MEPS 581:119-133 (2017)  -  DOI: https://doi.org/10.3354/meps12318

ABSTRACT: To assess the relationship among environmental stressors, seagrass susceptibility and Labyrinthula virulence, specimens of Thalassia testudinum were exposed to common abiotic stressors (hypersalinity, elevated temperature, nighttime hypoxia and elevated sulfide). Stressors were applied in isolation and in combination using an orthogonal design, incorporating either pulsed (Days 1-7) or sustained (Days 1-14) stress, with pathogen exposure occurring on Days 8-14. Seagrass infection responses were variable yet contingent upon environmental conditions affecting Labyrinthula viability. Following a 1 wk exposure period to any abiotic stressor (pre-infection), T. testudinum samples failed to show any significant drop in effective quantum yield. However, plant photochemistry declined significantly in response to successful infection, which was most prevalent under ambient conditions. Hypersalinity appeared to be the major factor which inhibited in vitro Labyrinthula sp. growth and in planta virulence. These data suggest that in the absence of selected abiotic stressors, Labyrinthula sp. has an enhanced capability of successful infection and can efficiently diminish host health (i.e. suppress photochemistry and lead to enhanced lesions). In summary, relatively short-term exposure to common environmental stressors has a generally negative influence on Labyrinthula sp. viability and virulence that can outweigh the effects of reduced T. testudinum photosynthetic health, essentially yielding an asymmetry favoring host defenses, but this asymmetry also reverses when stress is alleviated. When considering further the potential for complex interactions in this multi-stress system, positive antagonism may be more likely than synergistic or negatively antagonistic outcomes.

KEY WORDS: Seagrass · Stress · Disease · Labyrinthula