Hydroclimatic extremes threaten groundwater quality and stability.

Heavy precipitation, drought, and other hydroclimatic extremes occur more frequently than in the past climate reference period (1961-1990). Given their strong effect on groundwater recharge dynamics, these phenomena increase the vulnerability of groundwater quantity and quality. Over the course of the past decade, we have documented changes in the composition of dissolved organic matter in groundwater.

Tailor-made polymer tracers reveal the role of clay minerals on colloidal transport in carbonate media.

Hypothesis: Host rock weathering and incipient pedogenesis result in the exposition of minerals, e.g., clay minerals in sedimentary limestones.

Genome streamlining in Parcubacteria transitioning from soil to groundwater.

Background: To better understand the influence of habitat on the genetic content of bacteria, with a focus on members of Candidate Phyla Radiation (CPR) bacteria, we studied the effects of transitioning from soil via seepage waters to groundwater on genomic composition of ultra-small Parcubacteria, the dominating CPR class in seepage waters, using genome resolved metagenomics.

Results: Bacterial metagenome-assembled genomes (MAGs), (318 total, 32 of Parcubacteria) were generated from seepage waters and compared directly to groundwater counterparts. The estimated average genome sizes of members of major phyla Proteobacteria, Bacteroidota and Cand.

Iron coatings on carbonate rocks shape the attached bacterial aquifer community.

Most studies of groundwater ecosystems target planktonic microbes, which are easily obtained via water samples. In contrast, little is known about the diversity and function of microbes adhering to rock surfaces, particularly to consolidated rocks. To investigate microbial attachment to rock surfaces, we incubated rock chips from fractured aquifers in limestone-mudstone alternations in bioreactors fed with groundwater from two wells representing oxic and anoxic conditions.

A glimpse of the paleome in endolithic microbial communities.

Background: The terrestrial subsurface is home to a significant proportion of the Earth's microbial biomass. Our understanding about terrestrial subsurface microbiomes is almost exclusively derived from groundwater and porous sediments mainly by using 16S rRNA gene surveys. To obtain more insights about biomass of consolidated rocks and the metabolic status of endolithic microbiomes, we investigated interbedded limestone and mudstone from the vadose zone, fractured aquifers, and deep aquitards.

Importance of inner-sphere P-O-Fe bonds in natural and synthetic mineral-organic associations.

Sorption of organic molecules on mineral surfaces can occur through several binding mechanisms of varying strength. Here, we investigated the importance of inner-sphere P-O-Fe bonds in synthetic and natural mineral-organic associations. Natural organic matter such as water extracted soil organic matter (WESOM) and extracellular polymeric substances (EPS) from liquid bacterial cultures were adsorbed to goethite and examined by FTIR spectroscopy and P K-edge NEXAFS spectroscopy.

Reductive transformation of birnessite by low-molecular-weight organic acids.

Soil biogeochemistry is intrinsically coupled to the redox cycling of iron and manganese. Oxidized manganese forms various (hydr)oxides that may reductively transform and dissolve, thereby serving as electron acceptors for microbial metabolisms. Furthermore, manganese oxides might reduce purely abiotically by oxidation of dissolved Mn in a specific route of transformation from birnessite (MnO) into metastable feitknechtite (β-MnOOH) and stable manganite (γ-MnOOH).

Groundwater metabolome responds to recharge in fractured sedimentary strata.

Understanding the sources, structure and fate of dissolved organic matter (DOM) in groundwater is paramount for the protection and sustainable use of this vital resource. On its passage through the Critical Zone, DOM is subject to biogeochemical conversions. Therefore, it carries valuable cross-habitat information for monitoring and predicting the stability of groundwater ecosystem services and assessing these ecosystems' response to fluctuations caused by external impacts such as climatic extremes.

Exposure of humic acid-coated goethite colloids to groundwater does not affect their adsorption of metal(loid)s and their impact on Daphnid mobility.

Engineered humic acid-coated goethite (HA-Goe) colloids find increasing application in in situ remediation of metal(loid)-polluted groundwater. Once introduced into the subsurface, the colloids interact with groundwater altering their physicochemical properties. In comparison to freshly synthesized, unreacted HA-Goe colloids, such alterations could reduce the adsorption affinity towards metal(loid)s and also result in altered ecotoxicological effects.

Organic Matter from Redoximorphic Soils Accelerates and Sustains Microbial Fe(III) Reduction.

Microbial reduction of Fe(III) minerals is a prominent process in redoximorphic soils and is strongly affected by organic matter (OM). We herein determined the rate and extent of microbial reduction of ferrihydrite (Fh) with either adsorbed or coprecipitated OM by . We focused on OM-mediated effects on electron uptake and alterations in Fh crystallinity.

Groundwater bacterial communities evolve over time in response to recharge.

Time series analyses are a crucial tool for uncovering the patterns and processes shaping microbial communities and their functions, especially in aquatic ecosystems. Subsurface aquatic environments are perceived to be more stable than surface oceans and lakes, due to the lack of sunlight, the absence of photosysnthetically-driven primary production, low temperature variations, and oligotrophic conditions. However, periodic groundwater recharge should affect the structure and succession of groundwater microbiomes.

The mechanisms of gravity-constrained aggregation in natural colloidal suspensions.

Colloidal settlement in natural aqueous suspensions is effectively compensated by diffusive movement if particles resist aggregation - a state known as colloidal stability. However, if the settling velocity increases upon aggregation, complex structural features emerge from the directional movement induced by gravity. We present a comprehensive modeling study on the evolution of an aggregated three-dimensional structure due to diffusion, surface interactions, and gravity.

Event-driven dynamics of the total mobile inventory in undisturbed soil account for significant fluxes of particulate organic carbon.

Considerable portions of the total mobile inventory of soil seepage are the diverse colloidal and larger suspended materials that essentially contribute to pedogenesis, soil functioning, and nutritional supply of subsurface ecosystems. However, the size- and material-spectra of the total mobile inventory, and field-scale factors controlling its long-term seasonal and episodic dynamics in undisturbed soil, are scarcely investigated so far. In a 4.

Well-defined poly(ethylene glycol) polymers as non-conventional reactive tracers of colloidal transport in porous media.

Hypothesis: A prominent fraction of mobile organic matter in natural aqueous soil solutions is formed by molecules in sizes that seamlessly exceed the lower end of what is defined as a colloid. The hydrodynamics and the functional diversity of these molecules result in a transport behavior that is fundamentally different from smaller compounds. However, there is a lack of "reactive tracers" that allow for the study of colloidal transport phenomena appropriately.

Environmental selection shapes the formation of near-surface groundwater microbiomes.

Hydrodynamics drives both stochastic and deterministic community assembly in aquatic habitats, by translocating microbes across geographic barriers and generating changes in selective pressures. Thus, heterogeneity of hydrogeological settings and episodic surface inputs from recharge areas might play important roles in shaping and maintaining groundwater microbial communities. Here we took advantage of the Hainich Critical Zone Exploratory to disentangle mechanisms of groundwater microbiome differentiation via a three-year observation in a setting of mixed carbonate-siliciclastic alternations along a hillslope transect.

Complex food webs coincide with high genetic potential for chemolithoautotrophy in fractured bedrock groundwater.

Groundwater ecosystems face the challenge of energy limitation due to the absence of light-driven primary production. Lack of space and low oxygen availability might further contribute to generally assumed low food web complexity. Chemolithoautotrophy provides additional input of carbon within the subsurface, however, we still do not understand how abundances of chemolithoautotrophs, differences in surface carbon input, and oxygen availability control subsurface food web complexity.

Predominance of . Patescibacteria in Groundwater Is Caused by Their Preferential Mobilization From Soils and Flourishing Under Oligotrophic Conditions.

Despite the widely observed predominance of . Patescibacteria in subsurface communities, their input source and ecophysiology are poorly understood. Here we study mechanisms of the formation of a groundwater microbiome and the subsequent differentiation of .

The endolithic bacterial diversity of shallow bedrock ecosystems.

Terrestrial subsurface microbial communities are not restricted to the fluid-filled void system commonly targeted during groundwater sampling but are able to inhabit and dwell in rocks. However, compared to the exploration of the deep biosphere, endolithic niches in shallow sedimentary bedrock have received little interest so far. Despite the potential contribution of rock matrix dwellers to matter cycling and groundwater resource quality, their identity and diversity patterns are largely unknown.

C-Free Carbon Is a Major Contributor to Cellular Biomass in Geochemically Distinct Groundwater of Shallow Sedimentary Bedrock Aquifers.

Despite the global significance of the subsurface biosphere, the degree to which it depends on surface organic carbon (OC) is still poorly understood. Here, we compare stable and radiogenic carbon isotope compositions of microbial phospholipid fatty acids (PLFAs) with those of in situ potential microbial C sources to assess the major C sources for subsurface microorganisms in biogeochemical distinct shallow aquifers (Critical Zone Exploratory, Thuringia Germany). Despite the presence of younger OC, the microbes assimilated C-free OC to varying degrees; ~31% in groundwater within the oxic zone, ~47% in an iron reduction zone, and ~70% in a sulfate reduction/anammox zone.

First insights into the living groundwater mycobiome of the terrestrial biogeosphere.

Although fungi play important roles in biogeochemical cycling in aquatic ecosystems and have received a great deal of attention, much remains unknown about the living fractions of fungal communities in aquifers of the terrestrial subsurface in terms of diversity, community dynamics, functional roles, the impact of environmental factors and presence of fungal pathogens. Here we address this gap in knowledge by using RNA-based high throughput pair-end illumina sequencing analysis of fungal internal transcribed spacer (ITS) gene markers, to target the living fractions of groundwater fungal communities from fractured alternating carbonate-/siliciclastic-rock aquifers of the Hainich Critical Zone Exploratory. The probed levels of the hillslope multi-storey aquifer system differ primarily in their oxygen and nitrogen content due to their different connections to the surface.

Nitrogen Loss from Pristine Carbonate-Rock Aquifers of the Hainich Critical Zone Exploratory (Germany) Is Primarily Driven by Chemolithoautotrophic Anammox Processes.

Despite the high relevance of anaerobic ammonium oxidation (anammox) for nitrogen loss from marine systems, its relative importance compared to denitrification has less been studied in freshwater ecosystems, and our knowledge is especially scarce for groundwater. Surprisingly, phospholipid fatty acids (PLFA)-based studies identified zones with potentially active anammox bacteria within two superimposed pristine limestone aquifer assemblages of the Hainich Critical Zone Exploratory (CZE; Germany). We found anammox to contribute an estimated 83% to total nitrogen loss in suboxic groundwaters of these aquifer assemblages at rates of 3.

Disentangling the root- and detritus-based food chain in the micro-food web of an arable soil by plant removal.

Soil food web structure and function is primarily determined by the major basal resources, which are living plant tissue, root exudates and dead organic matter. A field experiment was performed to disentangle the interlinkage of the root-and detritus-based soil food chains. An arable site was cropped either with maize, amended with maize shoot litter or remained bare soil, representing food webs depending on roots, aboveground litter and soil organic matter as predominant resource, respectively.

Archaeal Diversity and CO Fixers in Carbonate-/Siliciclastic-Rock Groundwater Ecosystems.

Groundwater environments provide habitats for diverse microbial communities, and although Archaea usually represent a minor fraction of communities, they are involved in key biogeochemical cycles. We analysed the archaeal diversity within a mixed carbonate-rock/siliciclastic-rock aquifer system, vertically from surface soils to subsurface groundwater including aquifer and aquitard rocks. Archaeal diversity was also characterized along a monitoring well transect that spanned surface land uses from forest/woodland to grassland and cropland.

Attached and Suspended Denitrifier Communities in Pristine Limestone Aquifers Harbor High Fractions of Potential Autotrophs Oxidizing Reduced Iron and Sulfur Compounds.

Oxygen and nitrate availability as well as the presence of suitable organic or inorganic electron donors are strong drivers of denitrification; however, the factors influencing denitrifier abundance and community composition in pristine aquifers are not well understood. We explored the denitrifier community structure of suspended and attached groundwater microorganisms in two superimposed limestone aquifer assemblages with contrasting oxygen regime in the Hainich Critical Zone Exploratory (Germany). Attached communities were retrieved from freshly crushed parent rock material which had been exposed for colonization in two groundwater wells (12.

Quantification of pH-dependent speciation of organic compounds with spectroscopy and chemometrics.

Fluorescence and UV/Vis spectra of aqueous solutions with numerous organic compounds are a superposition of single spectra of the chemical species present. Thus, an isolation of individual spectra with chemometrics is required for their quantification. We investigated UV/Vis spectra and fluorescence excitation-emission matrices of vanillic acid, salicylic acid, phenoxyacetic acid and phthalic acid with positive matrix factorization (PMF) and non-negativity constrained parallel factor analysis (PARAFAC) in combination with the law of mass action.