Ecosystem Interactions Shape Spread of Wasting Disease

The expansive, dense eelgrass beds that line much of the U.S. coastline provide invaluable habitat and other services to support rich coastal ecosystems.

However, globally, eelgrass is struggling, thanks in part to seagrass wasting disease, an illness caused by the fungus-like protist Labyrinthula zosterae, which creates telltale black lesions on leaves that inhibit photosynthesis, hinder growth, and can prove fatal in some cases.

Bigelow Laboratory for Ocean Sciences researcher Maya Groner is co-leading a multi-institutional project to understand the spread of this disease and how it’s connected to changing environmental conditions. The project, which is helping train the next generation of disease ecologists, will help scientists understand and model the feedback loops between infectious diseases and the ecosystems in which they spread.

“You take a step in an eelgrass meadow, and life explodes in all directions,” Groner said. “You can see how critical this species is, but I rarely go into the field now without observing some evidence of wasting disease. We need to understand how that impacts biodiversity.”

Eelgrass is the most widespread species of seagrass in the Northern Hemisphere. Eelgrass meadows stabilize sediments, improve water quality, mitigate sea level rise, and provide protected habitat for a range of important fish and shellfish species.

Yet, past wasting disease outbreaks have had devastating impacts. Most meadows today show signs of at least mild chronic infection, and scientists don’t yet have tools to look at a mild outbreak and know for sure whether it will become lethal.

Scientists haven’t found a direct link between the disease and mortality. Instead, the problem seems to be that the disease is energetically taxing, eating into the sugar supplies the plant stores in its underground stem, which Groner likens to a battery.

“That battery helps them through the winter so how full that battery is — how much sugar the plant has stored — is a measure of how well it’s doing,” Groner said. “Our early data suggests it can take a few years, or a few outbreaks, to fully deplete that battery, which is when you see meadows disappear.”

The risk is particularly high when plants are already trying to cope with other environmental stressors, such as warming seawater. In this way, wasting disease is like many infectious marine diseases that may not be immediately fatal but are associated with significant population declines of foundational species in a changing ocean. Yet, also like those other diseases, it’s not routinely monitored.

That’s where Groner and her collaborators have stepped in.

The team, which includes researchers from University of California Bodega Marine Lab, Old Dominion University, Cornell University, and University of Washington, have been studying Washington’s eelgrass meadows since 2012. With that near-decade of baseline data and experience, they received significant funding from the National Science Foundation in 2021.

They’re genetically sequencing different strains of the pathogen to understand why some are more infectious than others. They’re also running experiments to monitor the disease in the wild, including “mark recapture” studies where they monitor the same individual shoots of eelgrass every two weeks to quantify mortality rates and how fast infections progress.

Led by Groner, and featuring Bigelow Laboratory Research Scientist Reyn Yoshioka, the team just published a paper this fall in Biology Letters that provides the first comprehensive review of what scientists know so far about seagrass wasting.

Between the review and their results to date, the team has uncovered just how important the broader ecological community appears to be. Oysters, for example, seem to filter out the pathogen from the water; certain bacteria associated with the eelgrass leaf, meanwhile, may facilitate infection. Better understanding these community interactions is essential for quantifying the impacts of the disease and developing management strategies to combat it.

“There are just so many feedbacks between the environment and the disease,” Groner said. “We can see all these different levers mediating the spread of this disease from other organisms, in both good and bad ways. It shows just how important it is to look at the whole community.”

The ultimate goal is to create models of disease spread and associated mortality for a range of environmental conditions and pathogen strains. So far, they’ve modeled how lesions grow and spread along individual leaves, which they hope to expand into models of multiple leaves and, eventually, an entire meadow. Those tools will help reveal how the disease is influenced by warming water, which has accelerated the spread of many pathogens and may leave meadows more vulnerable to other disturbances, like storms and heatwaves.

The team has invested heavily in education as well, including high school outreach programs in Washington. Groner and collaborators, including Yoshioka, have also taught intensive graduate-level summer workshops at Friday Harbor Laboratories on infectious diseases that use seagrass wasting as a model system.

“The disease is relatively easy to study in terms of getting out into the intertidal ecosystem and growing the pathogen in the lab, which makes it perfect for teaching,” Groner said. “It’s a great learning experience for students to see a pattern clearly in nature and then be able to directly test hypotheses about it in the lab.”

Between a slew of research papers and these outreach efforts, the project is helping elevate marine disease ecology, a field Groner admits has lagged behind the study of land-based diseases. Insight from this project will inform the study of diseases in other foundational species like corals and kelps. Between rising temperatures and growing investment in restoration, understanding how these systems respond to disease is more important than ever.

“If you can see all the levers that are influencing the spread of the disease, and how those levers are changing based on other drivers like temperature, we can start to think about what parts of the system we can tweak to maximize restoration or conservation efforts,” Groner said.

Photo 1: Intern Shayla Ferreiro (left) and Maya Groner tag and inspect eelgrass shoots in Padilla Bay, WA during low tide (Credit: Lindsay Alma).

Photo 2: A black lesion on eelgrass is indicative of infection with Labyrinthula zosterae (Credit: Olivia Graham).

Photo 3: Graduate students in the summer marine disease workshop inspect cultures of Labyrinthula zosterae (Credit: Maya Groner).

Video: Chloroplasts in eelgrass die as a wasting disease infection spreads (Credit: Lindsay Alma and John Burns).