Thalassia testudinum response to the interactive stressors hypersalinity, sulfide and hypoxia

A large-scale mesocosm (sixteen 500 L tanks) experiment was conducted to investigate the effects of hypersalinity (45–65 psu), porewater sulfide (2–6 mM) and nighttime water column hypoxia (5–3 mg L−1) on the tropical seagrass Thalassia testudinum Banks ex König. We examined stressor effects on growth, shoot survival, tissue sulfur (S0, TS, δ34S) and leaf quantum efficiencies, as well as, porewater sulfides (∑TSpw) and mesocosm water column O2 dynamics. Sulfide was injected into intact seagrass cores of T. testudinum exposing below-ground tissues to 2, 4, and 6 mM S2−, but rapid oxidation resulted in ∑TSpw < 1.5 mM. Hypersalinity at 65 psu lowered sulfide oxidation and significantly affected plant growth rates and quantum efficiencies (Fv/Fm < 0.70). The most depleted rhizome δ34S signatures were also observed at 65 psu, suggesting increased sulfide exposure. Hypoxia did not influence ∑TSpw and plant growth, but strengthened the hypersalinity response and decreased rhizome S0, indicating less efficient oxidation of ∑TSpw. Following nighttime hypoxia treatments, ecosystem level metabolism responded to salinity treatments. When O2 levels were reduced to 5 and 4 mg L−1, daytime O2 levels recovered to approximately 6 mg L−1; however, this recovery was more limited when O2 levels were lowered to 3 mg L−1. Subsequent to O2 reductions to 3 mg O2 L−1, nighttime O2 levels rose in the 35 and 45 psu tanks, stayed the same in the 55 psu tanks, and declined in the 65 psu tanks. Thus, hypersalinity at 65 psu affects T. testudinum's oxidizing capacity and places subtle demands on the positive O2 balance at an ecosystem level. This O2 demand may influence T. testudinum die-off events, particularly after periods of high temperature and salinity. We hypothesize that the interaction between hypersalinity and sulfide toxicity in T. testudinum is their synergistic effect on the critical O2 balance of the plant.