Tipping the balance: the impact of eelgrass wasting disease in a changing ocean

Infectious disease has the potential to cause devastating damage to valuable marine organisms and habitats. Eelgrass wasting disease (EGWD), caused by the pathogenic protist Labyrinthula zosterae (LZ), has caused mass die-offs in Zostera marina at regional and global scales. Despite this, little is known about the host-pathogen interaction or disease drivers in the Salish Sea. To determine the regional impact of EGWD, we measured summer prevalence and severity in the San Juan Islands, Padilla Bay, Hood Canal, South Puget Sound, and Willapa Bay. We used cultures and quantitative PCR to verify results, measuring LZ load in lesioned tissue from multiple sites. EGWD was present at all 16 sites surveyed, with prevalence ranging from 80% disease prevalence. Recent data suggest water temperature increases the virulence of LZ, indicating possible climate sensitivity. At our sites, water temperatures influenced both EGWD prevalence and severity, suggesting environmental conditions and climate change could impact the eelgrass-LZ relationship and lead to increased virulence. We ran a three-week controlled experiment to examine the impact of LZ infection on eelgrass shoots over time. We exposed half the eelgrass shoots to LZ infection and sampled shoots at seven time points. All exposed shoots showed signs of infection. EGWD severity and lesion number increased through time, corresponding with a measurable decrease in leaf and root growth and increased phenols. Our results show EGWD is widespread in Washington state eelgrass beds and suggests that EGWD severity is positively correlated with water temperature. Furthermore, EGWD has a detrimental effect on eelgrass health, potentially contributing to decreased density and meadow declines. While levels of EGWD in the field are variable, we identified four sites that are experiencing high prevalence. Further research is needed to understand the conditions leading to EGWD outbreaks

Life of an opportunistic marine pathogen, Labyrinthula zosterae

Infectious disease is a critical component of healthy ecosystem function, however recent evidence indicates increased mortalities due to infectious disease in both terrestrial and marine environments. Mass mortalities affecting ecosystem engineers, such as corals, oysters, and eelgrass due to disease have lead to widespread ecosystem change. In eelgrass, the primary pathogen of concern is Labyrinthula zosterae, a fungus-like protist, a Labyrinthulmycete, which is the causative agent of eelgrass wasting disease. Eelgrass wasting disease is of global concern, detected first in Europe and the US Atlantic Coast in the 1930’s in association with mass mortalities, and has been detected in the Salish Sea. L. zosterae is considered an opportunistic pathogen, and like other opportunistic pathogens, L. zosterae are ubiquitous in the environment (waters, sediments, and hosts). By definition, opportunistic pathogens cause disease only in immune-compromised or stressed hosts, however preliminary data indicate multiple L. zosterae strains of varying virulence may exist, and a combination of stress (such as temperature, salinity, or nutrients) and strain virulence may lead to development of eelgrass wasting disease. Eelgrass wasting disease is commonly identified by the presence of black lesions on eelgrass blades (also a general sign of stress), but as an opportunist, L. zosterae can be detected in the tissues of healthy and diseased individuals. Given the challenges of diagnosing wasting disease, improved diagnosis of L. zosterae is of particular importance. In order to better diagnose L. zosterae infections in eelgrass beds in the Salish Sea, we have been using multiple diagnostic tests, L. zosterae isolations and challenges to confirm the infectious nature of L. zosterae isolated from Salish Sea eelgrass. Improved detection of wasting disease and L. zosterae strain identification is of importance to eelgrass management and restoration in the Salish Sea, and may be an area of interest for community outreach

Identification of demographic and environmental risk factors associated with eelgrass wasting disease in the Salish Sea

As we increase our studies of marine diseases and their population- and community- level impacts, the role of opportunist pathogens in causing disease is often overlooked. These pathogens are often ubiquitous in the environment, but cause disease only under certain conditions such as immunosuppression due to environmental stress. While direct management of pathogens is often unrealistic in the marine environment, management of the environmental stressors leading to increased vulnerability to disease may be possible. Eelgrass wasting disease, caused by the opportunist pathogen, Labyrinthula zosterae, is commonly associated with severe population declines of the temperate seagrass Zostera marina at times resulting in a local extinction event. While this pathogen is omnipresent, disease outbreaks only occur occasionally. Factors that lead to these outbreaks are largely unknown, however some phenolic compounds have been shown to inhibit L. zosterae in vitro. It has been hypothesized that environmental stressors causing decreased phenolic concentrations will cause more disease. Here we present survey data of a current outbreak of eelgrass wasting disease in the San Juan Archipelago and Northern Puget Sound regions of the Salish Sea. We confirm that lesions in the field are due to infection with Labyrinthula using histology and culturing techniques. Then we describe the relationship between climatic factors including water movement and temperature, host factors including demography and phenolic concentrations and disease prevalence across 11 sites. At some of these sites eelgrass standing stock is in decline. We discuss the relevance of our work to seagrass declines in the Salish Sea and regional practices designed to protect Z. marina

Latitudinal variation in seagrass wasting disease from Puget Sound to Alaska

Widely regarded as critical marine habitat, seagrass provides ecologically and economically valuable services in coastal areas worldwide. Eelgrass (Zostera marina) is one of five seagrass species native to the Pacific Northwest and is threatened by outbreaks of the marine protist Labyrinthula zosterae, the causative agent of seagrass wasting disease. Infection from L. zosterae causes necrotic lesions, limiting growth and compromising the health of eelgrass beds. To determine levels of wasting disease in subtidal eelgrass beds across a broad latitudinal gradient, we measured and compared disease prevalence and severity in 5 subtidal eelgrass beds in Puget Sound, 5 in the San Juan Islands, 4 on Calvert Island, British Columbia, and 4 in south eastern Alaska. Disease prevalence, the proportion of diseased blades to health blades, was highest in Puget Sound, and steadily decreased in sites further north. Severity, the proportion of diseased tissue to healthy tissue on infected blades, also varied between sites and geographic regions. To determine potential environmental drivers of disease across broad spatial scales, we also measured site characteristics: density, canopy height, water temperature, and salinity. Understanding broad-scale patterns of disease and environmental drivers of wasting disease outbreaks can inform key decisions regarding eelgrass conservation within and beyond the Salish Sea