate our findings through the usual publication channels, but will also strive to include other publications in journals that have wider and less specialised audiences, such as New Scientist, the Microbiologist, Microbiology Today and Scientific American.

As described in Pathway to Impact, there is clear evidence that the media/general public found our previous NERC-funded work into DMS production to be interesting e.g. the Todd et al (2007) Science paper led to appearances on TV, both in the UK and in Germany, radio interviews and press reports throughout the world. We will continue to disseminate our findings to the public and media through the UEA Communications Office and NERC.

Our Pathway to Impact focuses on delivering its outcomes mainly to our younger generation. This will be done through the SAW (Science Art Writing) scheme (http://www.sawtrust.org/) and Natural England, and will involve several local primary and secondary schools near UEA. We also plan to build on the "Beacon of Public Engagement" award "CueEast" (Community University Engagement East) to UEA and its partners. We will host visits of school students at 6th form level in our labs and continue our outreach work to schoolchildren in the Norwich area through visits, talks and student projects. Development and maintenance of an interactive website detailing our work will allow us to reach a wider audience that we cannot reach via the above "local" proposals.