DMSP SYNTHESIS VIA A NOVEL ENZYME IN CYANOBACTERIA AND DIVERSE BACTERIA

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Dimethylsulfoniopropionate (DMSP) is an important and highly abundant organo-sulfur compound. It is synthesised by many algae, bacteria and some higher plants, where it is thought to be involved in chemotaxis, grazer deterrence, osmoprotection, cryoprotection, hydrostatic pressure protection and/or resistance to oxidative stress. DMSP, and its gaseous breakdown products, dimethyl sulfide (DMS) and methanethiol (MeSH) are the major biosources of sulfur transferred from the oceans to the atmosphere. Atmospheric DMS and MeSH are climate active gases (CAGs) that form aerosols and cloud condensation nuclei, which reduce the global radiation budget and 'cool' the local climate.It was previously thought that only Trichodesmium species of cyanobacteria and low proportions of marine bacteria produce DMSP at significant levels. However, our pilot work shows that many cyanobacteria, e.g. highly abundant marine Synechococcus and saltmarsh species, produce DMSP, as well as other important and abundant bacterial phyla (e.g. gammaproteobacteria). These microbes contain a gene that we term dsyC, which we show encodes the key S-methyltransferase enzyme that catalyses the committed and rate limiting step of DMSP synthesis. This work is important because dsyC genes occur in up to 5% of marine bacteria and are highly transcribed in Earth's photic waters and surface sediment, established from our analysis of the Tara Oceans and local datasets. In comparison, the other DMSP synthesis genes that we and others discovered (dsyB, DSYB, mmtN and TpMMT), encoding the key S-methyltransferase of alternate bacterial and algal DMSP biosynthesis pathways, are collectively far less abundant and transcribed than dsyC. Therefore, cyanobacteria and other diverse bacteria (cyano/bacteria) with DsyC could be very significant contributors to global production of DMSP, and, thus, of CAGs derived from it. This would be a paradigm-shifting finding, showing that cyano/bacteria, pretty much ignored as significant DMSP producers, are large-scale contributors to global production. However, without the multidisciplinary work planned here we are unable to make such statements.The first major goal will be to establish the environmental conditions under which DMSP is produced in cyano/bacteria that have DsyC homologues. We will then generate dsyC knockouts in marine Synechococcus species and a selection of other bacteria with dsyC. Growth of wild-type and mutant strains will then be compared under a range of environmental or stress conditions to assess the role of DMSP in these bacteria and importantly the major environmental drivers of DsyC-dependent DMSP synthesis. In conjunction, we will determine the amount of DMSP per cell and synthesis rates in wild-type and mutant samples. The next goal will be to determine the key features of the DsyC enzyme. To address this, we will examine the enzymatic activity and substrate affinity of DsyC from a range of cyano/bacteria to identify the key amino acid residues that could determine whether they are of a high activity or low activity type. Finally, via analysis of cyano/bacterial samples from a broad range of oligotrophic and coastal waters, and a seasonal study of saltmarsh cyanobacterial mats, we will quantify DMSP and CAG levels and synthesis rates and compare these to laboratory cultured model organisms. This will allow us to estimate annual, global production rates of DMSP by marine Synechococcus and potentially Prochlorococcus species, localised production by all species in Western Pacific and Eastern Indian oceans, and localised saltmarshes and potential production of DMS and therefore the environmental impact of cyanobacterial DMSP production.