Enriching electroactive microorganisms from ferruginous lake waters - Mind the sulfate reducers!

Electroactive microorganisms are pivotal players in mineral transformation within redox interfaces characterized by pronounced oxygen and dissolved metal gradients. Yet, their systematic cultivation from such environments remains elusive. Here, we conducted an anodic enrichment using anoxic ferruginous waters from a post-mining lake as inoculum. Weak electrogenicity (j = ∼5 µA cm-2) depended on electroactive planktonic cells rather than anodic biofilms, with a preference for formate as electron donor. Addition of yeast extract decreased the lag phase but did not increase current densities. The enriched bacterial community varied depending on the substrate composition but mainly comprised of sulfate- and nitrate-reducing bacteria (e.g., Desulfatomaculum spp. and Stenotrophomonas spp.). A secondary enrichment strategy resulted in different bacterial communities composed of iron-reducing (e.g., Klebsiella spp.) and fermentative bacteria (e.g., Paeniclostridium spp.). Secondary electron microscopy and energy-dispersive X-ray spectroscopy results indicate the precipitation of sulfur- and iron-rich organomineral aggregates at the anode surface, presumably impeding current production. Our findings indicate that (i) anoxic waters containing geogenically derived metals can be used to enrich weak electricigens, and (ii) it is necessary to specifically inhibit sulfate reducers. Otherwise, sulfate reducers tend to dominate over EAM during cultivation, which can lead to anode passivation due to biomineralization.

Soil solution data from Bohemian headwater catchments record atmospheric metal deposition and legacy pollution.

Soil solution chemistry depends largely on mineralogy and organic matter properties of soil horizons with which they interact. Differing lithologies within a given catchment area can influence variability in soil cation exchange capacities and affect solute transport. Zero-tension and tension lysimeters were used to evaluate the fast transport of solutes in the topsoil vs. slow diffusional matrix flow at the subsoil of three contrasting lithology catchments in a mid-elevation mountain forest. Our aim was to test the feasibility of lysimeters' hydrochemical data as a gauge for legacy subsoil pollution. Due to contrasting lithologies, atmospheric legacy pollution prevailing at the soil-regolith interface is differently yet consistently reflected by beryllium, lead, and chromium soil solution concentrations of the three catchments. Geochemical (dis)equilibrium between the soil and soil matrix water governed the hydrochemistry of the soil solutions at the time of collection, potentially contributing to decreased dissolved concentrations with increased depths at sites with higher soil pH. A complementary isotopic δ18O runoff generation model constrained potential seasonal responses and pointed to sufficiently long water-regolith interactions as to permit important seasonal contributions of groundwater enriched in chemical species to the topsoil levels. Our study also reflects subsoil equilibration with atmospheric solutes deposited at the topsoil and thus provides guidance for evaluating legacy pollution in soil profiles derived from contrasting lithology.

Winter arsenic pollution in 10 forest ecosystems in the mountainous border regions of the Czech Republic.

Arsenic (As) concentrations and deposition fluxes were measured in snow and rime at 10 mountain-top sites near the borders between the Czech Republic and Austria, Germany, Poland, and Slovakia during three consecutive winter seasons (2009-2011). Our study was performed at a time following several decades of sharply decreasing regional atmospheric pollution and following the 2006 implementation of stricter air quality standards across Europe. Our objective was to compare vertical and horizontal depositions of soluble and insoluble As forms throughout the Czech Republic and define a recent Central European As pollution gradient. Arsenic soluble in weak nitric acid contributed 83 to 85% to the total As deposition, with the remaining 17-15% bound to stable particulate forms. The highest As deposition rates were recorded in the eastern Czech Republic near the borders with Poland and Slovakia. Complementary hydrochemical monitoring in four mountain-slope catchments situated near selected main study sites revealed a further decrease in open-area As deposition by the end of 2018 in the east of the country. In contrast, spruce canopy throughfall flux did not change significantly between 2009-2011 and 2016-2018. The site-specific relative roles of coal-burning-derived and ore-smelting-derived atmospheric As are discussed.

Iron and nitrogen cycling, bacterioplankton community composition and mineral transformations involving phosphorus stabilisation in the ferruginous hypolimnion of a post-mining lake.

Lake Medard is an oligotrophic post-mining lake characterised by ferruginous bottom waters, with marked redox gradients resulting from iron (Fe) and nitrogen (N) speciation and accompanying depth-dependent variations in the abundance of volatile fatty acids (VFAs), pH and alkalinity. The lacustrine system is meromictic, featuring a dysoxic hypolimnion and an anoxic monimolimnion with relatively high concentrations of sulfate (SO42-, 19 ± 2 mM) and Fe(ii) (127 ± 17 µM). An increase in dissolved manganese is also observed with increasing depth, together with a general lack of sulfide, which can only be detected at the sediment-water interface at concentrations of ∼0.30 µM. In the hypolimnion, nitrate (NO3-) becomes progressively depleted and ammonium (NH4+) dominates the dissolved N inventory (up to 185 ± 13 µM). Here we describe the biogeochemical disequilibrium conditions governing critical mineralogical transformations involving Fe and phosphorus (P) co-precipitation in the dysoxic-to-anoxic bottom water column. A combination of mineral equilibrium modelling and synchrotron-based diffraction and spectroscopic techniques was applied to investigate the minerals comprising the upper anoxic sediments. The combined dataset indicates that elemental recycling on and below the hypolimnion promote the precipitation of FeOOH polymorphs that accumulate as heterogeneous mineral clusters. Changes in the relative abundance of bacterioplankton taxa with increasing water depth point to a link between the activity of certain members of Proteobacteria and the co-recycling of carbon (C), N, and Fe stocks. Such a redox recycling process seems to lead to P stabilisation into organic-rich Fe-(oxyhydr)oxides near and above the anoxic sediment-water interface (SWI).