Microbial mediators of the sulfur, nitrogen, and iron cycles in freshwater ecosystems

摘要

Human activities and concominant sulfur and nitrogen pollution endanger freshwater ecosystem quality. Improved knowledge on wetland biogeochemistry is a necessity to protect these valuable and fragile ecosystems. Effects of increased nitrate concentrations (stimulation of sulfide mineral oxidation, inhibition of sulfate reduction) and sulfate concentrations (internal eutrophication, sulfide toxicity, formation sulfide minerals) are known, but data on the associated microbiology are scarce. Therefore, freshwater microbial mediators of sulfur oxidation, iron reduction and iron oxidation were studied with culture-dependent and culture-independent methods with major emphasis on nitrate-dependent iron sulfide mineral oxidation. A bioreactor study with soil from the freshwater nature reserve Het Zwart Water showed that amorphous iron sulfide did stimulate denitrification whereas crystalline pyrite did not. The type of iron sulfide mineral seems an important determinant in the occurrence of anoxic iron sulfide mineral oxidation. Denitrifying sulfur-oxidizing Thiobacilli were shown to be likely promotors of anoxic iron sulfide mineral dissolution, based on integral molecular and chemical analyses of soil and groundwater samples from Het Zwart Water combined with enrichment and pure culture studies. Acidovorax sp., associated with nitrate-dependent iron oxidation, were detected in an enrichment culture on iron sulfide and nitrate and were abundant in Het Zwart Water groundwater. These bacteria may also contribute to anoxic iron sulfide mineral dissolution. Findings from an enrichment culture of Het Zwart Water ground water and molecular analyses of iron seep material from the nature reserve De Bruuk showed that iron reduction by Geobacter may not be restricted to atmospheric-oxygen-free conditions. Simultaneous detection of iron oxidizers and reducers indicated tight coupling of iron conversions in De Bruuk. The detection and abundance of bacteria that promote anoxic iron sulfide dissolution in our studies indicates that anoxic conditions alone do not suffice to protect freshwater ecosystems from adverse processes such internal eutrophication and release of potentially toxic metals.