This Special Issue presents a selection of papers on recent advances in the knowledge of the biological and environmental chemistry of methylated sulfur compounds, mainly dimethylsulfoniopropionate, dimethylsulfide and dimethylsulfoxide, in the ocean and other aquatic environments. Research topics include algal, coral and mollusc physiology, microbial metabolism, inter-organism associations, sulfur biogeochemistry under current environmental conditions and ocean acidification, air–sea exchange and numerical modeling from the individual cell to the global ocean.

Environmental context. Dimethylsulfoniopropionate (DMSP), a small sulfur compound biosynthesised by algae, plays an important role in global climate, particularly in polar regions. We investigated the environmental controls on DMSP levels, and provide the first experimental measurements of DMSP and associated physiological changes in a polar diatom exposed to a range of gradual salinity shifts representative of sea-ice conditions. Quantitative estimates of DMSP in polar diatoms following salinity changes will facilitate new mathematical models to predict seasonal responses and reactions to climate change.

Environmental context. Low iron concentrations and solar ultraviolet radiation can affect the growth of marine algae. We observed reduced growth and substantial increases in dissolved dimethylsulfoxide and cellular acrylate concentrations in low-iron cultures of a prevalent Southern Ocean algal species, Phaeocystis antarctica, with comparatively small increases observed for cellular dimethylsulfoniopropionate concentrations. Exposure of P. antarctica to high levels of ultraviolet and visible light had very little effect on concentrations of these compounds in culture, even under iron-limitation. Our results highlight the importance of iron to P. antarctica.

Environmental context. Dimethylsulfoxide (DMSO) is important in the biogeochemical cycle of sulfur. Using a mathematical flux model of DMSO production and loss rates, we find that the high intracellular DMSO concentrations measured in phytoplankton cannot be produced without invoking unrealistically high intracellular concentrations of the precursor dimethylsulfoniopropionate, or much lower phytoplankton cellular efflux rates than currently reported. Our study emphasises the need for further investigations of DMSO fluxes across intracellular and outer cell membranes.

Environmental context. Dimethyl sulfide (DMS) is released by marine algae and is important to sulfur transfer between the oceans and the atmosphere. We measured DMS emissions from algae that form large blooms, and found that the hydration of the plants, seawater temperatures and salinity affect DMS release, but their effects were species-specific. Thus, the effect of algal blooms on sulfur transfer will depend on the bloom’s species composition and the environmental conditions experienced by the algae.

Environmental context. Animals that eat marine algae strongly affect the rate at which a compound that algae synthesise, dimethylsulfoniopropionate (DMSP), is converted to the important atmospheric climate gas dimethylsulfide (DMS). In studying the processing of DMSP by algae-eating molluscs, we have discovered that some molluscs accumulate and retain DMSP exceptionally well, but this can be very variable. With this knowledge, investigators will be able to design improved experiments to understand the effects of molluscs on DMS production in local ecosystems.

Environmental context. Dimethylated sulfur compounds can exert multiple biological and environmental effects including climate regulation. Climate change and other anthropogenic factors are predicted to affect coral-reef ecosystems where these sulfur compounds are particularly abundant. We review the processes that regulate the production of dimethylated sulfur compounds in coral reefs and the potential consequences of environmental changes on their biogenic cycle in such fragile ecosystems under future climate change scenarios.

Environmental context. DMSP is one of the most important substrates for marine bacteria and its cycling contributes substantially to fluxes of carbon and sulfur in the ocean. Accurate determination of the concentration of DMSP available to bacteria is essential to quantifying DMSP consumption rates, and this work improves those determinations by identifying non-bioavailable pools of DMSP that have previously gone unrecognised. Improved estimates of DMSP consumption rates will lead to better understanding of its role in ocean food web and biogeochemical dynamics.

Dimethylsulfoniopropionate (DMSP) comprises an important fraction of the organic carbon produced by phytoplankton, and is a major source of carbon and sulfur for heterotrophic bacteria. Here, we show that a non-bioavailable fraction of DMSP recently discovered in coastal waters also exists in oligotrophic open-ocean waters. Taking account of the non-bioavailable pool improved estimates of cycling rates of DMSP and its contribution to bacterial nutrition.

Environmental context. Cobalamin, or vitamin B₁₂, is receiving increased attention as a critical trace nutrient in the growth and metabolic processes of oceanic phytoplankton and bacterial communities. We present evidence that indicates B₁₂ has a more significant role in the biogeochemical cycling of the climatically important compounds dimethylsulfide and dimethylsulfoniopropionate than previously understood. Several possible mechanisms are examined that link cellular-level processes involving B₁₂ to global-scale biogeochemical processes involving the oceanic cycling of dimethylsulfoniopropionate and dimethylsulfide.

Environmental context. Approximately 25 % of CO₂ released to the atmosphere by human activities has been absorbed by the oceans, resulting in ocean acidification. We investigate the acidification effects on marine phytoplankton and subsequent production of the trace gas dimethylsulfide, a major route for sulfur transfer from the oceans to the atmosphere. Increasing surface water CO₂ partial pressure (pCO₂) affects the growth of phytoplankton groups to different degrees, resulting in varying responses in community production of dimethylsulfide.

Environmental context. Ocean acidification affects marine algae and bacteria, which can produce climate active trace gases such as methane or dimethylsulfide from marine dimethylsulfoxide. We conducted field experiments simulating future ocean acidification, and showed that dimethylsulfoxide concentrations decreased with increasing acidification. Less dimethylsulfoxide in the future can affect climate by influencing the concentration of methane and dimethylsulfide.

Environmental context. The volatile sulfur compound, dimethylsulfide (DMS), plays a major role in the global sulfur cycle by transferring sulfur from aquatic environments to the atmosphere. Compared to marine environments, freshwater environments are under studied with respect to DMS cycling. The goal of this study was to assess the formation pathways of DMS in a freshwater lake using natural stable isotopes of sulfur. Our results provide unique sulfur isotopic evidence for the multiple DMS sources and dynamics that are linked to the various biogeochemical processes that occur in freshwater lake water columns and sediments.

Environmental context. Dimethylsulfide (DMS), the main biogenic sulfur compound in the atmosphere, is produced by the marine biosphere and plays an important role in the atmospheric sulfur cycle. This study recorded the spatial variability of DMS and dissolved and particulate dimethylsulfoniopropionate (DMSP) in the water column of the southern Gulf of Mexico. The results suggest that the spatial variability of DMS and DMSP is directly related to the hydrodynamics of the study area.

The trace gas dimethylsulfide (DMS) is emitted from surface ocean waters to the overlying atmosphere, where it forms aerosols that promote cloud formation and influence Earth’s climate. We present an updated climatology of DMS emissions from the vast Southern Ocean, demonstrating how the inclusion of new data yields higher regional sources compared with previously derived values. Our work provides an important step towards better quantifying the oceanic emissions of an important climate-active gas.

Environmental context. Models are needed to predict the importance of the changes in marine emissions of dimethylsulfide (DMS) in response to ocean warming, increased stratification and acidification, and to evaluate the potential effects on the Earth’s climate. We use complementary simulations to further our understanding of the marine cycle of DMS in subtropical waters, and show that a lack of phosphorus may exert a more important control on surface DMS concentrations than an excess of light.

Environmental context. As climate models increasingly include detailed, process-based models of aerosol formation, they need to represent dimethylsulfide emissions from the ocean. Options for this include data-based climatologies and empirical or process-based models; there are diverse examples of each in the literature. This paper presents the first global-scale comparison of all available approaches and evaluation of their skill relative to observations and their possible roles in future climate models.

Environmental context. Future changes in marine biogenic aerosol emissions in Arctic seas are likely to affect the radiative budget of the region. Here we employ a calibrated biogeochemical model to simulate change in sulfate aerosol emissions in the Barents Sea, and find strong increases occur by the late 21st century. If replicated across the Arctic Ocean, such increases in sulfate aerosol loading to the Arctic atmosphere may decrease the rate of warming at polar latitudes.