Role of salinity in the susceptibility of eelgrass Zostera marina to the wasting disease pathogen Labyrinthula zosterae

Eelgrass wasting disease, caused by the net-forming protist Labyrinthula zosterae, is suspected as the causative agent in the North Atlantic population declines of the temperate seagrass Zostera marina in the 1930s, as well as in more recent localized die-offs. During these declines, populations of eelgrass in low salinity areas were less likely to decline and quicker to recover. The goal of this study was to understand the relationships between eelgrass wasting disease and salinity, and how phenolic acids, putative plant defense compounds, varied with salinity and infection. We studied these relationships through mesocosm experiments in which eelgrass shoots were inoculated with L. zosterae under different salinity treatments (5, 10, 15, 25 and 40). We used lesion area as a measure of the degree of infection. Phenolic acids were separated and quantified with reversed phase HPLC. Three phenolic acids, rosmarinic, gallic and ferulic, were identified in leaf and rhizome tissue. Lesion area decreased with a decrease in salinity, and the concentration of phenolic acids increased with lesion area in inoculated leaves. Concentrations of phenolic acids were not good predictors of resistance to L. zosterae infection. Our study supports the hypothesis that observed decreases in eelgrass wasting disease in low salinity areas are due to the inhibition of L. zosterae at lower salinities.