Sources, Transformation, and Fate of Dissolved Organic Matter in the Gravel Bar of a Prealpine Stream

Gravel bars (GBs) are hotspots of biogeochemical activity, likely impacting carbon dynamics instreams and rivers. However, it remains unclear how GBs process dissolved organic matter (DOM) receivedfrom stream water and groundwater. Here we investigate the spatial and temporal variability of DOMconcentration and composition within a GB using DOM absorbance andfluorescence measurements. Wefound clear seasonal and diurnal patterns in DOM composition within the GB, indicating changingcontributions of DOM sources and transformation processes. While DOM composition was characterized bymore protein‐like DOM in summer, with increasing contributions of humic‐like DOM toward winterwithin the stream water and GB, groundwater DOM exhibited a strong aromatic and humic‐like characteryear‐round. DOM composition and concentration varied diurnally during autumn and winter within theGB and stream water, while seasonally higher groundwater inputs in summer likely muted the diurnalpattern, pointing to the importance of seasonal hydrological controls on DOM dynamics. Mixing modelanalysis showed GB DOM characteristics to differ from that predicted by stream water‐groundwater mixingwithin the GB, particularly in summer. Dissolved organic carbon concentration decreased alongflow pathsthrough the GB with increasing residence time, likely pointing to microbial uptake and/or adsorption tosediment surfaces within the GB, with concurrent clear shifts in DOM composition. While freshly producedand more humified DOM increased along GBflow paths, protein‐likefluorescence (C3) was removed,indicating the simultaneous production and removal of DOM. Together, ourfindings highlight the role of GBsin DOM removal and transformation, influenced by seasonal shifts in temperature and hydrodynamics.

Travel Time and Source Variation Explain the Molecular Transformation of Dissolved Organic Matter in an Alpine Stream Network

Tom Ian Battin, Hannes Markus Peter, Amber Joy Ulseth

Streams and rivers are important components of the carbon cycle as they transport and transform dissolved organic matter (DOM). Using high resolution Fourier transform ion cyclotron resonance mass spectrometry, we studied the spatial distribution of DOM at the molecular level at more than 100 sites across a stream network during summer and winter basefow. We developed a model approximating the time DOM spent in the fluvial network, a key constraint on the biogeochemical processing of DOM. Discharge-weighted travel time explained the compositional changes of DOM, which differed markedly in summer and winter. We attribute these seasonal differences to variation in source material, putatively reflecting the dynamics of freshly produced DOM in summer and DOM with an imprint of leaf litter in winter. Hydrological mixing was an important driver of the spatial dynamics of DOM. From the convergence rate of DOM compound intensities to the network wide average, we inferred the spatial distribution of sources within the catchment. Finally, we estimated network-wide apparent mass transfer coefficients (vf app) of individual DOM compounds, which describe the vertical velocity at which DOM compounds are removed by biotic and abiotic processes. We identified the oxidative state of carbon as an important factor explaining vf app , which we consequently attribute to biological uptake of thermodynamically favorable DOM compounds. This work contributes to our understanding of the spatial processes, temporal constraints, and chemical properties of DOM that regulate the transformation and diagenesis of DOM at the fluvial network scale.

2020

Light availability affects stream biofilm bacterial community composition and function, but not diversity

Tom Ian Battin

Changes in riparian vegetation or water turbidity and browning in streams alter the local light regime with potential implications for stream biofilms and ecosystem functioning. We experimented with biofilms in microcosms grown under a gradient of light intensities (range: 5-152 μmole photons s-1 m-2 ) and combined 454-pyrosequencing and enzymatic activity assays to evaluate the effects of light on biofilm structure and function. We observed a shift in bacterial community composition along the light gradient, whereas there was no apparent change in alpha diversity. Multifunctionality, based on extracellular enzymes, was highest under high light conditions and decoupled from bacterial diversity. Phenol oxidase activity, involved in the degradation of polyphenolic compounds, was twice as high on average under the lowest compared with the highest light condition. This suggests a shift in reliance of microbial heterotrophs on biofilm phototroph-derived organic matter under high light availability to more complex organic matter under low light. Furthermore, extracellular enzyme activities correlated with nutrient cycling and community respiration, supporting the link between biofilm structure-function and biogeochemical fluxes in streams. Our findings demonstrate that changes in light availability are likely to have significant impacts on biofilm structure and function, potentially affecting stream ecosystem processes.

Wiley-Blackwell2015

Biogeochemical Cycling by a Low-Diversity Microbial Community in Deep Groundwater

Louise Julia Balmer, Emma Bell, Rizlan Bernier-Latmani, Manon Frutschi, Guillaume Sommer

Olkiluoto, an island on the south-west coast of Finland, will host a deep geological repository for the storage of spent nuclear fuel. Microbially induced corrosion from the generation of sulphide is therefore a concern as it could potentially compromise the longevity of the copper waste canisters. Groundwater at Olkiluoto is geochemically stratified with depth and elevated concentrations of sulphide are observed when sulphate-rich and methane-rich groundwaters mix. Particularly high sulphide is observed in methane-rich groundwater from a fracture at 530.6 mbsl, where mixing with sulphaterich groundwater occurred as the result of an open drill hole connecting two different fractures at different depths. To determine the electron donors fuelling sulphidogenesis, we combined geochemical, isotopic, metagenomic and metaproteomic analyses. This revealed a low diversity microbial community fuelled by hydrogen and organic carbon. Sulphur and carbon isotopes of sulphate and dissolved inorganic carbon, respectively, confirmed that sulphate reduction was ongoing and that CO2 came from the degradation of organic matter. The results demonstrate the impact of introducing sulphate to a methane-rich groundwater with limited electron acceptors and provide insight into extant metabolisms in the terrestrial subsurface.

2018