Smell the sea air

Scientists spotlight the coolest gas you’ve probably never heard of but have likely smelt

How many times have you arrived at the coast and heard ‘breathe in that sea air’? That distinctive smell of the sea is mainly attributed to a little known marine trace gas, dimethylsulfide (DMS), but it does far more than give us that marvellous seaside smell.

DMS is a volatile organic compound that has an important role in ecosystem functioning and the Earth’s climate, as well as serving as an ‘infochemical’ that helps seabirds, turtles and marine mammals 'sniff' their way to feeding hotspots. As important as it is, there is still a lot to uncover about this wonder gas.

Smell the Sea Air

Where does dimethylsulfide come from?

DMS is a byproduct of the microbial breakdown  of its precursor, dimethylsulfoniopropionate (DMSP). DMSP is a useful protective chemical, produced in the cells of phytoplankton and bacteria to help these organisms cope with living in salty environments, or to guard against the stress caused by natural environmental fluctuations in nutrient levels, light or free radicals. 

As these tiny organisms die or are grazed by zooplankton, they release DMSP into the surrounding seawater, where it is rapidly broken down by bacteria eager to take advantage of this rich source of carbon and sulphur. This microbial meal leads to the formation of two major volatile sulphur products: approximately 75% becomes methanethiol, which is absorbed by other marine organisms, and the rest largely becomes DMS. 

Around 90% of marine DMS formed in this way is degraded or consumed in the water column through bacterial or light-driven processes, never escaping the marine realm. The small fraction of DMS which remains ventilates to the atmosphere, amounting to around 3 billion grams of sulphur every hour of every day, when scaled up across the global oceans.

How does it affect our climate?

Once in the atmosphere, DMS has a relatively short lifespan. In under two days it is rapidly oxidised, leading to the formation of sulphate aerosols. Water vapour then condenses around the aerosol particles, known as cloud condensation nuclei or cloud seeds, and cloud begins to form.

Clouds have a significant part to play in the regulation of the Earth’s climate by reflecting solar radiation back into space, preventing the full force of the Sun’s light from reaching the Earth’s surface and providing a cooling effect. Clouds are even more important over the ocean, which is both more extensive and darker than land, and therefore, absorbs a majority of the heat hitting the planet. 

Amazingly, the natural climate cooling effect of DMS is estimated to be of a similar magnitude to the warming that has been driven by human CO2 emissions, although the overall climate warming from increasing CO2 will always outweigh any cooling. Nevertheless, DMS is considered a crucial player in the delicate balance of Earth’s climate and is long overdue extra attention. 

“DMS is considered a crucial player in the delicate balance of Earth’s climate and is long overdue extra attention”

Due to the challenges of measuring DMS, the intricate biological processes involved in its production, and the complex role of aerosols in climate regulation, this vital trace gas is still crudely represented in climate models. As a result, there is much more uncertainty concerning the climate influence of DMS, compared to the well-understood role of CO2.

A greater scientific focus

There is significantly more data available on CO2 sources and sinks, leading to a vastly better understanding of how oceanic CO2 uptake impacts climate. When comparing CO2 sampling with DMS, up to 40 million CO2 measurements have been made across the global oceans since 1957, whereas DMS has only ever been sampled about 1 million times, with many large areas of ocean completely unsampled.

A new comprehensive review of DMS research emphasises the urgent need for a greater scientific focus on unravelling the key biological processes controlling DMS production and understanding its role in climate regulation, as well as improving the accuracy of DMS flux and climate models. 

The first comprehensive marine DMS review in recent years, led by Plymouth Marine Laboratory in partnership with Bigelow Laboratory for Ocean Sciences (USA) and the University of East Anglia, also highlights other areas that would benefit from further study and a more robust comprehension of DMS. These include studying the effects of multiple stressors on the biological production of DMS, and evaluating how climate change mitigation strategies might influence DMS production, which would again feed in to improving the accuracy of climate models.

Research challenges

The challenges in regularly measuring DMS centres mainly around technology. Autonomous CO2 measuring devices can be installed on almost any seagoing vessel, whereas DMS sampling equipment is much less autonomous, with lower sampling frequency and often needing a trained expert to operate. This greatly reduces opportunities for data collection. However, in recent years the technology has been catching up and DMS measurements can now be made at much higher frequency, resulting in greater global data coverage. 

Smell the Sea Air

The team emphasise the need for further development of low-cost, low-energy autonomous DMS sensors for deployment on buoys or unmanned vehicles to target under-sampled regions or capture sporadic events, such as dust-stimulated algal blooms that could increase the amount of DMS being produced. 

Furthermore, advances in artificial intelligence (AI) have been used to delve into DMS production and fate, with the aim of capturing the complex, nonlinear relationships between environmental variables and DMS concentrations. An advantage of combining observed data with AI approaches is that they can draw attention to regionally-specific differences in seawater DMS in terms of magnitude and timing. Over a 14-year period of satellite observations in the North Atlantic, for example, this approach has estimated that DMS concentrations during the productive season appear to vary threefold and the annual peak ranged over 2–3 months.

“an advantage of combining observed data with AI approaches is that they can draw attention to regionally-specific differences in seawater DMS”

A second issue for DMS research is the type of data that has been collected. Genetics and molecular biology-based techniques have revealed the multitude of marine microorganisms that are capable of influencing global DMS production. However, there are limitations and challenges in using this information, as molecular data only highlights the potential for changes in seawater DMS concentrations, and often lacks useful accompanying measurements of DMS production and cycling. Future research must be interdisciplinary to ensure that the most useful DMS data is collected and integrated into the developing knowledge base.

To complicate matters further, the impact on DMS of increasing CO2 absorption by the ocean, leading to ocean acidification, is still being investigated. In seven of the nine published mesocosm experiments, DMS concentrations tended to decline under future ocean acidification conditions, suggesting that net DMS production is influenced by increased acidity in seawater. However, unravelling the complex processes that could be driving this response is challenging and could include changes to plankton community structure, grazing rates on phytoplankton, the activity of DMSP breakdown and bacterial metabolism of DMSP to DMS. As modelling projects the DMS flux into the future under different climate change scenarios by applying and upscaling the results of these mesocosm experiments, longer scale and more sophisticated experiments are needed. 

Additionally, ocean acidification is not occurring in isolation so a future research priority should be in gaining a better understanding of multiple factors, such as ocean acidification, deoxygenation, nutrient availability, temperature and light over multiple generations. Although technically and logistically challenging, longer-term, multi-stressor, ecological-level experiments are essential to fill key knowledge gaps.

Smell the Sea Air

At sea and on land

But it is not just the marine science community that would benefit from a greater understanding of this fabulous trace gas. Back on land, research is ongoing into the production of DMSP and DMS in agriculture. In an exciting new project, scientists from Plymouth Marine Laboratory, Cranfield University and the University of East Anglia are investigating the importance of DMSP in agricultural landscapes, its role in crop stress tolerance and yield, and how much climate-cooling DMS is produced as a result through these kinds of environments.

The team have installed autonomous sensors in fields of key crops to automatically measure the levels of DMSP and DMS. Early data shows that DMSP is present in the soil around crop roots and microbial DMS production is similar to levels of those in seawater, but a full analysis will be undertaken once all data is collected.

Carbon concerns

DMS research is also important for climate change mitigation strategies. Carbon Dioxide Removal (CDR) measures, including carbon capture and storage, artificial upwelling of nutrients, alkalinity enhancement, albedo enhancement and iron fertilisation, have all been proposed to help mitigate the climate crisis. Any CDR activity has potential consequences for the marine sulphur cycle, but these responses are poorly quantified and the CDR field is progressing exceptionally quickly so there is an urgency to understand more.

“comprehensive environmental impact assessments are vital for any developing CDR scheme”

The DMS response to CDR is comparatively most understood within the realms of oceanic iron fertilisation and it has responded in 

various ways: increased DMS production was observed in the Equatorial Pacific and Southern Ocean experiments, whereas little change or a decline was observed in the sub-Arctic North Pacific. There is debate as to whether DMS emission changes, arising during iron fertilisation, would be sufficient to impact the regional radiative balance.

Smell the Sea Air

For the Southern Ocean, an assumed average 10% increase in DMS concentrations over the course of a month-long iron addition experiment in the austral summer would result in a small (0.005°C) decrease in global average annual temperature. The large uncertainties and limited understanding of DMS biogeochemistry, emissions and climate effects, as well as the risks of unexpected changes and imbalance in the regional energy budget, add to the uncertainties associated with CDR strategies and comprehensive environmental impact assessments are vital for any developing CDR scheme.

Final thoughts

DMS clearly has a profound influence on the atmosphere and climate, but the biological processes behind its production and fate are complex and difficult to model. This leaves us with fundamental uncertainties about the variations in space and time of surface ocean DMS concentrations and its sea-to-air fluxes. These uncertainties prevent precise simulation of DMS in Earth System Models and hinder the prediction of DMS’ influence on the future climate of the Earth. It is vital to have an accurate understanding of global DMS surface ocean concentrations and fluxes at scales relevant to the biological processes which drive them. A combination of increased DMS sampling frequency and global coverage via low-cost and autonomous equipment combined with a broader measurement collection and novel AI approaches will hopefully help fill the gaps in DMS understanding.