The occurrence of oxic methane production (OMP) has been reported for numerous aquatic ecosystems. Its seasonal dynamics and contribution to global methane () emissions remains uncertain, however, due to the lack of measurements constraining the spatial and temporal variability of OMP. In this study, we used data collected over 4 yrs with three types of models to estimate the net production/consumption of () in the surface mixed layer of a eutrophic lake. These models allowed us to assess the lateral contribution and the surface fluxes' variability on estimations. All model results show positive rates suggesting that OMP occurs consistently during the stratified period and was often the dominant source of surface diffusive emissions. In years with monthly data, we observed a consistent pattern of high rates at the start of stratification and a decrease towards the end of the stratified season. Along with the trends and in agreement with recent findings, correlations between vs. Secchi depth, chlorophyll a concentrations and light climate suggest that photoautotrophs play a role in production. Finally, using an isotopic mass balance model, we determined that the stable carbon isotopic signature of in this lake was likely between and , which aligns with published work. Clearly, OMP can be a significant component of an aquatic budget of eutrophic systems, but can vary temporally; thus, temporal variability should be considered when assessing the global contribution of OMP to aquatic emissions, particularly in systems experiencing or threatened by eutrophication.

Studying exoplanet atmospheres is essential for assessing their potential to host liquid water and their capacity to support life (their habitability). Each atmosphere uniquely influences the likelihood of surface liquid water, defining the habitable zone (HZ)—the region around a star where liquid water can exist. However, being within the HZ does not guarantee habitability, as life requires more than just liquid water. In this study, we adopted a two-pronged approach. First, we estimated the surface conditions of planets near the HZ’s inner edge under various atmospheric compositions. By utilizing a 3D climate model, we refined the inner boundaries of the HZ for planets with atmospheres dominated by H2 and CO2 for the first time. Second, we investigated microbial survival in these environments, conducting laboratory experiments on the growth and survival of E. coli K-12, focusing on the impact of different gas compositions. This innovative combination of climate modeling and biological experiments bridges theoretical climate predictions with biological outcomes. Our findings indicate that atmospheric composition significantly affects bacterial growth patterns, highlighting the importance of considering diverse atmospheres in evaluating exoplanet habitability and advancing the search for life beyond Earth.

Contrasting the paradigm that methane is only produced in anoxic conditions, recent discoveries show that oxic methane production (OMP, aka the methane paradox) occurs in oxygenated surface waters worldwide. OMP drivers and their contribution to global methane emissions, however, are not well constrained. In four adjacent pre-alpine lakes, we determine the net methane production rates in oxic surface waters using two mass balance approaches, accounting for methane sources and sinks. We find that OMP occurs in three out of four studied lakes, often as the dominant source of diffusive methane emissions. Correlations of net methane production versus chlorophyll-a, Secchi and surface mixed layer depths suggest a link with photosynthesis and provides an empirical upscaling approach. As OMP is a methane source in direct contact with the atmosphere, a better understanding of its extent and drivers is necessary to constrain the atmospheric methane contribution by inland waters.

EDITORIAL article Front. Environ. Sci., 22 May 2023Sec. Biogeochemical Dynamics Volume 11 - 2023 | https://doi.org/10.3389/fenvs.2023.1218878

Toxic cyanobacteria, such as Planktothrix rubescens, came to dominate temperate lakes in the mid‐to‐late 20th century, as a result of eutrophication. Even after decades of re‐oligotrophication, where phosphorus levels were reduced by 1–2 orders of magnitude, P. rubescens remains present in various lakes. In this study, we examine the persistence and changes of P. rubescens deep chlorophyll maximum (DCM) in Lake Hallwil (Switzerland) over 35 yr of steadily decreasing phosphorous concentrations. Although lake transparency increased and the euphotic layer deepened during this period, the P. rubescens population maximum moved even deeper. It is now found ca. 7.7 m deeper than its shallowest position in the 2000s, and this depth no longer coincides with the depth of maximal water column stability. P. rubescens neutral buoyancy has now driven it beneath the stable metalimnion into the hypolimnion, where buoyancy regulation is restricted due to reduced metabolic activity at low light and low temperature. If P. rubescens DCM continues to deepen each year, it will eventually reach a region of lower stability in the hypolimnion where turbulent conditions are strong enough to disperse the DCM. We also explore the mechanisms that ensure P. rubescens ongoing presence in peri‐alpine lakes despite strong re‐oligotrophication and ongoing climate change. We find that P. rubescens in the lake is mainly sustained by growth during fully mixed conditions in winter, not during stratification in summer. This may contradict another commonly made prediction that periods of longer stratification will promote future blooms of this cyanobacterium.

Since the detection of the first exoplanet orbiting a star like the Sun, the University of Geneva has been at the forefront of exoplanet research. Starting from an extensive expertise in planet detection (with radial velocity), the observatory is also now an important actor in the atmosphere characterization of exoplanets (e.g. Ehrenreich et al. 2020). Today the focus is shifting towards the atmospheric characterization of small temperate planets, such as Proxima-b and the TRAPPIST-1 planets. The university is therefore actively participating to the instruments RISTRETTO@VLT and ANDES@E-ELT which aim at characterizing the atmosphere of Proxima-b (among other goals) using a technique based on high-contrast imaging and high-resolution spectroscopy. One of the objectives of these instruments is to detect biosignatures in the atmosphere of rocky temperate planets. However, to be able to correctly identify a biosignature, one needs to be able to identify false positives. So, one needs to know how the atmosphere interacts with planetary interior, with incoming stellar radiation, and with many different other processes. A multi-disciplinary approach is therefore necessary. Recently, and following the 2019 Nobel prize in physics attributed to Michel Mayor and Didier Queloz for the discovery of 51 Peg b, the University of Geneva decided to create a faculty center: “Centre pour la Vie dans l’Univers” in French or “Center for Life in the Universe” (https://www.unige.ch/sciences/cvu/). The members of the center include experts in astrophysics, geophysics, environmental physics, chemistry, climatology and biology. The center aims at leading interdisciplinary projects on the origin of life on Earth and the search for life in our solar system and in exoplanetary systems to contribute to the world research on fundamental questions: How did life emerge and how did it diversify on Earth? Is the Universe full of life? What is the nature of life? How can we detect life elsewhere than on Earth? Several projects are starting and will start in the near future in the center on the following topics: * The rise of molecular complexity on primitive Earth * Multi-stability of climates and habitability * Evolution under extraterrestrial conditions * The atmosphere as a mirror for geological processes I will present these new interdisciplinary scientific projects, with an increased focus on the second and third ones which are already underway.

Methane oxidation in lakes removes a large portion of methane (CH4). To date, methane oxidation estimates in lakes have often been derived at low spatial resolution in the water column, preventing understanding of the links to the physicochemical gradients in the stratified regions. We applied a mass balance approach with measured dissolved CH4 and sediment CH4 fluxes to derive high-resolution depth profiles of specific CH4 oxidation rates (kox$$ {k}_{\mathrm{ox}} $$) in the water column during the stratified period of a small eutrophic lake (Soppensee, Switzerland). Estimated kox$$ {k}_{\mathrm{ox}} $$ ranged from 0 to 1 d(-1) and the kox$$ {k}_{\mathrm{ox}} $$ profiles agreed well with previous studies, and were also in agreement with rates from concurrent in situ oxidation experiments. A sensitivity analysis revealed that sediment CH4 flux is the largest source of uncertainty when deriving k(ox). Although previous studies have estimated methane oxidation based on delta C-13(CH4), we showed with numerical modeling that delta C-13(CH4) measurements could not be used to resolve the relative contributions of methane oxidation and sediment fluxes to the water column CH4 balance. Exploration of alternative approaches to derive methane oxidation is needed to reveal potentially unknown or misunderstood drivers of methane oxidation in lakes. The presented mass balance approach has the potential to calculate methane oxidation at high vertical resolution and enhance the spatial limitations of established incubation methods. As methane oxidation is responsible for removing most of the produced methane in lakes, it is as important to accurately resolve the key drivers to predict responses to future climate scenarios.

Methane (CH4) from aquatic ecosystems contributes to about half of total global CH4 emissions to the atmosphere. Until recently, aquatic biogenic CH4 production was exclusively attributed to methanogenic archaea living under anoxic or suboxic conditions in sediments, bottom waters, and wetlands. However, evidence for oxic CH4 production (OMP) in freshwater, brackish, and marine habitats is increasing. Possible sources were found to be driven by various planktonic organisms supporting different OMP mechanisms. Surprisingly, submerged macrophytes have been fully ignored in studies on OMP, yet they are key components of littoral zones of ponds, lakes, and coastal systems. High CH4 concentrations in these zones have been attributed to organic substrate production promoting classic methanogenesis in the absence of oxygen. Here, we review existing studies and argue that, similar to terrestrial plants and phytoplankton, macroalgae and submerged macrophytes may directly or indirectly contribute to CH4 formation in oxic waters. We propose several potential direct and indirect mechanisms: (1) direct production of CH4; (2) production of CH4 precursors and facilitation of their bacterial breakdown or chemical conversion; (3) facilitation of classic methanogenesis; and (4) facilitation of CH4 ebullition. As submerged macrophytes occur in many freshwater and marine habitats, they are important in global carbon budgets and can strongly vary in their abundance due to seasonal and boom-bust dynamics. Knowledge on their contribution to OMP is therefore essential to gain a better understanding of spatial and temporal dynamics of CH4 emissions and thus to substantially reduce current uncertainties when estimating global CH4 emissions from aquatic ecosystems.

To determine the drivers of phytoplankton biomass, we collected standardized morphometric, physical, and biological data in 230 lakes across the Mediterranean, Continental, and Boreal climatic zones of the European continent. Multilinear regression models tested on this snapshot of mostly eutrophic lakes (median total phosphorus [TP] = 0.06 and total nitrogen [TN] = 0.7 mg L-1), and its subsets (2 depth types and 3 climatic zones), show that light climate and stratification strength were the most significant explanatory variables for chlorophyll a (Chl a) variance. TN was a significant predictor for phytoplankton biomass for shallow and continental lakes, while TP never appeared as an explanatory variable, suggesting that under high TP, light, which partially controls stratification strength, becomes limiting for phytoplankton development. Mediterranean lakes were the warmest yet most weakly stratified and had significantly less Chl a than Boreal lakes, where the temperature anomaly from the long-term average, during a summer heatwave was the highest (+4 degrees C) and showed a significant, exponential relationship with stratification strength. This European survey represents a summer snapshot of phytoplankton biomass and its drivers, and lends support that light and stratification metrics, which are both affected by climate change, are better predictors for phytoplankton biomass in nutrient-rich lakes than nutrient concentrations and surface temperature.

Climate warming is causing changes in the physics of deep lakes, such as longer summer stratification, increased water column stability, reduced ice cover, and a shallower depth of winter overturns. An ultimate consequence of warming would be a transition to a different mixing regime. Here we investigate the role of physical, chemical, and biological feedback mechanisms that unfold during a shift in mixing regime, and whether these feedbacks could prompt and stabilise the new regime. Although climate, interannual temperature variation, and lake morphometry are the main determinants of a mixing regime, when climate change causes shifts in mixing regime, internal feedback mechanisms may gain in importance and modify lake ecosystem functioning. We review the role of these feedbacks in three mixing regime shifts: from polymictic to seasonally stratified, from dimictic to monomictic, and from holomictic to oligomictic or meromictic. Polymictic lakes of intermediate depth (c. 3-10 m mean depth) could experience seasonal stratification if a stratification event triggers phytoplankton blooms or dissolved organic matter release, reducing transparency and therefore further heating the surface layer. However, this feedback is only likely to have influence in small and clear lakes, it would be easily disturbed by weather conditions, and the resulting stratified state does not remain stable in the long term, as stratification is lost in winter. The ice-albedo feedback might cause an accelerated shift from ice-covered (dimictic) to ice-free (monomictic) winters in sufficiently deep (mean depth 50 m or more) lakes, where temperature memory is carried over from one winter to the next. Nevertheless, there is an ongoing debate into whether this process can persist during natural weather variations and overcome self-stabilising mechanisms such as thermal insulation by snow. The majority of studies suggest that a gradual transition from dimictic to monomictic is more likely than an abrupt transition. A shift from a holomictic to a meromictic regime can occur if anoxia is triggered by incomplete mixing and an increase in deep-water density-through the accumulation of solutes-exceeds a density decrease by hypolimnetic warming. A shift to meromixis would strongly alter the biology of a lake and might be difficult to reverse. If solutes accumulate only minimally in the hypolimnion, an oligomictic regime is formed, in which years with complete and incomplete mixing alternate. Understanding the importance of feedback mechanisms and the role of biogeochemistry when lakes shift in mixing regime could lead to a better understanding of how climate change affects lake ecosystems.

Worldwide, freshwater environments are impacted by inputs of nutrients and dissolved organic matter from human activities. Yet, the recovery of aquatic systems is usually focused only on nutrient management. In our work, we presented the case of an urban and hypereutrophic environment (Pampulha reservoir, Belo Horizonte, Brazil) that receives discharges from several streams and was treated with lanthanum modified bentonite (Phoslock®) and microbial bioremediation (Enzilimp®). Our goals were to evaluate whether the treatment could improve the water quality and characterize the spatiotemporal variation of dissolved organic matter sources and indices according to absorbance and fluorescence measurements from the reservoir and streams post-application months (2018). In our results, the reservoir showed a relative decrease in its phosphorus concentration compared to data from before the treatment. On the other hand, carbon concentrations reached expressive values in the post-application months following a similar pattern found in the streams. Our data showed that the reservoir's high resistance in its hypereutrophic condition was related to the elevated loading of external inputs coming from the streams. The parallel factor analysis (PARAFAC) identified four main carbon sources, two of them being potential tracers of organic pollution in the Pampulha reservoir and watershed, together with absorbance and fluorescence indices. Our findings suggest that carbon parameters can be essential tools to provide adequate monitoring and optimization of water recovery attempts in complex, polluted environments.

Differentiating microbial, anthropogenic, and thermogenic sources of carbon dioxide (CO2) and methane (CH4) in background air is an important element of understanding upper ocean ecosystem processes. Concentrations of these gases alone are not dispositive indicators of processes, so additional diagnostic parameters including meteorological data, related gas species measurements, and isotopic values can allow researchers to better investigate processes. Here we present data from the Fleur de Passion sailing research vessel which traveled from Dakar, Senegal to the Azores, and to Northwest Spain between early April and October of 2019 as part of the larger Ocean Mapping Expedition by the Geneva-based NPO Fondation Pacifique. The 33-meter-long ketch research vessel carried as part of its instrument suite a Picarro G2201-i high precision gas analyzer, measuring concentrations and ẟ13C values of CO2 and CH4. The high precision data collected by the isotopic carbon analyzer (which are being sampled as part of the University of Geneva’s Winds of Change program) allow for subtle differentiation of modalities separated by a per mil or less, signals that could be lost by infrequent flask measurements or low-precision analyzers. We present findings from this expedition, as well as a brief description of future efforts to measure underway dissolved gases.

Abstract Here, we investigate the importance of net CH4 production and emissions in the carbon (C) budget of a small productive lake by monitoring CH4, CO2, and O2 for two consecutive years. During the study period, the lake was mostly a net emitter of both CH4 and CO2, while showing positive net ecosystem production. The analyses suggest that during the whole study period, 32% ± 26% of C produced by net ecosystem production was ultimately converted to CH4 and emitted to the atmosphere. When converted to global warming potential, CH4 emission (in CO2 equivalents) was about 3–10 times higher than CO2 removal from in‐lake net ecosystem production over 100‐yr and 20‐yr time frames, respectively. Although more work in similar systems is needed to generalize these findings, our results provide evidence of the important greenhouse gas imbalance in human‐impacted aquatic systems.

Controlling factors of CH4 ebullitive flux are still poorly understood within daily timescales in aquatic environments. Here, we explored if Chaoborus larvae bioturbation can influence the timing of daily CH4 ebullitive flux and how this emission pathway is affected considering direct observations in a tropical reservoir. For this, we collected CH4 bubbles on two different days and three sampling periods: 1–4 pm, 4–7 pm and 7–10 pm. In the middle of each period, we collected Chaoborus larvae, chemical, and environmental variables and acoustically surveyed the underwater environment. We found similar temporal patterns of CH4 emissions between the periods in both days. The intervals of higher CH4 ebullitive flux (1–4 pm and 4–7 pm) appeared to correspond with the exit period of Chaoborus larvae from sediment (2:30 pm to 5:30 pm). Furthermore, the density of benthic larvae, a proxy of the bioturbation intensity, was the only variable that significantly correlated with the ebullitive flux, presenting a high and positive relation. Our findings suggest that the stochastic nature of the CH4 ebullitive flux may not be absolute truth, and this emission pathway might be enhanced daily by a Chaoborus-driven physical disturbance.

The upper reaches of the Swan River estuary (Perth, Australia) has a history of eutrophication-related oxygen depletion, which has contributed to poor water quality and fish deaths. To alleviate hypoxic conditions, a trial side-stream supersaturation (SSS) oxygenation plant was established at Guildford (39 km upstream of the estuary mouth) in 2009. After notable success, a second plant was constructed at Caversham (44.2 km upstream of the estuary mouth) in 2011. Oxygenation plants have more commonly been used to treat deep, freshwater lakes and reservoirs and this is a pioneer application to a shallow estuary. We report on the effect of the Caversham plant on water and sediment condition over a 12-day experiment: before, during and post-plant operation. We monitored several physical and chemical parameters collected from daily longitudinal transects, moored continuous loggers, an acoustic Doppler current profiler and an in-situ sediment microprofiler. Oxygenation immediately improved dissolved oxygen concentrations in the water column and the distance over which the effect was observed was strongly influenced by the hydrodynamics of the estuary. After five days of oxygenation, water column dissolved oxygen had increased over a distance in excess of 11.5 km. In addition, oxygenation improved dissolved oxygen concentrations at the sediment-water interface, thereby increasing oxygen fluxes into the sediment. Ultimately, artificially oxygenating the Swan River estuary provides a refuge for fauna while facilitating aerobic decomposition of organic matter and enhancing nutrient cycling at the sediment-water interface. In light of the increasingly critical state of urbanised estuaries world-wide, results from this study highlights 1) the effectiveness of oxygenation in improving water quality and its potential for facilitating ecosystem restoration, and 2) the diversity of environments in which artificial oxygenation can be applied.

Recent discovery of oxic methane production in sea and lake waters, as well as wetlands, demands re-thinking of the global methane cycle and re-assessment of the contribution of oxic waters to atmospheric methane emission. Here we analysed system-wide sources and sinks of surface-water methane in a temperate lake. Using a mass balance analysis, we show that internal methane production in well-oxygenated surface water is an important source for surface-water methane during the stratified period. Combining our results and literature reports, oxic methane contribution to emission follows a predictive function of littoral sediment area and surface mixed layer volume. The contribution of oxic methane source(s) is predicted to increase with lake size, accounting for the majority (>50%) of surface methane emission for lakes with surface areas >1 km(2).

Methane (CH4), a potent greenhouse gas, is produced in and emitted from lakes at globally significant rates. The drivers controlling the proportion of produced CH4 that will reach the atmosphere, however, are still not well understood. We sampled a small eutrophic lake (Soppensee, Switzerland) in 2016-2017 for CH4 concentrations profiles and emissions, combined with water column hydrodynamics to investigate the fate of CH4 produced in hypolimnetic sediments. Using a mass balance approach for the periods between April and October of both years, net CH4 production rates in hypolimnetic sediments ranged between 11.4 and 17.7 mmol m(-2) d(-1), of which 66-88% was stored in the hypolimnion, 13-27% was diffused to the epilimnion, and 6-7% left the sediments via ebullition. Combining these results with a process-based model we show that water column turbulent diffusivity (K-z) had a major influence on the fate of produced CH4 in the sediments, where higher K-z values potentially lead to greater proportion being oxidized and lower K-z lead to a greater proportion being stored. During fall when the water column mixes, we found that a greater proportion of stored CH4 is emitted if the lake mixes rapidly, whereas a greater proportion will be oxidized if the water column mixes more gradually. This work highlights the central role of lake hydrodynamics in regulating CH4 dynamics and further suggests the potential for CH4 production and emissions to be sensitive to climate-driven alterations in lake mixing regimes and stratification.

This paper provides a novel report of methane hydrates rising from bottom sediments to the surface of Lake Baikal, validated by photo and video records. The ascent of hydrates in the water column was confirmed by hydroacoustic data showing rising objects with velocities significantly exceeding the typical speeds (18–25 cm s−1) of gas bubbles. Mathematical modelling along with velocity and depth estimates of the presumed methane hydrates coincided with values observed from echograms. Modelling results also showed that a methane hydrate fragment with initial radius of 2.5 cm or greater could reach the surface of Lake Baikal given summer water column temperature conditions. Results further show that while methane bubbles released from the deep sedimentary reservoir would dissolve in the Lake Baikal water column, transport in hydrate form is not only viable but may represent a previously overlooked source of surface methane with subsequent emissions to the atmosphere. Methane hydrates captured within the ice cover may also cause the formation of unique ice structures and morphologies observed around Lake Baikal. Sampling of these ice structures detected methane content that exceeded concentrations measured in surrounding ice and from the atmosphere demonstrating a link with the methane transport processes described here.

Atmospheric methane (CH4) concentrations have more than doubled in the past similar to 250 yr, although the sources of this potent greenhouse gas remain poorly constrained. Freshwaters contribute similar to 20% of natural CH4 emissions, about half attributed to ebullition. Estimates remain uncertain as ebullition is stochastic, making measurements difficult, time consuming, and costly with current methods (e.g., floating chambers, funnel gas traps, and hydroacoustics). We present a novel approach to quantify basin-wide hypolimnetic CH4 fluxes at the sediment level based on measurements of bubble gas content and modeling of dissolved pore-water gases. We show that the relative ebullition flux pathway can be resolved by knowledge of only bubble gas content. As sediment CH4 production, diffusion, and ebullition are interrelated, the addition of a second observation allows closing the entire sediment CH4 balance. Such measurements could include bubble formation depth, sediment diffusive fluxes, ebullition, sediment CH4 production, or the hypolimnetic CH4 mass balance. The measurement of bubble gas content is particularly useful for identifying local ebullitive hotspots and integrating spatial heterogeneity of CH4 fluxes. Our results further revealed the crucial effect of water column depth, production rates, and hypolimnetic dissolved CH4 concentrations on sediment CH4 dynamics. Although we apply the model to cohesive sediments in an anoxic hypolimnion, the model can be applied to shallow, oxic settings by altering the CH4 production rate curve to account for oxidation. Utilizing our approach will provide a deeper understanding of in-lake CH4 budgets, and thus improve CH4 emission estimates from inland freshwaters at the regional and global scales.