Molecular discoveries in microbial DMSP synthesis.

Dimethylsulfoniopropionate (DMSP) is one of the Earth's most abundant organosulfur compounds because many marine algae, bacteria, corals and some plants produce it to high mM intracellular concentrations. In these organisms, DMSP acts an anti-stress molecule with purported roles to protect against salinity, temperature, oxidative stress and hydrostatic pressure, amongst many other reported functions. However, DMSP is best known for being a major precursor of the climate-active gases and signalling molecules dimethylsulfide (DMS), methanethiol (MeSH) and, potentially, methane, through microbial DMSP catabolism. DMSP catabolism has been extensively studied and the microbes, pathways and enzymes involved have largely been elucidated through the application of molecular research over the last 17 years. In contrast, the molecular biology of DMSP synthesis is a much newer field, with the first DMSP synthesis enzymes only being identified in the last 5 years. In this review, we discuss how the elucidation of key DMSP synthesis enzymes has greatly expanded our knowledge of the diversity of DMSP-producing organisms, the pathways used, and what environmental factors regulate production, as well as to inform on the physiological roles of DMSP. Importantly, the identification of key DMSP synthesis enzymes in the major groups of DMSP producers has allowed scientists to study the distribution and predict the importance of different DMSP-producing organisms to global DMSP production in diverse marine and sediment environments. Finally, we highlight key challenges for future molecular research into DMSP synthesis that need addressing to better understand the cycling of this important marine organosulfur compound, and its magnitude in the environment.

Insights into methionine S-methylation in diverse organisms.

Dimethylsulfoniopropionate (DMSP) is an important marine anti-stress compound, with key roles in global nutrient cycling, chemotaxis and, potentially, climate regulation. Recently, diverse marine Actinobacteria, α- and γ-proteobacteria were shown to initiate DMSP synthesis via the methionine (Met) S-methyltransferase enzyme (MmtN), generating S-methyl-Met (SMM). Here we characterize a roseobacterial MmtN, providing structural and mechanistic insights into this DMSP synthesis enzyme. We propose that MmtN uses the proximity and desolvation mechanism for Met S-methylation with two adjacent MmtN monomers comprising the Met binding site. We also identify diverse functional MmtN enzymes in potentially symbiotic archaeal Candidatus Woesearchaeota and Candidate Phyla Radiation (CPR) bacteria, and the animalcule Adineta steineri, not anticipated to produce SMM and/or DMSP. These diverse MmtN enzymes, alongside the larger plant MMT enzyme with an N-terminus homologous to MmtN, likely utilize the same proximity and desolvation mechanism. This study provides important insights into the catalytic mechanism of SMM and/or DMSP production, and proposes roles for these compounds in secondary metabolite production, and SMM cycling in diverse organisms and environments.

Mechanistic insights into the key marine dimethylsulfoniopropionate synthesis enzyme DsyB/DSYB.

Marine algae and bacteria produce approximately eight billion tonnes of the organosulfur molecule dimethylsulfoniopropionate (DMSP) in Earth's surface oceans annually. DMSP is an antistress compound and, once released into the environment, a major nutrient, signaling molecule, and source of climate-active gases. The methionine transamination pathway for DMSP synthesis is used by most known DMSP-producing algae and bacteria. The S-directed S-adenosylmethionine (SAM)-dependent 4-methylthio-2-hydroxybutyrate (MTHB) S-methyltransferase, encoded by the dsyB/DSYB gene, is the key enzyme of this pathway, generating S-adenosylhomocysteine (SAH) and 4-dimethylsulfonio-2-hydroxybutyrate (DMSHB). DsyB/DSYB, present in most haptophyte and dinoflagellate algae with the highest known intracellular DMSP concentrations, is shown to be far more abundant and transcribed in marine environments than any other known S-methyltransferase gene in DMSP synthesis pathways. Furthermore, we demonstrate in vitro activity of the bacterial DsyB enzyme from Nisaea denitrificans and provide its crystal structure in complex with SAM and SAH-MTHB, which together provide the first important mechanistic insights into a DMSP synthesis enzyme. Structural and mutational analyses imply that DsyB adopts a proximity and desolvation mechanism for the methyl transfer reaction. Sequence analysis suggests that this mechanism may be common to all bacterial DsyB enzymes and also, importantly, eukaryotic DSYB enzymes from e.g., algae that are the major DMSP producers in Earth's surface oceans.

Bacterial Dimethylsulfoniopropionate Biosynthesis in the East China Sea.

Dimethylsulfoniopropionate (DMSP) is one of Earth's most abundant organosulfur molecules. Recently, many marine heterotrophic bacteria were shown to produce DMSP, but few studies have combined culture-dependent and independent techniques to study their abundance, distribution, diversity and activity in seawater or sediment environments. Here we investigate bacterial DMSP production potential in East China Sea (ECS) samples. Total DMSP (DMSPt) concentration in ECS seawater was highest in surface waters (SW) where phytoplankton were most abundant, and it decreased with depth to near bottom waters. However, the percentage of DMSPt mainly apportioned to bacteria increased from the surface to the near bottom water. The highest DMSP concentration was detected in ECS oxic surface sediment (OSS) where phytoplankton were not abundant. Bacteria with the genetic potential to produce DMSP and relevant biosynthesis gene transcripts were prominent in all ECS seawater and sediment samples. Their abundance also increased with depth and was highest in the OSS samples. Microbial enrichments for DMSP-producing bacteria from sediment and seawater identified many novel taxonomic groups of DMSP-producing bacteria. Different profiles of DMSP-producing bacteria existed between seawater and sediment samples and there are still novel DMSP-producing bacterial groups to be discovered in these environments. This study shows that heterotrophic bacteria significantly contribute to the marine DMSP pool and that their contribution increases with water depth and is highest in seabed surface sediment where DMSP catabolic potential is lowest. Furthermore, distinct bacterial groups likely produce DMSP in seawater and sediment samples, and many novel producing taxa exist, especially in the sediment.

Bacteria are important dimethylsulfoniopropionate producers in marine aphotic and high-pressure environments.

Dimethylsulfoniopropionate (DMSP) is an important marine osmolyte. Aphotic environments are only recently being considered as potential contributors to global DMSP production. Here, our Mariana Trench study reveals a typical seawater DMSP/dimethylsulfide (DMS) profile, with highest concentrations in the euphotic zone and decreased but consistent levels below. The genetic potential for bacterial DMSP synthesis via the dsyB gene and its transcription is greater in the deep ocean, and is highest in the sediment.s DMSP catabolic potential is present throughout the trench waters, but is less prominent below 8000 m, perhaps indicating a preference to store DMSP in the deep for stress protection. Deep ocean bacterial isolates show enhanced DMSP production under increased hydrostatic pressure. Furthermore, bacterial dsyB mutants are less tolerant of deep ocean pressures than wild-type strains. Thus, we propose a physiological function for DMSP in hydrostatic pressure protection, and that bacteria are key DMSP producers in deep seawater and sediment.

Publisher Correction: Marine microbiology: A day in the life of marine sulfonates.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Bacteria are important dimethylsulfoniopropionate producers in coastal sediments.

Dimethylsulfoniopropionate (DMSP) and its catabolite dimethyl sulfide (DMS) are key marine nutrients1,2 that have roles in global sulfur cycling2, atmospheric chemistry3, signalling4,5 and, potentially, climate regulation6,7. The production of DMSP was previously thought to be an oxic and photic process that is mainly confined to the surface oceans. However, here we show that DMSP concentrations and/or rates of DMSP and DMS synthesis are higher in surface sediment from, for example, saltmarsh ponds, estuaries and the deep ocean than in the overlying seawater. A quarter of bacterial strains isolated from saltmarsh sediment produced DMSP (up to 73 mM), and we identified several previously unknown producers of DMSP. Most DMSP-producing isolates contained dsyB8, but some alphaproteobacteria, gammaproteobacteria and actinobacteria used a methionine methylation pathway independent of DsyB that was previously only associated with higher plants. These bacteria contained a methionine methyltransferase gene (mmtN)-a marker for bacterial synthesis of DMSP through this pathway. DMSP-producing bacteria and their dsyB and/or mmtN transcripts were present in all of the tested seawater samples and Tara Oceans bacterioplankton datasets, but were much more abundant in marine surface sediment. Approximately 1 × 108 bacteria g-1 of surface marine sediment are predicted to produce DMSP, and their contribution to this process should be included in future models of global DMSP production. We propose that coastal and marine sediments, which cover a large part of the Earth's surface, are environments with high levels of DMSP and DMS productivity, and that bacteria are important producers of DMSP and DMS within these environments.

Author Correction: DSYB catalyses the key step of dimethylsulfoniopropionate biosynthesis in many phytoplankton.

In the version of this Letter originally published, the Methods incorrectly stated that all phytoplankton cultures were sampled in mid-exponential phase. The low-nitrogen cultures were sampled in early stationary phase and at the point at which Fv/Fm values decreased, to indicate that cultures were experiencing low-nitrogen conditions. All other phytoplankton cultures were sampled in exponential phase. Growth and Fv/Fm data are provided here on high- and low-nitrogen cultures (Figs 1, 2 and 3) to clarify and support this correction. The Methods also stated that cell counting was done using a Beckman Multisizer 3 Coulter Counter, but a CASY Model TT Cell Counter was used.

DSYB catalyses the key step of dimethylsulfoniopropionate biosynthesis in many phytoplankton.

Dimethylsulfoniopropionate (DMSP) is a globally important organosulfur molecule and the major precursor for dimethyl sulfide. These compounds are important info-chemicals, key nutrients for marine microorganisms, and are involved in global sulfur cycling, atmospheric chemistry and cloud formation1-3. DMSP production was thought to be confined to eukaryotes, but heterotrophic bacteria can also produce DMSP through the pathway used by most phytoplankton 4 , and the DsyB enzyme catalysing the key step of this pathway in bacteria was recently identified 5 . However, eukaryotic phytoplankton probably produce most of Earth's DMSP, yet no DMSP biosynthesis genes have been identified in any such organisms. Here we identify functional dsyB homologues, termed DSYB, in many phytoplankton and corals. DSYB is a methylthiohydroxybutryate methyltransferase enzyme localized in the chloroplasts and mitochondria of the haptophyte Prymnesium parvum, and stable isotope tracking experiments support these organelles as sites of DMSP synthesis. DSYB transcription levels increased with DMSP concentrations in different phytoplankton and were indicative of intracellular DMSP. Identification of the eukaryotic DSYB sequences, along with bacterial dsyB, provides the first molecular tools to predict the relative contributions of eukaryotes and prokaryotes to global DMSP production. Furthermore, evolutionary analysis suggests that eukaryotic DSYB originated in bacteria and was passed to eukaryotes early in their evolution.

Methanethiol-dependent dimethylsulfide production in soil environments.

Dimethylsulfide (DMS) is an environmentally important trace gas with roles in sulfur cycling, signalling to higher organisms and in atmospheric chemistry. DMS is believed to be predominantly produced in marine environments via microbial degradation of the osmolyte dimethylsulfoniopropionate (DMSP). However, significant amounts of DMS are also generated from terrestrial environments, for example, peat bogs can emit ~6 µmol DMS m-2 per day, likely via the methylation of methanethiol (MeSH). A methyltransferase enzyme termed 'MddA', which catalyses the methylation of MeSH, generating DMS, in a wide range of bacteria and some cyanobacteria, may mediate this process, as the mddA gene is abundant in terrestrial metagenomes. This is the first study investigating the functionality of MeSH-dependent DMS production (Mdd) in a wide range of aerobic environments. All soils and marine sediment samples tested produced DMS when incubated with MeSH. Cultivation-dependent and cultivation-independent methods were used to assess microbial community changes in response to MeSH addition in a grassland soil where 35.9% of the bacteria were predicted to contain mddA. Bacteria of the genus Methylotenera were enriched in the presence of MeSH. Furthermore, many novel Mdd+ bacterial strains were isolated. Despite the abundance of mddA in the grassland soil, the Mdd pathway may not be a significant source of DMS in this environment as MeSH addition was required to detect DMS at only very low conversion rates.

Dimethylsulfoniopropionate biosynthesis in marine bacteria and identification of the key gene in this process.

Dimethylsulfoniopropionate (DMSP) is one of the Earth's most abundant organosulfur molecules, a signalling molecule1, a key nutrient for marine microorganisms2,3 and the major precursor for gaseous dimethyl sulfide (DMS). DMS, another infochemical in signalling pathways4, is important in global sulfur cycling2 and affects the Earth's albedo, and potentially climate, via sulfate aerosol and cloud condensation nuclei production5,6. It was thought that only eukaryotes produce significant amounts of DMSP7-9, but here we demonstrate that many marine heterotrophic bacteria also produce DMSP, probably using the same methionine (Met) transamination pathway as macroalgae and phytoplankton10. We identify the first DMSP synthesis gene in any organism, dsyB, which encodes the key methyltransferase enzyme of this pathway and is a reliable reporter for bacterial DMSP synthesis in marine Alphaproteobacteria. DMSP production and dsyB transcription are upregulated by increased salinity, nitrogen limitation and lower temperatures in our model DMSP-producing bacterium Labrenzia aggregata LZB033. With significant numbers of dsyB homologues in marine metagenomes, we propose that bacteria probably make a significant contribution to oceanic DMSP production. Furthermore, because DMSP production is not solely associated with obligate phototrophs, the process need not be confined to the photic zones of marine environments and, as such, may have been underestimated.

The relationship between emotion recognition ability and social skills in young children with autism.

This study assessed the relationship between emotion recognition ability and social skills in 42 young children with autistic disorder aged 4-7 years. The analyses revealed that accuracy in recognition of sadness, but not happiness, anger or fear, was associated with higher ratings on the Vineland-II Socialization domain, above and beyond the influence of chronological age, cognitive ability and autism symptom severity. These findings extend previous research with adolescents and adults with autism spectrum disorders, suggesting that sadness recognition is also associated with social skills in children with autism.

Teaching emotion recognition skills to young children with autism: a randomised controlled trial of an emotion training programme.

BACKGROUND: Children with autism have difficulties in emotion recognition and a number of interventions have been designed to target these problems. However, few emotion training interventions have been trialled with young children with autism and co-morbid ID. This study aimed to evaluate the efficacy of an emotion training programme for a group of young children with autism with a range of intellectual ability. METHODS: Participants were 55 children with autistic disorder, aged 4-7 years (FSIQ 42-107). Children were randomly assigned to an intervention (n = 28) or control group (n = 27). Participants in the intervention group watched a DVD designed to teach emotion recognition skills to children with autism (the Transporters), whereas the control group watched a DVD of Thomas the Tank Engine. Participants were assessed on their ability to complete basic emotion recognition tasks, mindreading and theory of mind (TOM) tasks before and after the 4-week intervention period, and at 3-month follow-up. RESULTS: Analyses controlled for the effect of chronological age, verbal intelligence, gender and DVD viewing time on outcomes. Children in the intervention group showed improved performance in the recognition of anger compared with the control group, with few improvements maintained at 3-month follow-up. There was no generalisation of skills to TOM or social skills. CONCLUSIONS: The Transporters programme showed limited efficacy in teaching basic emotion recognition skills to young children with autism with a lower range of cognitive ability. Improvements were limited to the recognition of expressions of anger, with poor maintenance of these skills at follow-up. These findings provide limited support for the efficacy of the Transporters programme for young children with autism of a lower cognitive range.