A Microbial Pathway for Methanesulfonic Acid Biosynthesis from DMSP Expands the Marine Sulfur Cycle

The oceans are vast, silent engines that drive the climate of our planet. Hidden within the microscopic life that drifts through these waters is a complex chemical cycle involving sulfur, an element essential to life and a key player in how the Earth regulates its temperature. For decades, scientists have understood one major route this sulfur takes: tiny marine plants produce a specific compound to protect themselves, and when bacteria break it down, they release a gas that rises into the air. Once in the atmosphere, sunlight and chemistry transform this gas into an acid that helps form clouds, which in turn reflect sunlight back into space. This process has long been thought to be the only way this particular acid, known as methanesulfonic acid, enters the marine environment, arriving only after a long journey through the sky.

A team of researchers at Friedrich Schiller University Jena has now uncovered a second, entirely different path for this acid, one that happens entirely underwater and does not require the atmosphere at all. By studying marine bacteria in the lab and in the wild, they discovered that these microbes can convert a common sulfur compound directly into methanesulfonic acid inside their own cells. This finding rewrites the map of the marine sulfur cycle, showing that the ocean produces this climate-relevant acid on its own, independent of the air above it.

The story begins with a molecule called dimethylsulfoniopropionate, or DMSP. This substance is produced in staggering quantities by phytoplankton, the microscopic plants that form the base of the ocean food web. It serves as a kind of internal shield for these plants, protecting them from salt stress and oxidation. When these plants die or are eaten, bacteria break down the DMSP. Until now, scientists believed this breakdown happened in one of three ways: the bacteria could split the molecule to release a gas, strip away a methyl group to create a different sulfur compound, or oxidize it into a substance called dimethylsulfoxonium propionate, or DMSOP. The last of these, DMSOP, was known to be a stable intermediate that could be broken down further into other compounds, but it was not thought to lead directly to methanesulfonic acid.

The researchers suspected that the story was incomplete. They began by screening sixteen different strains of marine bacteria to see which ones could turn DMSP into DMSOP. They found that several species, including a bacterium named Celeribacter baekdonensis, were very efficient at this first step. To test what happened next, they fed these bacteria a version of DMSOP that had been chemically tagged with a heavy isotope of carbon, allowing them to track the molecule's fate with extreme precision. When they analyzed the bacteria after incubation, they found something unexpected: the bacteria had converted the tagged DMSOP into a new, labeled compound.

Using high-resolution mass spectrometry, a technique that identifies molecules by their precise weight, the team determined that this new compound was methanesulfonic acid. This was a significant discovery because, until this study, methanesulfonic acid was considered exclusively a product of atmospheric chemistry, formed when the gas dimethylsulfide is oxidized by sunlight in the air. The researchers confirmed that the bacteria were not just absorbing the acid from their environment but were actively manufacturing it. They observed the acid accumulating inside the bacterial cells and being released into the surrounding water.

To understand how this transformation occurred, the team isolated the specific enzymes responsible. They identified three proteins, known as DmdA, DmdC, and DmdD, which are part of the bacteria's standard toolkit for breaking down sulfur compounds. In a series of experiments, they showed that these enzymes could work together to strip a methyl group from DMSOP and rearrange the remaining pieces. This process creates a highly unstable intermediate that quickly reacts with oxygen to form methanesulfonic acid. The team demonstrated this by mixing the purified enzymes with the necessary chemical helpers in a test tube and watching the acid form, proving that the biological machinery alone is sufficient to drive the reaction.

The significance of this discovery extends far beyond the laboratory. To see if this process happens in the real ocean, the researchers analyzed water samples collected from the Weddell Sea in Antarctica, a region known for its high biological productivity. They separated the water into two fractions: the dissolved chemicals floating freely and the contents of the tiny organisms themselves. In both fractions, they found substantial amounts of methanesulfonic acid. In some locations, the concentration of this acid inside the cells was higher than that of the original DMSP, suggesting that the conversion is rapid and efficient. The presence of the acid in the dissolved water further indicated that the bacteria were releasing it into the environment, adding a direct biological source to the ocean's chemical inventory.

This finding challenges the long-held view that all methanesulfonic acid in the ocean comes from the sky. While atmospheric deposition still plays a role, the study shows that marine bacteria are a major, independent source. This means that the sulfur cycle is more interconnected than previously thought, with a direct link between the metabolism of microscopic life and the chemical composition of the seawater. The acid produced by these bacteria can be used by other microbes as a source of carbon and energy, creating a new loop in the food web.

The implications for climate science are profound. Methanesulfonic acid is a key component in the formation of cloud condensation nuclei, the tiny particles around which cloud droplets form. Clouds reflect sunlight, cooling the planet. If a significant portion of this acid is generated directly in the ocean rather than arriving from the atmosphere, it changes how scientists calculate the balance of sulfur between the sea and the sky. It suggests that the biological activity of the ocean itself is a more active driver of cloud formation and climate regulation than models have accounted for.

By tracing the path of a single molecule from a protective shield in a phytoplankton cell to a climate-influencing acid in the water, this research connects the microscopic world of bacterial enzymes to the global scale of climate dynamics. It reveals that the ocean is not just a passive recipient of atmospheric chemicals but an active generator of them, driven by the metabolic processes of its invisible inhabitants. The work provides a clearer picture of how life in the sea shapes the chemistry of the planet, reminding us that even the smallest organisms play a critical role in the Earth's systems.