Département Ressources aquatiques et eau potable

Oxygen availability is a key environmental cue that shapes bacterial physiology, growth, and surface colonization, yet how individual bacteria respond when oxygen landscapes change over time remains poorly understood. Microfluidic devices provide powerful platforms to generate and control dissolved gas gradients at microscopic scales, enabling systematic investigations of their effects on biological and chemical processes. However, existing systems, utilizing diffusion for gradient formation, often limit rapid modulation of dissolved gas over a large field of view, constrain possible flow rates, and require multilayer fabrication. Here, we developed a single-layer, microfluidic platform that enables rapid equilibration of dissolved gas concentrations over a large chamber width, compared to diffusion-based systems with the same width. Using this device, we investigated the response of the soil bacterium Pseudomonas putida to dissolved oxygen gradients that vary both spatially and temporally. We tracked surface colonization by individual cells over periods of up to 7 h. Our results reveal how spatiotemporal fluctuations in dissolved oxygen shape colony spatial organization, growth rates, and single-cell dynamics, including elongation, attachment, and detachment dynamics. The experiments show that P. putida maintains exponential growth for several generations after an oxygen downshift, indicating a buffered response likely supported by metabolic memory and internal reserves while low-oxygen adaptation unfolds. This delayed physiological shift drives spatial reorganization into “settler” and “disperser” subpopulations, linking single-cell adaptation to biofilm-scale structure.

Despite the importance of coastal areas for the transfer of selenium (Se) from rivers to the open ocean and its bioavailability to food webs, dissolved Se concentrations and speciation in these ecosystems remain poorly understood. We conducted a 3-yr study with monthly water column sampling at five stations spanning from the North Sea to the central Baltic Sea, capturing large gradients of salinity, redox conditions, and biological productivity to quantify dissolved Se species and characterize Se speciation across environmental gradients over time. Despite remarkably similar total dissolved Se concentrations (66 ± 17 ng·L⁻¹) across all sub-basins, Se speciation showed contrasting spatial and vertical patterns. Marine-influenced waters from the North Sea were dominated by bioavailable inorganic oxidized Se species (selenite: Se^IV and selenate: Se^VI), while the brackish Baltic Proper was enriched in reduced organic and elemental Se (Se^−II + Se⁰). Vertical profiles showed consistent Se^IV depletion in surface waters, indicating biological uptake across all stations, while Se^−II + Se⁰ exhibited both production during periods of high phytoplankton biomass and subsequent vertical transport, leading to accumulation in hypoxic and euxinic waters. These contrasting patterns suggest fundamentally different speciation and transformation regimes: well-mixed marine systems maintain efficient recycling of bioavailable inorganic Se through frequent water exchange, whereas the permanently stratified Baltic system creates a significant sink for reduced Se species in oxygen-depleted deep waters. Our results demonstrate how water column structure and redox conditions control Se bioavailability and fate in coastal systems, with implications for understanding micronutrient dynamics in expanding oxygen minimum zones and climate-driven changes in coastal stratification.

Eine nationale Umfrage unter Fachpersonen im Bereich Hydrogeologie zeigt: Die Herausforderungen im Schweizer Grundwassermanagement sind bekannt – doch ihre Umsetzung bleibt eine zentrale Hürde. Besonders relevant sind Fragen der Wasserqualität, Governance und Datennutzung. Der Beitrag ordnet die Schweizer Ergebnisse im Kontext globaler Entwicklungen ein.

Von einer Arbeitsgruppe der Schweizerischen Gesellschaft für Hydrogeologie (SGH) wurde ein neuer Leitfaden aufgestellt, der zeigt, wie hydrogeologische und hydrologische Daten systematisch erhoben, interpretiert und modellgerecht aufbereitet werden können. Er beschreibt die nachvollziehbare Ableitung belastbarer Modellparameter sowie die Entwicklung hydrogeologischer Konzeptmodelle als Grundlage numerischer Grundwassermodelle. Der Leitfaden unterstützt Fachpersonen, Behörden und Auftraggebende bei der Planung, Bearbeitung sowie Einordnung von Modellergebnissen und Unsicherheiten.