Aqueous system-level processes and prokaryote assemblages in the ferruginous and sulfate-rich bottom waters of a post-mining lake

Abstract

Abstract. In the aqueous oligotrophic ecosystem of a post-mining lake (Lake Medard, Czechia), reductive Fe(II) dissolution outpaces sulfide generation from microbial sulfate reduction (MSR), and ferruginous conditions occur without quantitative sulfate depletion. An isotopically constrained estimate of the rates of sulfate reduction (SRR) suggests that despite a high genetic potential, this respiration pathway is limited by the rather low amounts of metabolizable organic carbon. This points to substrate competition exerted by iron and nitrogen respiring prokaryotes. Yet, the microbial succession across the nitrogenous and ferruginous zones of the bottom water column also indicates sustained genetic potential for chemolithotrophic sulfur oxidation. Therefore, our isotopic SRR estimates could be rather portraying high rates of anoxic sulfide oxidation to sulfate, probably accompanied by microbially induced disproportionation of S intermediates. Near and at the anoxic sediment-water interface, vigorous sulfur cycling can be fuelled by ferric and manganic particulate matter and redeposited siderite stocks. Sulfur oxidation and disproportionation then appear to prevent substantial stabilization of iron monosulfides as pyrite but can enable the interstitial precipitation of small proportions of equant microcrystalline gypsum. This latter mineral isotopically fingerprints sulfur oxidation proceeding at near equilibrium with the ambient anoxic waters, whilst authigenic pyrite-sulfur displays a 38 to 27 ‰ isotopic offset from ambient sulfate, suggestive of incomplete MSR and likely reflective also of an open sulfur cycling system. Pyrite-sulfur fractionation decreases with increased reducible reactive iron in the sediment. In the absence of ferruginous coastal zones today, the current water column redox stratification in the post-mining Lake Medard has scientific value for (i) testing emerging hypotheses on how a few interlinked biogeochemical cycles operated in nearshore paleoenvironments during redox transitional states; and (ii) to acquire insight on how similar early diagenetic redox proxy signals developed in sediments affected by analogue transitional states in ancient water columns.

Anaerobic dissolved As(iii) removal from metal-polluted waters by cathode-stabilized Fe(iii)-oxyhydroxides

“…and Rhodoferax spp.). 34,36 In addition, the anoxic lake sediments exhibit As enrichment into easily reducible and highly crystalline Fe(III) oxyhydroxides. 35 In November 2019, the redox stratified bottom water column at the central area of Lake Medard was characterized by using a multiparameter sonde (6600 V2-2, YSI, USA).…”

“…This was followed by bottom water sampling for cation and anion concentration analyses. 34 Results from these analyses guided the preparation of electrolyte solutions featuring environmentally relevant hydrochemistry that considers the measured bottom water dissolved As and Fe concentrations, as well as its conductivity and dissolved acetate and RNS levels. These experimental solutions were used to evaluate the feasibility of a BES for directly sequestering As(III) from the anoxic portion of the Lake Medard bottom water column (Fig.…”

“…38 This mineraloid is among the main oxidative breakdown products of lignite. 39 Dissolved breakdown products of lignite are thought to be present in the bottom waters of Lake Medard 34,36 (S1.3, Table S1 †). The conductivities of our experimental solutions were in the range of 7854 ± 335 μS cm −1 , also resembling the model lake system.…”

“…The lake bottom water column has an abundance of humic substance derivatives released during lignite degradation in the anoxic sediments. 34 Minor changes in the redox state of the water column affect As speciation and, at times augment the release of As from the sediments. 35 In this model site, arsenic and iron are rapidly cycled between the sediment-water interface and the aqueous redoxcline.…”

“…35 In this model site, arsenic and iron are rapidly cycled between the sediment-water interface and the aqueous redoxcline. 34,36 Another relevant feature that influences the cycling of iron and arsenic in Lake Medard is the coexistence of dissolved inorganic N species. 36 Changes in the dissolved water column N inventory can affect Fe(III) mineral transformations.…”