Sulfide as a soil phytotoxin-a review.

In wetland soils and underwater sediments of marine, brackish and freshwater systems, the strong phytotoxin sulfide may accumulate as a result of microbial reduction of sulfate during anaerobiosis, its level depending on prevailing edaphic conditions. In this review, we compare an extensive body of literature on phytotoxic effects of this reduced sulfur compound in different ecosystem types, and review the effects of sulfide at multiple ecosystem levels: the ecophysiological functioning of individual plants, plant-microbe associations, and community effects including competition and facilitation interactions. Recent publications on multi-species interactions in the rhizosphere show even more complex mechanisms explaining sulfide resistance. It is concluded that sulfide is a potent phytotoxin, profoundly affecting plant fitness and ecosystem functioning in the full range of wetland types including coastal systems, and at several levels. Traditional toxicity testing including hydroponic approaches generally neglect rhizospheric effects, which makes it difficult to extrapolate results to real ecosystem processes. To explain the differential effects of sulfide at the different organizational levels, profound knowledge about the biogeochemical, plant physiological and ecological rhizosphere processes is vital. This information is even more important, as anthropogenic inputs of sulfur into freshwater ecosystems and organic loads into freshwater and marine systems are still much higher than natural levels, and are steeply increasing in Asia. In addition, higher temperatures as a result of global climate change may lead to higher sulfide production rates in shallow waters.

Multi-level effects of sulphur-iron interactions in freshwater wetlands in The Netherlands.

Although sulphur deposition rates in Europe have considerably decreased over the last decades, sulphate concentrations in freshwater wetlands are still high, as a result of drainage, nitrate pollution, and increased sulphur loads in rivers. High sulphur fluxes may cause sulphide toxicity and eutrophication, and strongly interfere with the biogeochemical cycling of iron and phosphorus. In the present study the ecotoxicological interactions between sulphur, phosphate, iron, and trace metals in freshwater wetlands are reviewed.

Predicting metal uptake by wetland plants under aerobic and anaerobic conditions.

Metal pollution can be a serious threat to ecosystems at a global scale. Although the bioavailability of potentially toxic metals is determined by many biotic and abiotic factors, including pH and redox potential, total metal concentrations in the soil are used widely to assess or predict toxicity. In the present study we tested the effect of desiccation of soils differing in acidification potential and total heavy metal contamination on the growth and metal uptake of three typical, common wetland species: Caltha palustris, Juncus effusus, and Rumex hydrolapathum. We found that plant growth in wet soils mainly was determined by nutrient availability, though in dry soils the combined effects of acidification and increased metal availability prevailed. Metal uptake under anaerobic conditions was best predicted by the acidification potential (sediment S/[Ca + Mg] ratio), not by total metal concentrations. We propose that this is related to radial oxygen loss by wetland plant roots, which leads to acidification of the rhizosphere. Under aerobic conditions, plant metal uptake was best predicted by the amount of CaCl2-extractable metals. We conclude that total metal concentrations are not suitable for predicting bioavailability and that the above diagnostic parameters will provide insight into biogeochemical processes involved in toxicity assessment and soil policy.

Differential responses of the freshwater wetland species Juncus effusus L. and Caltha palustris L. to iron supply in sulfidic environments.

Sulfur pollution can lead to serious problems in freshwater wetlands, including phosphorus eutrophication and sulfide toxicity. We tested the effects of anaerobic iron-rich groundwater discharge in fens, simulated by iron injection, on two characteristic species (Juncus effusus and Caltha palustris) in a sulfidic environment. Biomass production of C. palustris roots showed an optimum response to the combined addition of iron and sulfide, with highest values at intermediate concentrations of both substances. Iron deficiency apparently occurred at low iron concentrations, while at high iron concentrations, growth was decreased. For J. effusus, in contrast, no toxic effects were found of both iron and sulfide. This could be explained by larger radial oxygen loss (ROL) of J. effusus and could not be explained by differences in phosphorous concentrations. The results of our experiments confirm that iron-rich groundwater discharge has the potential to affect vegetation composition through toxicity modification in sulfidic environments.

Evidence for the involvement of betaproteobacterial Thiobacilli in the nitrate-dependent oxidation of iron sulfide minerals.

The Thiobacilli are an important group of autotrophic bacteria occurring in nature linking the biogeochemical cycles of sulfur and nitrogen. Betaproteobacterial Thiobacilli are very likely candidates for mediating the process of nitrate-dependent anoxic iron sulfide mineral oxidation in freshwater wetlands. A Thiobacillus denitrificans-like bacterium was present in an enrichment on thiosulfate and nitrate, derived from an iron-sulfide- and nitrate-rich freshwater environment. Preliminary FISH analysis showed that the 16S rRNA gene-based bacterial probe mix showed great variation in intensity under different culture conditions. Furthermore, the widely applied 23S rRNA gene-based probe set BET42a/GAM42a incorrectly identified the T. denitrificans-like bacterium as a member of the Gammaproteobacteria. To circumvent these problems, the 23S rRNA genes of two T. denitrificans strains were partially sequenced and a new 23S rRNA gene-based probe (Betthio 1001) specific for betaproteobacterial Thiobacilli was designed. Use of this new probe Betthio 1001, combined with field measurements, indicates the involvement of Thiobacilli in the process of nitrate-dependent iron sulfide mineral oxidation.