Biogeochemistry of iron, nitrogen and phosphorus in a meromictic lake and its ferruginous sediments

In freshwater ecosystems, reactive iron (Fe) species fuel a very active microbially mediated elemental cycling driven both by autotrophic and heterotrophic metabolisms, including obligate and facultative nitrate (NO3-) and ammonium (NH4+) bioutilization. These microbial pathways exert a critical role over the solubility of phosphorus (P) via sorption/desorption reactions on Fe(III) oxide surfaces. The bioutilization of sulphur is also closely coupled to the transformations of P and Fe. The respiration of organic matter by sulphate reducers enhance the precipitation potential of Fe-sulphides, which deplete the reactive Fe oxide pool in the sediment to counteract the functioning of Fe(III) species in binding P.

At the Soil and Water Research Infrastructure (SoWa) we are evaluating the intimate relation between natural populations of iron- and nitrogen-utilizing microorganisms and their effect over sorption/desorption reactions of phosphorus as occurring in a meromictic lake with a ferruginous hypolimnion. This lake, known as Medard (Karlovy Vary, Czech Republic), was a former open lignite mine flooded with water from river Ohře as part of a major reclamation effort. In this unusual study site, the presence of bottom water sulphate and significant concentrations of dissolved iron suggest that denitrification and dissimilatory nitrate reduction to ammonium on and below the sediment water interface are largely driven by chemolithotrophy and marginally by carbon turnover. Under such anoxic — but non-sulfidic conditions, chemolithotrophic microbes may induce the precipitation of partially reduced Fe minerals with high phosphate (PO43−) adsorption capacity, including mixed valence Fe-hydroxides and siderite, and may even led to the localized precipitation of vivianite (Fe3(PO4)2·8H2O). As these mineral form, they maintain the P retention capacity of the sediment upon burial as compared with sulphidic sediments, which exhibit minimal affinity for PO43−.

In this seminar, I present our current state of knowledge regarding the biomineralization processes likely to be occurring in the ferruginous sediments of Lake Medard. I will also discuss our multi-analytical approach which aims at constraining the impact of discrete microbial niches over iron reactivity and phosphorus solubility and was designed to provide further evidence on the role of nitrogen- and iron-utilizing microorganisms as agents in geochemical cycling of iron-bound phosphorus.

For more details see http://kbe.prf.jcu.cz/en/seminars