Effect of increased sediment sulfide concentrations on the composition of stable sulfur isotopes (δ 34S) and sulfur accumulation in the seagrasses Zostera marina and Posidonia oceanica

Abstract

The effect of increased sediment sulfide concentrations on the sulfur isotopic composition (δ 34S), total sulfur (TS) and elemental sulfur (S 0) concentrations in plant tissues was studied for the two seagrasses Zostera marina (3 weeks in laboratory) and Posidonia oceanica (4 months in situ). Porewater sulfide concentrations were experimentally regulated and plants exposed to high sediment sulfide concentrations had δ 34S signals closer to the δ 34S of sulfide, whereas plants exposed to no / low sulfide concentrations had δ 34S signals closer to the δ 34S of seawater sulfate, indicating a higher sulfide invasion in plants exposed to high sulfide concentrations. The δ 34S varied between the plant tissues in both species with the leaves having more positive δ 34S signals than roots and rhizomes, indicating that sulfide was invading into the roots and moved to the other tissues through the lacunae. TS and S 0 concentrations were higher in plants exposed to sulfide in both experiments suggesting that sulfur derived from sediment sulfide accumulated in the plants. The δ 34S signal in S 0 was similar to sediment sulfide verifying that S 0 found in the seagrasses originated from sediment sulfide. Direct comparisons of δ 34S in the two different seagrasses and across the treatments were not possible due to large differences in δ 34S of the sulfur sources. F sulfide adjusted for these differences and may be a useful alternative, when δ 34S of the sulfur sources varies between study sites. There were no significant effects of sulfide exposure on plant growth and mortality in Z. marina and P. oceanica after 3 weeks and 8 weeks exposure, respectively, but P. oceanica showed indications of reduced growth and higher mortality after 16 weeks of sulfide exposure probably due to sulfide invasion/toxicity.