Sulfide effects on Thalassia testudinum carbon balance and adenylate energy charge

Abstract

Low iron content in tropical carbonate sediments limits the formation of iron–sulfide compounds such as pyrite. Thus, seagrasses in the tropics may be more susceptible to sulfide toxicity. Sediment sulfide levels greater than 2 mM and up to 13 mM have been hypothesized to cause widespread ‘die-back’ of the tropical seagrass Thalassia testudinum in a subtropical lagoon, Florida Bay. Hydroponic chambers were used to determine the effects of sulfide (0.0, 2.0, 4.0, 6.0 and 10.0 mM) on root and leaf adenylate ratios, energy charge (EC), leaf O 2 flux, and growth of T. testudinum under light-saturated conditions. T. testudinum did not suffer mortality under short-term (48 h) exposure to sulfide concentrations up to 10 mM, but several metabolic stress responses were observed. Root ATP and energy charge significantly declined ( P<0.05) as a function of increasing sulfide concentrations. Root EC was reduced from 0.78 in the control to 0.63–0.59 in the 2.0–6.0 mM treatments, and fell to 0.43 in the 10.0 mM treatment. Leaf elongation rates declined ( P<0.05) by an average of 43% in 2.0–6.0 mM sulfide and 67% in 10 mM sulfide. Although root EC, root ATP production and leaf elongation rates significantly declined under root sulfide exposure, sulfide concentrations of 2.0–10.0 mM failed to produce visual signs of acute sulfide toxicity, such as leaf chlorosis, leaf or root necrotic tissue development, or loss of leaf or root turgor. Photosynthesis and leaf EC remained high after sulfide treatments, suggesting a resilience of T. testudinum leaf metabolism to short-term sulfide exposure. Our data do not support the hypothesis that sulfide initiates rapid ‘die-off’ episodes of T. testudinum in Florida Bay, although this phytotoxin may play a critical role as a root carbon drain over long-term exposures.