Do Pathogens Colonize Microscopic ‘Islands?’
Oceanographers Believe Aggregate Specks in the Water May Provide Refuge to Disease-Causing Microorganisms
By Jim Raper
Imagine a batch of E. coli bacteria that have flowed into the Chesapeake Bay because of a sewage leak. On their own in the open waters—like shipwrecked sailors—the bacteria’s future is, at best, uncertain. But what if up ahead they detect a refuge—like the shipwrecked sailors spotting an island?
We are beginning to see the outline of a new, $2.3 million research project anchored by two oceanographers at Old Dominion University.
The funding agency is the National Science Foundation (NSF) and the ODU investigators are Fred Dobbs, professor, and Maille Lyons, postdoctoral researcher, in the Department of Ocean, Earth and Atmospheric Sciences. The title of the four-year project: “Microscopic Islands: Modeling the Theory of Island Biogeography for Aquatic Pathogens Colonizing Marine Aggregates.”
Marine aggregates, as well as their freshwater counterparts, are specks made up of even smaller bits of detritus and other components—some being living organisms—that usually aren’t visible to the naked eye. When these bitsy components come in contact with each other, they sometimes clump together, perhaps if one of them is a sticky grain of pollen. Scientists call the clumps organic aggregates, or simply snow, perhaps because aggregates resemble the flecks that swirl about in one of those souvenir snow globes.
So our E. coli cells latch onto an aggregate—like exhausted sailors pulling themselves up onto an island beach—and immediately their future looks brighter.
Island Visits Suit Bacteria
In the case of bacteria in general, previous research has shown them to be much more likely to prosper if they can find refuge on an aggregate. Research by Dobbs and Lyons will try to determine if this also applies to waterborne pathogens. Early results suggest that in addition to the more favorable resources that aggregates can offer to bacteria, the aggregates floating near the water’s surface may provide bacteria with protection from potentially harmful ultraviolet rays.
The ODU researchers speculate that concepts of the theory of island biogeography apply to the microbial community of aggregates and may help explain the persistence of aquatic pathogens in nature. In fact, a novel application of this theory of island biogeography makes the project of Dobbs and Lyons unique.
As long as there have been scientists, most likely there has been curiosity about how insects and animals come to settle on islands, and how the various colonies of critters co-exist in a defined space. A specific interest of science has been how the introduction of a new species can change the ecological balance on an island. In some instances, such as when a volcanic eruption kills all life on an island, researchers have been able to track how the terrain is repopulated from scratch.
Studies utilizing the island biogeography theory have not been limited to the prototypical islands of soil or rock that are surrounded by water. Discrete ecosystems can exist on any patch of hospitable living area sounded by inhospitable stretches, such as oases in a desert.
But Dobbs and Lyons say that almost all island biogeography research has focused on bugs, birds and mammals. “We believe this is the first project to be set up for the microworld, but with the findings being applied to the macroworld of disease ecology,” Lyons explains.
Do Persistent Pathogens Choose Aggregates?
In essence, the theory predicts a dynamic equilibrium between colonization of new species and extinction of resident species based on island size and distance from the source of potential colonizers. “Through laboratory experiments we hope to test that what has been shown for arthropods, lizards, birds and so forth on islands also applies to bacteria and other organisms on aggregates,” Dobbs adds.
The researchers’ central hypothesis is this: The way aggregate “islands” affect the promulgation, persistence and spread of pathogens in our waters could have a significant impact on the health of macro-organisms—especially humans.
They describe aggregates as compound specks often high in organic content that form in the water column, and they note that their project does not consider suspended sediment particles, such as sand and clay, that have relatively low organic content.
Dobbs couldn’t resist crediting Lyons with bringing the “germ” of the idea for the project with her when she came to ODU in 2008 as a postdoctoral researcher. Her doctoral thesis research at the University of Connecticut was based on clam pathogens. While doing this work she generated some interesting data that seem to implicate marine aggregates in the deaths of clams. Based on that, she thought bacterial pathogens that make people sick might also be found in marine aggregates.
Lyons’ previous research showed that marine aggregates definitely are one environmental reservoir for the hard-clam pathogen referred to as QPX (Quahog Parasite Unknown.)
Work Featured on NSF Web Site
Individual microscopic bacteria are so tiny they usually flow directly through clams when the bivalves are filtering water to trap bits of food. Because marine aggregates are larger, however, they can be trapped by the clams. And although filtered bits that don’t qualify as food can be spit out, researchers believe that bacteria living on an aggregate would have more of an opportunity than their free-roaming kin to settle inside the host clam.
At the NSF News Web site in October 2009, Dobbs and Lyons were featured in an article about recently funded ecology-of-infectious disease projects. “The concepts of island biogeography may apply to bacterial pathogens in aquatic environments, the scientists believe, especially to those disease-causing organisms that flow from point sources of pollution and are subsequently incorporated into aggregates of marine snow,” the article stated. “These ‘microscopic islands’ may carry pathogens as they’re transported in currents. Among the team’s goals is improving decisions made by water-quality managers about opening or closing beaches and shellfish beds.”
Dobbs points out that the research could bring changes in water-sampling protocols. “Currently, when public health microbiologists test water, say to determine whether to order a beach closure, they do it irrespective of aggregates or particles and just measure bacteria in a unit of water.”
Collaborators on the research include J. Evan Ward, a professor in the Department of Marine Sciences at the University of Connecticut and a mentor to Lyons. Others are Randall Hicks, professor of biology and director of the Center for Freshwater Research and Policy at the University of Minnesota, Duluth, and John Drake, an assistant professor at the Odom School of Ecology at the University of Georgia.
At ODU, which will receive about $850,000 of the NSF funding, Dobbs and Lyons have already begun experiments in which aggregates are created and then studied to determine how and under what conditions the specks come to be homes for microscopic organisms.
A thin cylinder that had been filled with relatively clear water from a local saltwater creek is speckled with dozens of aggregates after it has been tumbled for as little as a few hours to one day. Individual aggregates are then extracted, inspected for size, shape and material content, and isolated in tiny incubation chambers to determine how likely they are to be colonized by organisms. First-phase experiments also will measure how the size of aggregates and their distance from a microbial source point—say a source of pollution—can determine the richness of species on the aggregates. Other experiments will test the researchers’ prediction that the number of species on any given aggregate will be influenced by the arrival of new colonizers, as well as the loss of species due to factors such as competition with new colonizers.
Mathematical Model is the Goal
Eventually, the researchers will develop a mathematical model of island biogeography applied to individual aggregates that predicts bacterial species richness and the correlation of aggregate species richness to pathogen persistence. Experiments will then be conducted to determine how well the model captures the observed persistence of pathogens in the rolling cylinders.
Later experiments will test predictions of the models, and included in them will be studies of the influence of aggregates on the uptake of living bacteria by bivalves. Dobbs says the potential is great for this project to spur other ecology-of-infectious disease research. “Understanding the distribution of pathogens within aggregates may lead to novel insights into disease-transmission dynamics.”
The project has another aspect that is more child-friendly. As part of their outreach activities, the researchers will work closely with two science centers for schoolchildren, Port Discover in Elizabeth City, N.C., and Project Oceanography on the Avery Point campus of the University of Connecticut. Between them, the centers attract 30,000 visitors a year for science-related exhibits and demonstrations.
Lyons, who lives near Elizabeth City, serves as science adviser for Port Discover, and will be responsible for educational programs based on the NSF project that will be offered at that facility during the course of the grant. Titles of the programs that are planned are: “Healthy Ecosystems and Healthy People”; “Water, Water Everywhere, When Is It Safe to Drink?”; and “Let It Snow, Let It Snow, Let It Snow: Does It Snow Inside the Ocean?”
“Dr. Lyons’ relationship with Port Discover was central to our proposal,” Dobbs says. “All told with these outreach programs, we can assure NSF that 100,000 young students will be exposed to science during the life of this grant.”