Marine Microbes and Sulfur Regulation

Scientists have sought for long to learn more about how the Earth’s oceans absorb carbon dioxide and generally exchange gases with the atmosphere so they can better understand the corresponding effects on climate. To that end, many researchers are turning their attention to the microscopic organisms that help recycle carbon, nitrogen, sulfur and other elements through the oceans. Finding out exactly how and to what degree they do that is an ongoing scientific challenge, and scientists may first have to learn more about how the microbes interact with their environment at the scale of the individual microbe. In recent work, an international team of scientists led by Professor Roman Stocker of the MIT Department of Civil and Environmental Engineering opened a window into that microbial world. The team studied how certain strains of marine microbes find and use sulfur, an element vital to many of this type of microbe. Some microbes ingest the sulfur, convert it and pass it back into the ocean in altered form, keeping the chemical moving through the Earth’s sulfur cycle.

Using video microscopy, the scientists have captured digital images of the single-celled microbes swimming toward two forms of sulfur: dimethylsulfide (DMS), the chemical responsible for the slightly sulfuric smell of the sea, and its precursor dimethylsulfoniopropionate (DMSP), which can be converted to DMS by the microbes.

DMS is produced by from dying phytoplankton cells in the shallow levels of the ocean, and is the major natural source of sulfur gas emitted from the sea, where it is responsible for the distinctive smell of the sea along coastlines. DMS still only has a residence time of about one day in the atmosphere and a majority of it is redeposited in the oceans rather than making it to land.

DMS is known to influence climate and is believed to be the largest natural source of sulfur to the atmosphere; when it moves from the ocean to the atmosphere as a gas, it oxidizes, forming cloud condensation nuclei which promote cloud formation over the ocean. These clouds reflect sunlight rather than allowing it to heat the Earth’s surface. This then has its effects on climate.

DMS is oxidized in the marine atmosphere to various sulfur-containing compounds, such as sulfur dioxide and sulfuric acid. Among these compounds, sulfuric acid has the potential to create new aerosols which act as cloud condensation nuclei. Through this interaction with cloud formation, the massive production of atmospheric DMS over the oceans may have a significant impact on the Earth's climate. The CLAW hypothesis (a proposed feedback loop between the ocean ecosystems and climate) suggests that DMS may play a role in maintaining a planetary equilibrium.

Stocker, Justin Seymour, a former postdoctoral fellow at MIT who is now a research fellow at the University of Technology Sydney, Professor Rafel Simó of the Institute for Ocean Sciences in Barcelona, and MIT graduate student Tanvir Ahmed reported this research.

"It had been previously demonstrated that DMSP and DMS draw coral reef fish, sea birds, sea urchins, penguins and seals, suggesting that these chemicals play a prominent ecological role in the ocean. Now we know that they also attract microbes,” said Stocker. "But this is not simply adding a few more organisms to that list. The billions of microbes in each liter of seawater play a more important role in the ocean’s chemical cycles than any of the larger organisms."

Stocker has pioneered the use of microfluidic technology to study the behavior of marine microbes in the laboratory. He re-creates a microcosm of the ocean environment using a device about the size of a flash drive, made of clear rubbery material engraved with minuscule channels into which he injects ocean water, microbes and food in the form of dissolved organic matter. Then, using a camera attached to a microscope, he records the microbes’ response.

In the latest research, the scientists injected different chemicals into the channels of the device in a way that mimicked the bursting of a microbial cell. Although they performed the tests using several substances, including DMS, the scientists focused primarily on DMSP, which is produced by some phytoplankton and released into the water when a cell explodes (dies). That DMSP can dissolve in the water or be transformed by other microbes into DMS, which also dissolves in the water before being released as a gas into the atmosphere.

The research indicates that the chemical’s odor does draw microbial predators, much as its smelly cousin DMS does at larger scales. This is the first such study to make a visual record of microbial behavior in the presence of DMSP.

The team selected seven microbial species that are roughly analogous to plants, herbivores and predators in the animal kingdom: three photosynthetic types (phytoplankton), two types of bacteria that feed off the carbon produced by other microbes, and two microzooplankton that prey on other microbes.

Six of the seven microbial species tested were attracted to the DMSP in the microfluidic device; only one species — a phytoplankton — ignored it. Some of the species displayed the strongest swimming responses among any of the 100 or so cases yet tested by Stocker and Seymour in their research projects. This, Stocker said, is a clear indication that DMSP acts as a powerful chemical cue for a microbe and attracts the microbe to the food source.

The researchers also found that some marine microbes, including bacteria, are attracted to DMSP because they feed on it, while others, the microzooplankton, are drawn to the chemical because it signals the presence of prey. This challenges previous theories that DMSP might deter predators. "Our observations clearly show that, for some plankton, DMSP acts as an attractant towards prey rather than a deterrent," said Simó.

In other words what is being observed is a microscopic ecological system triggered by DMSP.

For further information: http://web.mit.edu/newsoffice/2010/chemical-cues-1102.html