Do bacteria occupying important ecological niches contribute to the entrance of mercury into the food webs?

Abstract

Bioavailable mercury represents a major threat locally to wildlife and also contributes to the global Hg pool and cycling of Hg. In this study, we focused on bacterial mobilization of inorganic Hg contained in sediment collected from the river Idrijca near the site of UNESCO's Idrija Mercury Mine. At the lowest trophic level, bacteria have an important role in Hg mobilization due to the cumulative nature of Hg in the food chain. Ignoring the most commonly studied sulfate reducing and methanogenic bacteria, we focused, among others, on the ubiquitous mineral weathering bacteria, Bacillus and Pseudomonas, the nitrifying Nitrobacter sp., the nitrogen fixing Rhizobium and Azotobacter, the aerobic Brevundimonas and the exoelectrogenic Schewanella to examine their potential for the mobilization of total (tHg) and the bioavailable Hg (bHg) fraction. We showed that several bacterial groups are able to mobilize the less soluble formes of Hg, i.e. HgS and smelting calcine in the sediment, either via close contact between cells and sediment particles or by the cell- excreted exudates resulting in increased levels of tHg and bHg in solution or in the cell biomass. The diversity of bacterial groups capable of mobilizing Hg implies that many different more general pathways and consequently many mobilization mechanisms, are possible apart from the Hg specific routes. In particular, the bacterial groups Pseudomonas and Bacillus, with their mineral weathering capabilities, were shown to be able to increase or decrease the availability of bHg depending on the peculiarities of their respective metabolisms and the nature of their exposure to the river sediment. Additionally, we showed that accumulation of Hg in the biofilm biomass is one type of mechanism influencing the bioavailability of Hg and can act as a point at which the insoluble Hg can enter into the foodweb.