Marsh Mud Hides a Key Ingredient for "Anti-Greenhouse Gas"

We may have far more of this anti-greenhouse gas than we thought. 

marsh mud, climate change

The smell that wafts off of the ocean is instantly recognizable: Sometimes it smells fresh and clean, but other times it’s a little clammy, briny, or sulfuric. That smell actually indicates a plentiful source of what some scientists call an “anti-greenhouse gas.”

The ocean already plays an important role in carbon capture, which can help offset the amount of carbon that lingers in the atmosphere. But the sea also has another effect on temperature regulation. That sea smell is partially the odor of dimethyl sulfide (DMS), which is released when marine microorganisms break down another sea nutrient called dimethylsulfoniopropionate (DMSP).

DMS has sometimes been called the “anti-greenhouse gas” because once it’s in the atmosphere it rapidly breaks down into aerosols that allow clouds to form. Those clouds can help scatter UV radiation back into space, creating a cooling effect.

In a paper published Monday in Nature Microbiology Jonathan Todd, Ph.D., and a team at the University of East Anglia have shown that bacteria hidden deep in marsh mud can manufacture DMSP, which is the key ingredient needed to make DMS.

“Previously DMSP was largely thought to be a molecule made by eukaryotic algae in Earth’s surface ocean,” Todd tells Inverse. “However, we show that DMSP concentration, DMSP and DMS synthesis rates are far higher in all tested coastal mud samples than in the surface water.”

marsh, saltwater, climate
Saltmarsh ponds had far higher rates of DMSP and DMS production than ocean surface water, according to these results. 

Interestingly, these bacteria seem to be scattered throughout the world’s underwater mud. Todd and his team collected mud samples from several saltwater marshes and estuaries scattered throughout eastern England and found that nearly a quarter of the bacteria living in those samples had the ability to produce DMSP. He also found traces of them in the Mariana Trench — the deepest natural ocean trench that descends nearly seven miles deep. These bacteria were thriving as far as 4 kilometers (2.4 miles) down.

In terms of climate, we can think of DMSP as fuel that may one day be turned into the anti-greenhouse gas, DMS. And these muddy stores seem to be a particularly rich source of it. This team estimates that there may be 100 million DMSP-producing bacteria in a single gram of marsh mud, which suggests that these coastal areas could play a far greater role in global DMS production than previously thought.

Can We Actually Use the “Anti-Greenhouse Gas”?

These findings lend themselves to a somewhat controversial idea: that we might be able to manipulate these marine ecosystems to produce more DMS and try to offset climate change that way.

This idea dates back to 1987, when James Lovelock (the person who came up with the “Gaia hypothesis”) proposed that we could actually use DMS-producing plankton to offset the warming climate. In 2007, he wrote a letter proposing an “emergency treatment for the pathology of global warming”: the creation of 100- to 200-meter-long pipes that could bring oceanic nutrients to the surface, jumpstarting DMS production.

In that letter, Lovelock admitted that a project like this “may fail perhaps on engineering or economic grounds.” And it has since been criticized because it could also cause dangerous algal blooms or other unintended consequences. But the idea has never totally disappeared. In 2015, another paper published in Scientific Reports used two climate models to show that increasing DMS production would actually offset some changes in warming.

Still, even that paper acknowledged that there would likely be an excessive “mixture of positive and negative impacts on the climate” to attempt such an aggressive geoengineering scheme.

Todd, in the context of this new paper, doesn’t see his team’s work that way.

“We feel our study does provide important knowledge required to understand the global production and cycling of DMSP and DMS,” he says. “Personally I do not think there is a geo-engineering angle to the work, others may disagree.”

Still, as we continue to search for a solution, some may be inspired to know that our saltwater marshes and estuaries could be far richer sources of the “anti-greenhouse gas” that we once thought.

Abstract:

Dimethylsulfoniopropionate (DMSP) and its catabolite dimethyl sulfide (DMS) are key marine nutrients1,2 that have roles in global sulfur cycling2, atmospheric chemistry3, signal- ling4,5 and, potentially, climate regulation6,7. The production of DMSP was previously thought to be an oxic and photic pro- cess that is mainly confined to the surface oceans. However, here we show that DMSP concentrations and 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 pro- ducers 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 meth- yltransferase 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 contribu- tion 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 productiv- ity, and that bacteria are important producers of DMSP and DMS within these environments.