This article was adapted from .
Mya Breitbart, a viral ecologist at the ˛ÝÝ®ĘÓƵ’s College of Marine Science, has teamed up with her colleague , a marine biologist at Oregon State University’s College of Earth, Ocean, and Atmospheric Sciences, to test a hypothesis about the role of viruses in iron cycling in the ocean.
Funded by the , the results of the three-year study could challenge some of our most basic understanding of ocean chemistry, with implications for global food webs and carbon cycling.
Iron, the fourth-most abundant element on Earth, is eroded from the land and deposited in the ocean via winds and rivers. It’s a good thing it is, because nearly all life on Earth needs iron to function, including marine life, from microbes to mammals.
But there’s a problem with elemental iron in the ocean: there’s very little of it. When iron comes in contact with oxygen, it turns to rust, making it insoluble in seawater. The rusted form of iron sinks through the water column and is not usable, or “bioavailable,” to any cells in the sunlit surface ocean where microbes need it the most.
Microbes have evolved a way to address this problem by producing siderophores, a biological fishing tackle of sorts. Siderophores on a cell’s surface can bind to iron in the environment, solubilizing it and then bringing it through a specialized receptor into the cell. Once iron enters microbes, it can move through the food web as predators eat prey.
Trojan Horse Hypothesis
The role of siderophores in introducing iron to the marine food web has been studied for decades. But Breitbart and Buck think there might be another pathway for iron to follow in the marine environment, an idea that occurred to them in a flash of inspiration following a research seminar given by Buck.
Kirsten Buck and Mya Breitbart will collaborate from labs located across the country. Image courtesy of Oregon State University.
“Kristen was talking about the importance of dissolved iron,” Breitbart recalls, “which she defined as the iron in seawater that passes through a particular size of filter. As I was listening I realized, wait a minute, my viruses are so small that they pass through those same size filters — meaning that in this context, viruses are dissolved, too! Could some of her dissolved iron that passes through those filters potentially be bound to my viruses?”
Although little is known about the chemical composition of marine viruses, several phages (viruses that infect bacteria specifically) in other ecosystems are known to incorporate iron into their “tail” structures, the pieces that look like landing gear when a phage sits on the surface of a cell.
What if marine viruses similarly include iron in their structures? And what if microbial siderophore receptors, evolved to capture siderophore-bound iron, were tricked into binding to the viral iron, pulling it into the cell, thereby aiding the virus’s ability to infect the cell and reproduce?
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This is what the duo refers to as their Ferrojan Horse Hypothesis – an iron-centric play on the Trojan horse. If a proportion of what was previously considered “dissolved” iron is instead bound to viruses, “Then suddenly we are realizing that viruses contain an element that is extremely rare in the marine environment and absolutely essential for all life,” Buck says. “If dissolved iron exists as biological entities, rather than chemical complexes, that represents a radical change in how we think about iron in the ocean.”
Testing the hypothesis
Buck and Breitbart will take a multi-pronged approach to answering the question of whether a significant proportion of the ocean’s iron is actually bound to viruses. They will start in the lab by culturing the commonly-studied marine bacterium, Vibrio natriegens, forcing it to incorporate a labeled form of iron that can then be tracked throughout the rest of the phases of the experiments and ultimately into the structures of new viruses. These experiments should provide evidence of whether iron is incorporated into newly produced viruses via the infection cycle.
They will also try to determine if what they find in the lab is applicable to the real world, no mean feat given the vast number — at least tens of thousands — of bacterial and viral species. To bridge the gap between the lab and the ocean, they will take water samples from the ocean and isolate viruses from the water, and then try to detect if there is iron in those viruses.
“This is trickier because we can’t use the labeled iron,” Buck points out. “And the concentrations of iron in the environment are already very low and hard to measure.”
High risk, high reward
“Tricky” describes most of this work — even the researchers, experts in their respective fields, know that these are technically challenging experiments. That’s where the Keck Foundation comes in: The Keck Foundation emphasizes high-risk, high-reward science, which is often not competitive at other funding agencies. The research partners are grateful for the $1.2 million grant from the foundation, and applaud Keck’s willingness to take a leap with them.
“No matter what, we’re going to learn a lot about the iron cycle from this work,” Breitbart says. “And I truly believe we’ll prove the hypothesis correct. I’d bet on it, and I’m so happy that Keck has decided to bet on it, too.”
The payoffs to this work could be significant, the two researchers say. First, they expect to come away with a more accurate and complete picture of iron cycling.
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“We already know that dissolved iron concentrations in the ocean are really low,” Breitbart says. “If some of that dissolved pool is tied up in viruses, does that mean the amount of bioavailable iron is even lower than we thought? Or is that iron still available to cells and does it actually play a role in infection, in which case, we will have a lot of questions about how that happens.”
Understanding iron could help us understand more about other elements in the ocean as well.
“We've never looked for any trace metals or any other elements in viruses,” Buck says. “And this work will also give us ideas about binding of other elements to viruses, like zinc, nitrogen, phosphorus.”
She adds that because iron is critical for all life, understanding iron chemistry in the marine environment is also fundamental to understanding the planet’s carbon cycle, and therefore, ultimately, understanding climate dynamics.
“Results of this study could completely upend how we think about iron chemistry,” Buck says, “And that is incredibly exciting.”
About the W. M. Keck Foundation
The W. M. Keck Foundation was established in 1954 in Los Angeles by William Myron Keck, founder of The Superior Oil Company. One of the nation’s largest philanthropic organizations, the W. M. Keck Foundation supports outstanding science, engineering and medical research. The Foundation also supports undergraduate education and maintains a program within Southern California to support arts and culture, education, health and community service projects.