Eelgrass disease dynamics : An experimental analysis of the eelgrass - Labyrinthula zosterae interaction

Infectious diseases in the marine realm may have severe ecologic and socio-economic implications. This thesis addresses disease in the foundation species Zostera marina, commonly known as eelgrass. In the 1930s ‘wasting disease’ hit trans-Atlantic Zostera marina L. (eelgrass) populations, provoking the biggest ever reported seagrass die-off. The proposed agent of this disease is the marine net-slime mold Labyrinthula zosterae. It has been suggested that wasting disease outbreaks are favored by unfavorable conditions for eelgrass. However, these hypotheses were hardly targeted by experimental investigations. In this thesis, I aimed to characterize the interaction of contemporary L. zosterae - eelgrass and to test the influence of diverse environmental factors. Therefore, I performed a series of experimental infections with naive eelgrass plants raised from seeds and L. zosterae isolates from the study area the south western Baltic and the North Sea. In three consecutive studies I investigated how disease dynamics were influenced by (1) geographic origin of host and pathogen, (2) nutrient limitation and (3) heat, shading and salinity including their interactive effects. The experiments revealed a fast and efficient infection of contemporary L. zosterae isolates, along with changes in host gene expression. However, infection levels dropped to pre-infection levels after 3 weeks even under stressful environmental conditions. This work corroborates the idea that contemporary L. zosterae isolates do not represent an immediate risk for eelgrass beds in the south-western Baltic. However, they might represent a reservoir from where more virulent forms may evolve.