Dimethylsulfoniopropionate (DMSP) is one of the most abundant sulfur-containing organic compounds on the earth, which is an important carbon and sulfur source and plays an important role in the global sulfur cycle. Marine microorganisms are an important group involved in DMSP metabolism. The strain Cobetia sp. D5 was isolated from seawater samples in the Yellow Sea area of Qingdao during an algal bloom. There is still limited knowledge on the capacity of DMSP utilization of Cobetia bacteria. The study reports the whole genome sequence of Cobetia sp. D5 to understand its DMSP metabolism pathway. The genome of Cobetia sp. D5 consists of a circular chromosome with a length of 4,233,985 bp and the GC content is 62.56%. Genomic analysis showed that Cobetia sp. D5 contains a set of genes to transport and metabolize DMSP, which can cleave DMSP to produce dimethyl sulphide (DMS) and 3-Hydroxypropionyl-Coenzyme A (3-HP-CoA). DMS diffuses into the environment to enter the global sulfur cycle, whereas 3-HP-CoA is catabolized to acetyl CoA to enter central carbon metabolism. Thus, this study provides genetic insights into the DMSP metabolic processes of Cobetia sp. D5 during a marine algal bloom, and contributes to the understanding of the important role played by marine bacteria in the global sulfur cycle.

Dimethylsulfoniopropionate (DMSP) is an abundant and ubiquitous organosulfur molecule in marine environments with important roles in global sulfur and nutrient cycling. Diverse DMSP lyases in some algae, bacteria, and fungi cleave DMSP to yield gaseous dimethyl sulfide (DMS), an infochemical with important roles in atmospheric chemistry. Here, we identified a novel ATP-dependent DMSP lyase, DddX. DddX belongs to the acyl-CoA synthetase superfamily and is distinct from the eight other known DMSP lyases. DddX catalyses the conversion of DMSP to DMS via a two-step reaction: the ligation of DMSP with CoA to form the intermediate DMSP-CoA, which is then cleaved to DMS and acryloyl-CoA. The novel catalytic mechanism was elucidated by structural and biochemical analyses. DddX is found in several Alphaproteobacteria, Gammaproteobacteria, and Firmicutes, suggesting that this new DMSP lyase may play an overlooked role in DMSP/DMS cycles.
The global sulfur cycle is a collection of geological and biological processes that circulate sulfur-containing compounds through the oceans, rocks and atmosphere. Sulfur itself is essential for life and important for plant growth, hence its widespread use in fertilizers. Marine organisms such as bacteria, algae and phytoplankton produce one particular sulfur compound, called dimethylsulfoniopropionate, or DMSP, in massive amounts. DMSP made in the oceans gets readily converted into a gas called dimethyl sulfide (DMS), which is the largest natural source of sulfur entering the atmosphere. In the air, DMS is converted to sulfate and other by-products that can act as cloud condensation nuclei, which, as the name suggests, are involved in cloud formation. In this way, DMS can influence weather and climate, so it is often referred to as ‘climate-active’ gas. At least eight enzymes are known to cleave DMSP into DMS gas with a few by-products. These enzymes are found in algae, bacteria and fungi, and are referred to as lyases, for the way they breakdown their target compounds (DMSP, in this case). Recently, researchers have identified some bacteria that produce DMS from DMSP without using known DMSP lyases. This suggests there are other, unidentified enzymes that act on DMSP in nature, and likely contribute to global sulfur cycling. Li, Wang et al. set out to uncover new enzymes responsible for converting the DMSP that marine bacteria produce into gaseous DMS. One new enzyme called DddX was identified and found to belong to a superfamily of enzymes quite separate to other known DMSP lyases. Li, Wang et al. also showed how DddX drives the conversion of DMSP to DMS in a two-step reaction, and that the enzyme is found across several classes of bacteria. Further experiments to characterise the protein structure of DddX also revealed the molecular mechanism for its catalytic action. This study offers important insights into how marine bacteria generate the climatically important gas DMS from DMSP, leading to a better understanding of the global sulfur cycle. It gives microbial ecologists a more comprehensive perspective of these environmental processes, and provides biochemists with data on a family of enzymes not previously known to act on sulfur-containing compounds.

Methyl sulfur compounds are a rich source of environmental sulfur for microorganisms, but their use requires redox systems. The bacterial sfn and msu operons contain two-component flavin-dependent monooxygenases for dimethylsulfone (DMSO2) assimilation: SfnG converts DMSO2 to methanesulfinate (MSI-), and MsuD converts methanesulfonate (MS-) to sulfite. However, the enzymatic oxidation of MSI- to MS- has not been demonstrated, and the function of the last enzyme of the msu operon (MsuC) is unresolved. We employed crystallographic and biochemical studies to identify the function of MsuC from Pseudomonas fluorescens. The crystal structure of MsuC adopts the acyl-CoA dehydrogenase fold with putative binding sites for flavin and MSI-, and functional assays of MsuC in the presence of its oxidoreductase MsuE, FMN, and NADH confirm the enzymatic generation of MS-. These studies reveal that MsuC converts MSI- to MS- in sulfite biosynthesis from DMSO2.

The precise interpretation of environmental sulfur isotope records requires a quantitative understanding of the biochemical controls on sulfur isotope fractionation by the principle isotope-fractionating process within the S cycle, microbial sulfate reduction (MSR). Here we provide the only direct observation of the major ((34)S/(32)S) and minor ((33)S/(32)S, (36)S/(32)S) sulfur isotope fractionations imparted by a central enzyme in the energy metabolism of sulfate reducers, dissimilatory sulfite reductase (DsrAB). Results from in vitro sulfite reduction experiments allow us to calculate the in vitro DsrAB isotope effect in (34)S/(32)S (hereafter, [Formula: see text]) to be 15.3 ± 2‰, 2σ. The accompanying minor isotope effect in (33)S, described as [Formula: see text], is calculated to be 0.5150 ± 0.0012, 2σ. These observations facilitate a rigorous evaluation of the isotopic fractionation associated with the dissimilatory MSR pathway, as well as of the environmental variables that govern the overall magnitude of fractionation by natural communities of sulfate reducers. The isotope effect induced by DsrAB upon sulfite reduction is a factor of 0.3-0.6 times prior indirect estimates, which have ranged from 25 to 53‰ in (34)εDsrAB. The minor isotope fractionation observed from DsrAB is consistent with a kinetic or equilibrium effect. Our in vitro constraints on the magnitude of [Formula: see text] is similar to the median value of experimental observations compiled from all known published work, where (34)ε r-p = 16.1‰ (r-p indicates reactant vs. product, n = 648). This value closely matches those of MSR operating at high sulfate reduction rates in both laboratory chemostat experiments ([Formula: see text] 17.3 ± 1.5‰, 2σ) and in modern marine sediments ([Formula: see text] 17.3 ± 3.8‰). Targeting the direct isotopic consequences of a specific enzymatic processes is a fundamental step toward a biochemical foundation for reinterpreting the biogeochemical and geobiological sulfur isotope records in modern and ancient environments.