Biology Centre CAS, Soil and Water Infrastructure

Biogeochemistry of iron, nitrogen and phosphorus in a meromictic lake and its ferruginous sediments: Lake Medard, Czech Republic

Daniel Petráš, Dagmara Sirova, Jiří Jan, and Jakub Borovec

Biology Centre CAS, Soil and Water Infrastructure, Na Sádkách 7, 370 05 České Budějovice, Czech Republic

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, which exert a critical role over the solubility of phosphorus (P) via sorption/desorption reactions on Fe(III) oxide surfaces. The cycling of sulphur (S) is also closely coupled to the transformations of P and Fe, and 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 currently 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 an unusual meromictic lake with a ferruginous hypolimnion known as Lake Medard (Karlovy Vary, Czech Republic). This lake 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, such as mixed valence Fe-hydroxides and siderite, and may even increase the precipitation potential of vivianite (Fe3(PO4)2·8H2O). As these mineral form, they maintain the P retention capacity of the sediment upon burial as compared with sulfidic sediments, which exhibit minimal affinity for PO43−.

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

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