Biology subjects

Tischer, J.

Publications and source records attributed to Tischer, J..

4 recordsLinked to original sources

bioRxiv2025

Nitrogen and oxygen isotope effects during enzymatic nitrate reduction in vitro and by natural lake water consortia

Nitrate (NO3-) isotope ratios are useful indicators for nitrogen (N)-transformation processes if the associated isotope effects and their environmental controls are well-constrained. The NO3- isotope effects in natural environments may depend on the type of dissimilatory nitrate reductases involved and the degree of isotopic overprinting via NO3- regeneration. We measured the coupled N and oxygen (O) isotope effects of NO3- reduction in anoxic incubation experiments with laboratory cultures (Pseudomonas sp. and Escherichia coli) harboring different nitrate reductase enzymes (Nar and/or Nap) as well as with natural freshwater consortia from Lake Lugano (Switzerland) and Lake La Cruz (Spain). For comparison, isotope effects were also evaluated through coupled N and O isotope measurements in the redox transition zone of Lake Lugano North Basin. Incubation-based Rayleigh model N isotope effects ({varepsilon}N) were variable, ranging from 9 to 30{per thousand}. In comparison, in situ {varepsilon}N values (5 to 14{per thousand}) estimated by the closed system model were lower, likely due to substrate limitation in the water column. Experiments with Pseudomonas sp. and E. coli cultures possessing Nar yielded N isotope effects of similar magnitudes and, consistent with previous data, robust {Delta}{delta}18O:{Delta}{delta}15N enrichment ratios of [~]0.9 - 1.0. Nitrate reduction by cultures possessing solely Nap led to lower {Delta}{delta}18O:{Delta}{delta}15N of [~]0.7. In anoxic incubations of lake water, where the effect of nitrification could be excluded, {Delta}{delta}18O:{Delta}{delta}15N values between 0.6 and 1.0 suggest "community activity" of both Nar and Nap. Interestingly, stimulation of lithotrophic nitrate reduction in incubations with amended NO3- + sulfide resulted in a {Delta}{delta}18O:{Delta}{delta}15N slope of 0.90 {+/-} 0.03 (standard error, SE), indicating Nar as the more dominant nitrate-reducing enzyme. On the contrary, stimulation of organotrophic nitrate reduction in NO3- + acetate amended incubations resulted in a significantly lower slope of 0.72 {+/-} 0.03 SE, suggesting a greater contribution by Nap. In contrast to the nitrate-reduction incubation experiments, the in situ {Delta}{delta}18O:{Delta}{delta}15N value of 1.36 {+/-} 0.14 SE observed in the Lake Lugano water column was unusually high for a freshwater environment, likely reflecting the superimposed effect of NO3- production by nitrification. Our study thus underscores that both variations in activity of Nar versus Nap during nitrate reduction, as well as isotopic overprinting by nitrate regeneration may impact ecosystem NO3- isotope dynamics in natural denitrifying environments.

microbiology↗

bioRxiv2025

Microbial nitrogen removal versus recycling in the redox transition zone of a meromictic lake and its coupling to sulfur

Organotrophic denitrification is an important nitrogen (N) removal process in lakes, but alternative N reduction processes such as lithotrophic sulfur (S)-oxidizing denitrification may be greatly underappreciated. We studied the redox transition zone (RTZ) in the meromictic water column of the North Basin of Lake Lugano (Switzerland) to characterize N transformation pathways coupled to the S and carbon (C) cycles. Incubations with 15N-labeled and unlabeled nitrate showed low denitrification rates and a general limitation of organic electron donors. The most accessible fractions of exported primary production biomass may have been largely consumed in the oxic water column during sedimentation, and did not reach the RTZ at [~]100 m. Conversely, sulfide (H2S) and methane (CH4), major end products of anaerobic degradation of the more recalcitrant organic matter fractions in the sediment, represent a continuous source of energy to the RTZ, fostering the establishment of a community of S- and CH4-dependent nitrate reducers, dominated by Sulfuritalea and Candidatus Methylomirabilis over several years of observation. Anoxic incubation experiments with H2S amendments revealed a strong stimulation of dissimilatory nitrate reduction to ammonium (DNRA), but not denitrification. High relative abundances of the archaeal ammonia oxidizer Candidatus Nitrosopumilus and bacterial nitrifiers indicate intense nitrate regeneration by nitrification in the upper RTZ. The potential interaction between nitrification and S-driven DNRA is unclear. However, their importance in close proximity suggests that, at least under conditions of carbon limitation, N recycling between the nitrate and ammonium pools, predominates over N removal via complete denitrification in the Lake Lugano North Basin.

microbiology↗

bioRxiv2022

Molecular landscape and functional characterization of centrosome amplification in ovarian cancer

High grade serous ovarian carcinoma (HGSOC) is characterised by poor outcome and extreme chromosome instability (CIN). Therapies targeting centrosome amplification (CA), a key mediator of chromosome missegregation and CIN, may have significant clinical utility in HGSOC. However, the prevalence of CA in HGSOC, its relationship to genomic biomarkers of CIN and its potential impact on therapeutic response have not been defined. Using high-throughput multi-regional microscopy on 287 clinical HGSOC tumour tissues and 73 ovarian cancer cell lines, we show that CA through centriole overduplication is a highly recurrent and heterogeneous feature of HGSOC and is strongly associated with CIN and genome subclonality. Cell-based studies showed that high prevalence CA is phenocopied in ovarian cancer cell lines, and that high CA is associated with increased multi-treatment resistance; most notably to paclitaxel which is the most common treatment used in HGSOC. CA in HGSOC may therefore present a potential driver of tumour evolution and a powerful biomarker for response to standard-of-care treatment.

cancer biology↗

bioRxiv2021

Water column stability as an important factor controlling nitrite-dependent anaerobic methane oxidation in stratified lake basins

Anaerobic oxidation of methane (AOM) with nitrate/nitrite as the terminal electron acceptor may play an important role in mitigating methane emissions from lacustrine environments to the atmosphere. We investigated AOM in the water column of two connected but hydrodynamically contrasting basins of a south-alpine lake in Switzerland (Lake Lugano). The North Basin is permanently stratified with year-round anoxic conditions below 120 m water depth, while the South Basin undergoes seasonal stratification with the development of bottom water anoxia during summer. We show that below the redoxcline of the North Basin a substantial fraction of methane was oxidized coupled to nitrite reduction by Candidatus Methylomirabilis. Incubation experiments with 14CH4 and concentrated biomass from showed at least 43-52%-enhanced AOM rates with added nitrate/nitrite as electron acceptor. Multiannual time series data on the population dynamics of Candidatus Methylomirabilis in the North Basin following an exceptional mixing event in 2005/2006 revealed their requirement for lasting stable low redox-conditions to establish. In the South Basin, on the other hand, we did not find molecular evidence for nitrite-dependent methane oxidizing bacteria. Our data suggest that here the dynamic mixing regime with fluctuating redox conditions is not conducive to the development of a stable population of relatively slow-growing Candidatus Methylomirabilis, despite a hydrochemical framework that seems more favorable for nitrite-dependent AOM than in the North Basin. We predict that the importance of N-dependent AOM in freshwater lakes will likely increase in future because of longer thermal stratification periods and reduced mixing caused by global warming.

microbiology↗