Abteilung Oberflächengewässer

Mainstream anaerobic ammonium oxidation (anammox) represents one of the most promising energy-efficient mechanisms of fixed nitrogen elimination from wastewaters. However, little is known about the exact processes and drivers of microbial community assembly within the complex microbial biofilms that support anammox in engineered ecosystems. Here, we followed anammox biofilm development on fresh carriers in an established 8m³ mainstream anammox reactor that is exposed to seasonal temperature changes (∼25-12°C) and varying NH₄⁺ concentrations (5-25 mg/L). We use fluorescence in situ hybridization and 16S rRNA gene sequencing to show that three distinct stages of biofilm development emerge naturally from microbial community composition and biofilm structure. Neutral modelling and network analysis are employed to elucidate the relative importance of stochastic versus deterministic processes and synergistic and antagonistic interactions in the biofilms during their development. We find that the different phases are characterized by a dynamic succession and an interplay of both stochastic and deterministic processes. The observed growth stages (Colonization, Succession and Maturation) appear to be the prerequisite for the anticipated growth of anammox bacteria and for reaching a biofilm community structure that supports the desired metabolic and functional capacities observed for biofilm carriers already present in the system (∼100g_NH4-N m³ d⁻¹). We discuss the relevance of this improved understanding of anammox-community ecology and biofilm development in the context of its practical application in the start-up, configuration, and optimization of anammox biofilm reactors.

This study presents a novel concept for estimating net ecosystem production (NEP), the export of organic carbon (OC) from the productive surface layer to the deep‐water (hypolimnion) of 11 seasonally stratified lakes, varying in depth and trophic state. As oxygen remineralizes settling OC at a constant ratio, NEP is equivalent to the areal hypolimnetic mineralization rate (AHM) plus burial in the sediment. Two major interferences have to be considered, however. First, OC from terrestrial sources, not originating from primary production, consumes a fraction of oxidants. Second, sediment diagenetic processes of lakes in trophic transition (e.g., undergoing eutrophication or reoligotrophication) that are not in quasi‐steady‐state with actual fluxes of OC from the productive surface layer, bias the NEP estimation. In these cases, the flux of reduced substances diffusing from the sediment must be subtracted. This results in some overestimation for lakes with high allochthonous loads, and slight underestimation in lakes that are not in quasi‐steady‐state, because the actual sediment burial of autochthonous OC is small but not negligible. The presented approach requires data from routinely available monitoring and thus can be applied to historic data. The temporal integration over the productive season makes the estimation of NEP robust. Based on a historic 47 years long data record of Lake Geneva, NEP estimations (∼70 gC m^-2) from AHM rates agree well with P and N export budgets from the productive surface zone, which help to verify and constrain the uncertainty of the estimates.