Synergistic effects of high temperature and sulfide on tropical seagrass

To examine the synergism of high temperature and sulfide on two dominant tropical seagrass species, a large-scale mesocosm experiment was conducted in which sulfide accumulation rates (SAR) were increased by adding labile carbon (glucose) to intact seagrass sediment cores across a range of temperatures. During the initial 10 d of the 38 d experiment, porewater SAR in cores increased 2- to 3-fold from 44 and 136 mu mol L(-1) d(-1) at 28-29 degrees C to 80 and 308 mu mol L(-1) d(-1) at 34-35 degrees C in Halodule wrightii and Thalassia testudinum cores, respectively. Labile C additions to the sediment resulted in SAR of 443 and 601 mu mol L(-1) d(-1) at 28-29 degrees C and 758 to 1,557 mu mol L(-1) d(-1) at 34-35 degrees C in H. wrightii and T testudinton cores, respectively. Both T testudinum and H. wrightii were highly thermal tolerant, demonstrating their tropical affinities and potential to adapt to high temperatures, While plants survived the 38 d temperature treatments, there was a clear thermal threshold above 33 degrees C where T testudinum growth declined and leaf quantum efficiencies (Fv/Fm) fell below 0.7. At this threshold temperature, H. wrightii maintained shoot densities and leaf quantum efficiencies. Although H. wrigghtii showed a greater tolerance to high temperature, T testudinum had a greater capacity to sustain biomass and short shoots under thermal stress with labile C enrichment, regardless of the fact that sulfide levels in the T testudium cores were 2 times higher than in the H. wrightii cores. Tropical seagrass tolerance to elevated temperatures, predicted in the future with global warming, should be considered in the context of the sediment-plant complex which incorporates the synergism of plant physiological responses and shifts in sulfur biogeochemistry leading to increased plant exposure to sulfides, a known toxin. (c) 2006 Elsevier B.V. All rights reserved.