Sulfide as a phytotoxin to the tropical seagrass Thalassia testudinum: interactions with light, salinity and temperature

Abstract

Over the last several decades, sulfides have been identified as a potent phytotoxin. However, little is known about the effects of sulfide on Thalassia testudinum, a dominant tropical seagrass, or other seagrass species. It has been hypothesized that high sulfide exposure is a major contributor to T. testudinum “die-back” in the large subtropical lagoon of South Florida, Florida Bay. Three experiments were conducted to pursue the mechanism by which T. testudinum was resilient to sulfide exposure in our previous experiment and to investigate the levels of sulfide that cause T. testudinum mortality. Two low-light (∼150 μmol PAR m −2 s −1) experiments were conducted to evaluate the role of light and photosynthesis on sulfide toxicity. Secondly, we tested the effects of high salinity (HS) and high temperature (HT) on sulfide tolerance to determine if these interactions could synergistically create a “die-back” response. Leaf elongation rates were not significantly affected by below-ground sulfide exposure in the range of 1–10 mM when incubated at subsaturating light and ambient sediment pH (7.0). Leaf O 2 production rates were also unaffected by sulfide exposure. In fact, all plants post-treatment possessed rhizome-extractable O 2 concentrations greater than 30%. When sulfide treatments (6 mM) were combined with HS and HT treatments, however, we observed our first “die-back” response. Shoots exposed to 6.0 mM sulfide under HS and HT had 50% and 33% mortality rates, and those in the HT+HS treatment had 100% mortality. Interestingly, no mortality was seen in the HS or HT treatments without sulfides added. The first two experiments in this study and our previous experiment clearly suggest that T. testudinum may be tolerant to short-term (<28 days) below-ground tissue exposure to high sulfide concentrations. An important caveat appears to be, however, when sulfide exposure is combined with other stressors common in Florida Bay and other tropical/subtropical lagoons and estuaries. These results point to the importance of examining multiple interactive stressors when elucidating the factors causing “die-back” in seagrasses.