Sulfide intrusion in the seagrass Halodule wrightii as assessed by tissue sulfur isotopic composition

Seagrass populations are declining considerably due to anthropogenic pressures, such as nutrient loading into coastal waters. Excessive nutrient availability in these environments can induce eutrophication events that ultimately generate anoxia in marine sediments. This promotes the activity of anaerobic sulfate-reducing bacteria, which reduce sulfate (SO42-) to hydrogen sulfide (H2S) for energy production. When high levels of this bacterial activity persist, H2S accumulates in the sediment. This small molecule can be highly toxic to living cells; yet, seagrasses appear to withstand relatively high concentrations of H2S in their environments. The purpose of this study is to assess sulfide intrusion in the seagrass Halodule wrightii from various locations along the Texas Gulf Coast, and determine whether the uptake and distribution of sulfide-derived sulfur in H. wrightii vegetation differs between locations. Vegetation, sediment, and seawater samples were collected from three H. wrightii meadows within two estuaries near Corpus Christi, TX: two from the Upper Laguna Madre and one from Oso Bay. Root, rhizome, and leaf tissue from vegetation samples were separated, lyophilized, and homogenized into a fine powder. Sediment samples underwent a distillation procedure capable of liberating H2S gas, which was precipitated as Ag2S. Seawater samples were acidified and treated with BaCl2 to precipitate seawater sulfate as BaSO4. Tissue, Ag2S, and BaSO4 samples were analyzed for their sulfur stable isotopic composition by EA-IRMS. 34S values of H. wrightii samples taken from the Upper Laguna Madre, for all tissue types, were lower than those obtained for vegetation from Oso Bay, suggesting that the Upper Laguna Madre populations sourced more sulfur from sedimentary sulfide compared to the Oso Bay population. Results interpreted to date suggest a difference between H. wrightii populations from these two locations in sulfide uptake and assimilation, and allows for future research into the effects of local conditions on seagrass sulfur metabolism.