What Is Changing in the Marine Sulfur Cycle from Plankton to the Atmosphere?

The ocean is not only a major buffer for heat and carbon dioxide but also a central arena for many biogeochemical cycles. Among these, the marine biogenic sulfur cycle—centered on dimethyl sulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP)—has increasingly been recognized over the past few decades as an important bridge linking marine ecosystems with the climate system, particularly in remote ocean regions where background aerosol concentrations are low. Marine production of DMS accounts for roughly 80% of global natural biogenic sulfur emissions and represents the dominant reduced sulfur compound released from the ocean into the atmosphere. Once DMS enters the atmosphere, it is rapidly oxidized to sulfuric acid and sulfate aerosols. These aerosols can act as cloud condensation nuclei (CCN), influencing cloud droplet formation and the microphysical properties of clouds, and potentially affecting regional climate. For this reason, understanding how climate change alters the production, transformation, and emission of DMS and DMSP in the ocean has become an important scientific question.

The DMS cycle begins within marine organisms. Many marine organisms—from phytoplankton and macroalgae to corals and their symbiotic partners—are capable of synthesizing DMSP inside their cells. This compound was originally thought to function primarily as an osmolyte that helps cells regulate internal osmotic pressure. Subsequent studies, however, have shown that DMSP also plays a role in antioxidant defense, helping organisms cope with oxidative stress generated by high light intensity or elevated temperature. When phytoplankton cells are grazed upon, undergo senescence, experience viral infection, or rupture, the DMSP previously contained within the cells is released into the surrounding seawater.

Heterotrophic bacteria, such as members of the order Candidatus Pelagibacterales and the genus Roseobacter, can rapidly absorb dissolved DMSP from seawater and metabolize it through different biochemical pathways. One pathway is demethylation, in which DMSP is converted into compounds such as methanethiol. This pathway provides bacteria with reduced sulfur and carbon for growth but does not produce DMS. The alternative pathway is the cleavage pathway. In this case, to prevent the excessive accumulation of sulfur compounds within the cell, bacteria employ the enzyme DMSP lyase to split DMSP directly into DMS. Which pathway dominates strongly influences how much sulfur ultimately reaches the atmosphere as DMS. When DMSP concentrations in seawater are relatively low, bacteria tend to favor the demethylation pathway because it allows more efficient recovery of sulfur for cellular metabolism. When phytoplankton blooms produce an excess supply of DMSP, however, the cleavage pathway becomes more prominent, leading to increased production of DMS.

Even so, only a small fraction of the DMS produced in the ocean ultimately escapes to the atmosphere. Large amounts of DMS are consumed again by marine bacteria or transformed through photochemical reactions into other compounds such as dimethyl sulfoxide (DMSO). Consequently, the climatic influence of DMS does not depend solely on the total amount produced. Instead, it depends heavily on the balance between microbial community composition and their metabolic strategies. This means that even relatively small environmental changes capable of altering these microbial balances may significantly influence the flux of sulfur from the ocean to the atmosphere.

Schematic representation of the marine biogenic sulfur cycle（圖片來源：Jackson R and Gabric A. (2022)，採用 CC BY 4.0 授權）

Climate change is now disrupting this system through multiple pathways. First, ocean warming directly affects the physiology of both phytoplankton and bacteria. Higher temperatures often accelerate bacterial decomposition and remineralization of organic matter, potentially causing more DMS or DMSP to be consumed before it can escape into the atmosphere. Warming may also favor the proliferation of smaller microbial communities, including cyanobacteria. These organisms typically produce lower amounts of DMSP, which may reduce the overall supply of sulfur available for conversion within the cycle.

Global distribution of sea surface temperature from 2001 to 2020（圖片來源：Jackson R and Gabric A. (2022)，採用 CC BY 4.0 授權）

Second, climate change intensifies ocean stratification and causes the surface mixed layer to become shallower. This restricts the upward transport of nutrients from deeper waters to the surface and can reduce primary productivity. If phytoplankton productivity declines, the total production of DMSP may decrease as well, thereby limiting the potential formation of DMS. However, this pattern does not occur uniformly across all regions. In high-latitude oceans, declining sea ice may extend the growing season, potentially favoring the proliferation of certain high-DMSP-producing species. One example is the Antarctic phytoplankton species Phaeocystis antarctica, which produces large quantities of DMSP. Changes in the abundance of such species could influence how the Southern Ocean regulates regional and even global climate in the future. In contrast, conditions in low-latitude tropical waters—particularly coral reef ecosystems—are far more complex.

Global distribution of ocean mixed layer depth from 2001 to 2020（圖片來源：Jackson R and Gabric A. (2022)，採用 CC BY 4.0 授權）

Corals and their symbiotic algae often increase the synthesis and breakdown of DMSP when exposed to high temperature and strong light. This response helps counteract the accumulation of reactive oxygen species (ROS). In the short term, this stress response may increase DMS concentrations in surrounding seawater and even in the atmosphere above coral reefs. However, if temperatures exceed the tolerance limits of corals, large-scale bleaching or mortality may occur. Under such conditions, the entire sulfur source provided by the coral reef system could decline rapidly. Over the longer term, if corals cannot adapt to rising temperatures and ocean acidification through changes in their symbiotic partners or microbial communities, coral reefs may contribute less to the marine sulfur cycle.

Ocean acidification introduces another important variable. As the ocean absorbs excess atmospheric carbon dioxide, seawater chemistry shifts toward lower pH. Experimental studies have shown that acidification can influence the growth and metabolism of certain phytoplankton species such as Emiliania huxleyi. Changes in the physiology of these organisms may lead to reduced production and release of DMSP. However, the direction and magnitude of this effect depend strongly on the composition of the biological community and the duration of environmental change, meaning that further research is still needed to fully understand these dynamics.

Global distribution of mean ocean pH from 2001 to 2020（圖片來源：Jackson R and Gabric A. (2022)，採用 CC BY 4.0 授權）

Climate change is therefore reshaping the marine biogenic sulfur cycle in multiple ways. Ocean warming, increased stratification, and acidification simultaneously influence phytoplankton community structure and microbial metabolic pathways. These changes directly determine how much DMS is produced in the ocean and when and where it ultimately reaches the atmosphere. In the future climate system, the role of marine microorganisms will depend largely on how marine ecosystems reorganize themselves under persistent environmental stress, adjusting the biological sulfur cycle that links the ocean and atmosphere.

Author: Shui-Ye You

Reference:

Jackson R and Gabric A. (2022). Climate Change Impacts on the Marine Cycling of Biogenic Sulfur: A Review. Microorganisms.