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

Over the last several decades, sulfides have been identified as a potent ph ytotoxin. However, little is known about the effects of sulfide on Thalassi a testudinum, a dominant tropical seagrass, or other seagrass species. It h as been hypothesized that high sulfide exposure is a major contributor to T . testudinum "die-back" in the large subtropical lagoon of South Florida, F lorida Bay. Three experiments were conducted to pursue the mechanism by whi ch T. testudinum was resilient to sulfide exposure in our previous experime nt and to investigate the levels of sulfide that cause T. testudinum mortal ity. Two low-light (similar to 150 mu mol PAR m(-2) s(-1)) experiments were conducted to evaluate the role of light and photosynthesis on sulfide toxi city. Secondly, we tested the effects of high salinity (HS) and high temper ature (HT) on sulfide tolerance to determine if these interactions could sy nergistically create a "die-back" response. Leaf elongation rates were not significantly affected by below-ground sulfide exposure in the range of 1-1 0 mM when incubated at subsaturating light and ambient sediment pH (7.0). L eaf 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 mo rtality was seen in the HS or HT treatments without sulfides added. The fir st two experiments in this study and our previous experiment clearly sugges t that T. testudinum may be tolerant to short-term ( < 28 days) below-groun d tissue exposure to high sulfide concentrations. An important caveat appea rs to be, however, when sulfide exposure is combined with other stressors c ommon in Florida Bay and other tropical/subtropical lagoons and estuaries. These results point to the importance of examining multiple interactive str essors when elucidating the factors causing "die-back" in seagrasses. (C) 2 001 Elsevier Science B.V. All rights reserved.