Detoxifying toxicants: interactions between sulfide and iron toxicity in freshwater wetlands

In many Dutch freshwater wetlands, concentrations of sulfate in the surface water and groundwater have increased. It is especially in peaty areas that this can lead to problems, including the reduction of sulfate to toxic sulfide. Our aquarium experiments showed that even low sulfide concentrations of 50 micromol/L are toxic to the freshwater macrophyte Nitella flexilis and the freshwater oligochaete Ophidonais serpentina. Sulfide toxicity can be modified by the availability of free iron in soil moisture or sediment from iron-rich groundwater discharge. The iron precipitates the sulfide, thereby immobilizing it and decreasing its toxicity. However, iron itself can be a toxicant as well. We found a detoxifying effect of moderate iron concentrations on sulfide toxicity, as well as a toxic effect of high iron concentrations on the growth of Potamogeton acutifolius. At the intermediate range, the formation of metal sulfides can simultaneously decrease metal and sulfide toxicity. Results of the experiments are discussed in relation to hydrological changes in freshwater wetlands.