Sensitivity of Ruppia maritima and Zostera marina to sulfide exposure around roots

Ruppia maritima and Zostera marina often inhabit the same coastal areas and are often exposed to fairly high sulfide concentrations, with the former being most resilient. Our aim was to investigate differences in sulfide tolerance between the two species. Sensitivity towards sulfide in R. maritima and Z. marina was investigated in split chambers that allowed exposure of roots to a range of sulfide concentrations, while the leaves were kept under optimal light and oxygen conditions. Roots of R. maritima show a higher threshold towards sulfide (1.8-5.9mM H ₂S) than those of Z. marina (0-0.5mM H ₂S) after two weeks of incubation. The difference in sulfide sensitivity is attributed to different capacity of root oxygen leakage. Light and oxygen exposure enables oxygen transport from leaves down to the roots during both day and night. The oxic micro-shield around roots has a capacity to withstand sulfide intrusion until the sensitivity level is reached, when no other stressors are present. Leaves of exposed R. maritima plants are affected at similar sulfide concentrations as roots, while leaves of Z. marina are affected at higher sulfide concentrations (1-3.5mM H ₂S), reflecting different pathways of sulfide intrusion, root permeability and the ability to allocate energy away from exposed plant parts. Sensitivity towards sulfide occurs gradually and probably as a linear response to increasing sulfide concentration in R. maritima, but the exact pattern in Z. marina is still unclear. Other external stressors, such as anoxia, shading, or increased temperature may change the thresholds of the species by affecting radial oxygen loss from roots, and make especially R. maritima more prone to sulfide toxicity due to its highly permeable roots.