Ocean microbes’ big role in climate processes

A new study shows that “hotspots” of nutrients surrounding phytoplankton, which are tiny marine algae producing approximately half of the oxygen we breathe everyday, play an outsized role in the release of a gas involved in cloud formation and climate regulation.

Quantifying the key chemical DMSP

The new research quantifies the way specific marine bacteria process a key chemical called dimethylsulfoniopropionate (DMSP), which is produced in enormous amount by phytoplankton. This chemical plays a pivotal role in the way sulfur and carbon get consummed by microorganisms in the ocean and released into the atmosphere.

The work, published in the journal Nature Communications, is the result of a collaboration with MIT and UTS Climate Change Cluster researchers Dr. Jean-Baptiste Raina and Professor Justin Seymour from the Ocean Microbes and Healthy Oceans Research Group.

DMSP is a major nutrient for marine bacteria, it provides a large fraction of their carbon and sulfur requirements” - Dr Raina 

More than a billion tons of DMSP is produced annually by microorganisms in the oceans, accounting for 10 percent of the carbon that gets taken up by phytoplankton – a major “sink” for carbon dioxide, without which the greenhouse gas would be building up even faster in the atmosphere. But exactly how this compound gets processed and how its different chemical pathways figure into global carbon and sulfur cycles had not been well-understood until now.

“DMSP is a major nutrient for marine bacteria, it provides a large fraction of their carbon and sulfur requirements”, Dr Raina says. “So given the ubiquity and the abundance of DMSP, we expected that these microbial processes would have a significant role in the global sulfur and carbon cycles.”

Fluorescing marine bacteria and cloud formation

Lead author, MIT graduate Cherry Gao and her co-workers genetically modified a marine bacterium called Ruegeria pomeroyi, causing it to fluoresce when one of two different pathways for processing DMSP was activated, allowing the relative expression of the processes to be analyzed under a variety of conditions.

One of the two pathways is called demethylation, and produces carbon and sulfur based nutrients that can be used to sustain bacterial growth. The other pathway, called cleavage, produces a gas called dimethylsulfide (DMS), which Raina explains “is the compound that is responsible for the smell of the sea.”

Oceanic DMS emissions create the largest flux of biologically derived sulfur that enters the atmosphere. Once in the atmosphere, these sulfur molecules are cloud precursors, enabling the condensation of water molecules, so their concentration in the air affects both rainfall patterns and the overall reflectivity of the atmosphere through cloud formation.

Oceanic DMS emissions create the largest flux of biologically derived sulfur that enters the atmosphere. Once in the atmosphere, these sulfur molecules are cloud precursors, enabling the condensation of water molecules, so their concentration in the air affects both rainfall patterns and the overall reflectivity of the atmosphere through cloud formation. Understanding the process responsible for much of that production could be important in multiple ways for refining climate models.

Those climate implications are “the reason why we were interested in knowing when bacteria decide to use the cleavage pathway versus the demethylation pathway,” in order to better understand how much of the important DMS gets produced under what conditions, Gao says. “This has been an open question for at least two decades.”

The new study found that concentration of DMSP in their vicinity regulates which pathway the bacteria use. Below a certain concentration, demethylation was dominant, but above a level of about 10 micromoles, the cleavage process dominated.

“What was really surprising to us was, upon experimentation with the engineered bacteria, we found that the concentration of DMSP that leads to upregulation of the cleavage pathway is higher than expected — orders of magnitude higher than the average concentration in the ocean,” Gao says.

Microscale “hotspots”

That suggests that this process hardly takes place under typical ocean conditions, the researchers concluded. Rather, microscale “hotspots” of elevated DMSP concentration are probably responsible for a disproportionate amount of global DMS production. These microscale “hotspots” are areas surrounding certain phytoplankton cells where extremely high amounts of DMSP are present at about a thousand times greater than average oceanic concentration.

“We actually did a co-incubation experiment between the engineered bacteria and a DMSP producing phytoplankton,” Raina says. The experiment showed “that indeed, bacteria increased their expression of DMS producing genes, closer to the phytoplankton.”

This new work will help researchers understand how these microscopic marine organisms, through their collective behavior, are affecting global-scale biogeochemical and climatic processes, the researchers say.

Funding

The work was supported by the Gordon and Betty Moore Foundation, the Simons Foundation, the National Science Foundation, and the Australian Research Council.