The Origins of Slime

As a child growing up near the beach in Trieste, Italy, Francesca Malfatti knew all too well about mucilage, or mucillagine in Italian, a sticky whitish green slime that rises to the sea surface in translucent clouds and washes ashore on northern Adriatic Sea beaches.

Malfatti remembers that kids jumping off rocks into the water would emerge coated in the naturally forming mucus, which, though not harmful, is by all accounts quite disgusting. The biggest threat of mucilage may be to the region’s tourist industry, since northern Adriatic beaches are very popular among German and other European vacationers.

Severe mucilage events in 1989, 1997, and 2000 convinced some people that pollution was the cause. But scientists have been uncertain what’s behind the phenomenon that occurs mainly in the northern Adriatic. Now, the Scripps Institution of Oceanography at UC San Diego researcher who grew up swimming in the slime-tainted waters thinks she has developed a better understanding… or at least the beginning of the answer.

OF SLIME AND SEAWATER

Malfatti, who is working on her Ph.D., is using an atomic force microscope to examine the interactions between single-cell bacteria and even smaller particles known as marine microgels. Marine microgels were discovered only as recently as the 1990s. Scientists now believe that 10 percent of all dissolved organic matter in marine waters could be in the form of microgels.

Marine bacteria play an important role in the oceanic carbon cycle, interacting with all organic matter present in seawater. Gels are the smallest particles in the ocean and very abundant at 100 million parts per milliliter. This produces considerable interaction with the 1 million bacteria that also exist in the same milliliter of seawater. That interaction might be different than normal in the northern Adriatic where mucilage forms, according to Malfatti.

“Human impact may be part of the equation,” Malfatti said. “But the mucus formation has to be studied across the scale, from the micrometer where bacteria interact with gels to the mesoscale where water circulation, seawater temperature, and wind impact the carbon cycle in the northern Adriatic basin.”

Recurring mucilage phenomena are nothing new to the northern Adriatic, where the slime was first recorded in 1729. But major events in recent decades threatened to drive away tourists from Adriatic beaches. Mucilage also harms the fishing industry because it befouls nets.

Malfatti’s advisor, Farooq Azam, a distinguished professor in the Marine Biology Research Division at Scripps Oceanography, emphasizes that there is nothing terribly unusual about microgels in the sea. All areas of the sea have them. All organisms produce mucus as part of their survival strategies, such as for protection from microbial attack or to capture food. Some organisms such as algae and bacteria release mucus into the environment in large amounts, and many microbes also degrade and use it for their nutrition. So, there is a dynamic pool of mucus and microgels everywhere in the ocean.

Mucilage events are not the same as microalgae blooms, which are caused by excessive nutrients in seawater, usually from agricultural runoff. Instead, Malfatti believes that precursors of mucilage exist for almost the entire year in the northern Adriatic. On some occasions during warmer months, gels start to aggregate in massive amounts and rise to the surface. According to Malfatti’s research, bacteria might dictate the rate of microgels

coalescence in mid waters, forming the massive mucus clouds that rise and linger on the surface until they are broken up by wind and storms, and then decay and sink. Malfatti hypothesizes that in the northern Adriatic, bacteria enzymatically modify the surface of the microgels, changing their degree of stickiness. This would lead to the formation of bigger aggregates of gels, transforming into mucilage.

“It is critical to understand the role of the bacteria in shaping the organic matter in the sea at the microscale level to better predict mucilage events,” she said.

USING THE (ATOMIC) FORCE

Working under a grant from the U.S. National Science Foundation, Malfatti and Azam collaborate with scientists from Italy, Slovenia and Croatia. Major mucilage events impact the coastlines of all three countries. Malfatti, a graduate of University of Trieste in microbiology, also was a researcher at the Marine Biology Laboratory, today part of the Italian National Institute of Oceanography and Experimental Geophysics in Trieste.

Malfatti is studying samples of seawater taken from the coastlines of Italy, Slovenia, and Croatia to determine under controlled conditions in the laboratory how bacteria andmicrogels interact. She is the first to use atomic force microscopy, a very high-resolution type of scanning-probe microscope with resolution to a nanometer, to study the interactions between bacteria and microgels. Single-cell bacteria can be 600-1,000 nanometers while microgels, the smallest particles in the ocean, can be as small as 10 nanometers.

Discovering the cause of mucilage events in the Adriatic does not mean a solution will follow soon.

“Like the greenhouse effect, we need to understand how something works first before we can know if anything can be done about it,” Malfatti said. “But mucilage has been in these waters for a long time. It could be an important part of the ecosystem. It may be an over-accumulation of organic matter in the marine carbon cycle of the Adriatic Sea that is variable in time and space, and we just need to better understand it.”

TINY WORLDS PLAY A GLOBAL ROLE

For Farooq Azam, the mucilage project in the northern Adriatic Sea may speak volumes about the oceans despite its very specific geographic focus.

And the atomic force microscope will be a crucial tool that helps the scientists acquire their global view. Obtained with help from the Gordon and Betty Moore Foundation, it allows the researchers to zoom in and “feel” the architecture of molecular structures. Another instrument, called a laser confocal microscope, uses lasers to produce three-dimensional pictures of the microscopic world in which microbes live and interact.

“These new instruments are really bringing us closer to studying the ecology of microbes as you would study the ecology of larger organisms—animals, plants with which we are more familiar,” said Azam. “We are now beginning to acquire insights on these organisms where the environment itself is also viewed. So what we really want to do is see the architecture of the microscopic world in which microbes live.”

Such novel instruments are becoming more vital as scientists push to gain a deeper understanding of life and processes at the microscopic scale, according to Azam, especially as societies pay more attention to the role of marine life in the carbon cycle and how organisms respond to global warming.

Indeed, Azam said, microbial ecologists are increasingly focusing on whether the structure of the ocean at its most minuscule scales might be disrupted by climate change, and whether it could be modified by the ocean’s rising acidification due to increases in carbon dioxide.

To answer one regionally specific question, the northern Adriatic research will give the team a means for investigating why the gels are not degraded as in other areas. It’s possible that the Po River and its influx of nitrogen and phosphorous into the northern Adriatic may be behind the mucus blooms. Why isn’t the matter within the mucus decomposing as quickly as it might elsewhere? Malfatti and Azam think that peculiar environmental conditions in the northern Adriatic Sea cause the production and accumulation of slow-to-degrade dissolved organic matter. It may be left over from persistent blooms of algae supported by the nutrient load from the Po River. Time and research will tell.

But Azam sees the project providing much more than that. He views the accumulation and persistence of gels in the northern Adriatic as a model, and the northern Adriatic as a living laboratory to investigate questions of disturbance of the ocean’s carbon cycle due to human activities on a much larger scale.

“The northern Adriatic Sea gives us the opportunity to address questions about how the carbon cycle in the ocean may, under certain conditions, be disrupted to create conditions similar to the northern Adriatic,” said Azam. “The gels are huge accumulations of organic matter that is carbon rich. So with enormous amounts of carbon being accumulated the question becomes: Could it be that other areas of the ocean—due to human activity that we are now exerting—may actually become like the northern Adriatic Sea, because of disruption of the carbon cycle?”

Answers to these and other questions, he believes, will come—one microscopic step at a time.

— By Jim Gogek, Mario C. Aguilera contributed to this story.