Effects of sulfide concentration, pH, and anoxia on photosynthesis and respiration of Zostera marina

Sulfide produced in marine sediments by sulfate reduction is toxic for several macrophytes, and high sediment sulfide concentrations have been associated with seagrass die-off events. Sulfide has been shown to reduce growth in a number of seagrasses, but little is known about its effect on the photosynthetic and respiratory metabolism. Consequently, the aim of this research was to evaluate the effect of sulfide on photosynthesis and respiration of the seagrass Zostera marina. Photosynthetic oxygen evolution and respiration were determined polarographically, while the optimum quantum yield was used as a measure of the photosynthetic performance of photosystem 11 in the leaves of Z. marina. The results showed that sulfide concentrations of approximately 1000 mu M could be considered an upper threshold limit for the survival of Z. marina in the coastal lagoons of Baja California (Mexico) studied. Respiration was not inhibited by sulfide concentrations up to 1000 mu M during 48-h incubations, while photosynthetic performance was reduced by short exposure to sulfide concentrations of 25 mu M but also by long exposure to concentrations as low as 50 mu M. This is the first study that shows that the photosynthetic capacity of Z. marina is not recovered once the sulfide-free conditions have been re-established.