Abteilung Oberflächengewässer

Anoxygenic phototrophic sulfide oxidation by green and purple sulfur bacteria (PSB) plays a key role in sulfide removal from anoxic shallow sediments and stratified waters. Although some PSB can also oxidize sulfide with nitrate and oxygen, little is known about the prevalence of this chemolithotrophic lifestyle in the environment. In this study, we investigated the role of these phototrophs in light‐independent sulfide removal in the chemocline of Lake Cadagno. Our temporally resolved, high‐resolution chemical profiles indicated that dark sulfide oxidation was coupled to high oxygen consumption rates of ~9 μM O₂·h⁻¹. Single‐cell analyses of lake water incubated with ¹³CO₂ in the dark revealed that Chromatium okenii was to a large extent responsible for aerobic sulfide oxidation and it accounted for up to 40% of total dark carbon fixation. The genome of Chr. okenii reconstructed from the Lake Cadagno metagenome confirms its capacity for microaerophilic growth and provides further insights into its metabolic capabilities. Moreover, our genomic and single‐cell data indicated that other PSB grow microaerobically in these apparently anoxic waters. Altogether, our observations suggest that aerobic respiration may not only play an underappreciated role in anoxic environments but also that organisms typically considered strict anaerobes may be involved.

Measures to reduce lake phosphorus concentrations have been encouragingly successful in many parts of the world. Aftersignificant eutrophication in the twentieth century, nutrient concentrations have declined in many natural settings. In additionto these direct anthropogenic impacts, however, climate change is also altering various processes in lakes. Its effects onlacustrine nutrient budgets remain poorly understood. Here we investigate the total phosphorus (TP) concentrations in theepilimnion of the meromictic Lake Zug under present and future climatic conditions. Results are compared with those ofother deep lakes. Data showed that TP transported from the hypolimnion by convective winter mixing was the most importantsource of TP for the epilimnion, reaching values more than ten times higher than the external input from the catchment.We found a logarithmic relationship between winter mixing depth (WMD) and epilimnetic TP content in spring. Warmingclimate affects WMD mainly due to its dependence on autumn stratification. Model simulations predict a reduction of averageWMD from 78 (current) to 65 m in 2085 assuming IPCC scenario A2. Other scenarios show similar but smaller changes inthe future. In scenario A2, climate change is predicted to reduce epilimnetic TP concentrations by up to 24% during warmwinters and may consequently introduce significant year-to-year variability in primary productivity.