Effects of short-term hypersalinity exposure on the susceptibility to wasting disease in the subtropical seagrass Thalassia testudinum

Seagrass meadows are a vital component of coastal ecosystems and have experienced declines in abundance due to a series of environmental stressors including elevated salinity and incidence of disease. This study evaluated the impacts of short-term hypersalinity stress on the early stages of infection in Thalassia testudinum Banks ex König by assessing changes in cellular physiology and metabolism. Seagrass short shoots were exposed to ambient (30 psu) and elevated (45 psu) salinities for 7 days and subsequently infected for one week by the causative pathogen of wasting disease, Labyrinthula sp. The occurrence of wasting disease was significantly lower in the hypersalinity treatments. Additionally, while exposure to elevated salinity caused a reduction in chlorophyll a and b content, T. testudinum’s health, in terms of photochemical efficiency, was not significantly compromised by hypersalinity or infection. In contrast, plant respiratory demand was significantly enhanced as a function of infection. Elevated salinity caused T. testudinum to significantly increase its in vivo H2O2 concentrations to levels that exceeded those which inhibited Labyrinthula growth in a liquid in vitro assay. The results suggest that while short-term exposure to hypersalinity alters selected cellular processes this does not necessarily lead to an immediate increase in wasting disease susceptibility.