Biological ice nucleating particles at tropospheric cloud height

Airborne ice nucleating particles (INPs) promote the freezing of cloud droplets, which is relevant for the radiative properties of clouds and for the development of precipitation. A quantitative assessment of the impact of INPs on cloud processes and on their responsiveness to climate and land use change is still missing. This is particularly true for INPs of biological origin. They are made of molecules produced by bacteria, fungi, plants, lichens, which promote the freezing of droplets at temperatures above -15 °C. Bottom-up modelling studies based on the release of ice nucleation active cells from crops and plants have excluded any chance for biological INPs to impact climate. Nevertheless, recent observations point at the ubiquity across ecosystems of species capable of producing INPs and at the fact that such INPs can be released from cells and maintain their activity for instance when linked to soil particles. Here we employed a top-down approach to improve our understanding of the variability of biological INPs in precipitation. 16 sampling campaign were organised between 2012 and 2014 and over 100 precipitation samples were collected at the High Altitude Research Station Jungfraujoch (3580 m a.s.l., Switzerland). They have been analysed for their content in INPs active at moderate supercooling directly in field with our new immersion freezing apparatus LINDA. Several environmental parameters have been studied to derive more information on the most relevant factors responsible for the variability of INPs. The abundance of bacterial cells and the presence of the nucleation active plant pathogen bacterium Pseudomonas syringae have been determined as well, to know more on the nature of biological INPs in precipitation. By means of stable water isotopes, we demonstrate that INPs are rapidly and selectively removed by precipitating clouds. Focusing on INPs active at -8 °C (INPs-8), their concentrations varied between 0.21 and 434 INPs-8 mL-1. Up to 75% of this large temporal variability can be modelled and predicted by multiple linear regression models based on the combination of a few environmental parameters. These models point at the interaction of “source” (uptake) and “sink” (removal) processes as relevant to determine the variability of INPs-8. Large abundance of INPs-8 can be best expected with a coincidence of high wind speed and little precipitation lost from an air mass prior to sampling. Bacterial cells present more constant concentrations than INPs, from 2.4·103 to 6.8·104 cells mL-1, with their numbers increasing mainly under high wind speed. INPs are more efficiently removed than bacterial cells by precipitation, which implies a larger variability, a shorter residence time in the atmosphere and shorter lengths of dispersal for INPs rather than for bacterial cells. P. syringae has been successfully isolated at high-altitude and its presence seems to be influenced by uptake and removal processes, as it happens for INPs-8. This study constitutes a strong improvement of our understanding of the abundance, variability and nature of biological INPs in precipitation and points at the potential for this group of INPs to impact cloud processes. In fact, a coincidence of high wind speed and first precipitation often occurs at the passage of a front, where the meteorological conditions are also favourable to precipitation. This can be the ideal and frequent context where to expect large numbers of INPs-8 and to study their effects on cloud processes. Furthermore, bacterial cells can contribute to the number of INPs-8, but a large fraction of INPs-8 is potentially due to cellular fragments and macromolecules, both freely floating and attached to mineral and soil dust. This multiplies the possibility for biological INPs to be released and be present in the atmosphere

Carbon and methane cycling in arsenic-contaminated aquifers

Geogenic arsenic (As) contamination of groundwater is a health threat to millions of people worldwide, particularly in alluvial regions of South and Southeast Asia. Mitigation measures are often hindered by high heterogeneities in As concentrations, the cause(s) of which are elusive. Here we used a comprehensive suite of stable isotope analyses and hydrogeochemical parameters to shed light on the mechanisms in a typical high-As Holocene aquifer near Hanoi where groundwater is advected to a low-As Pleistocene aquifer. Carbon isotope signatures (δ$^{13}$C-CH$_{4}$, δ$^{13}$C-DOC, δ$^{13}$C-DIC) provided evidence that fermentation, methanogenesis and methanotrophy are actively contributing to the As heterogeneity. Methanogenesis occurred concurrently where As levels are high (>200 µg/L) and DOC-enriched aquitard pore water infiltrates into the aquifer. Along the flowpath to the Holocene/Pleistocene aquifer transition, methane oxidation causes a strong shift in δ$^{13}$C-CH$_{4}$ from -87‰ to +47‰, indicating high reactivity. These findings demonstrate a previously overlooked role of methane cycling and DOC infiltration in high-As aquifers

Spatial and temporal evolution of groundwater arsenic contamination in the Red River delta, Vietnam: Interplay of mobilisation and retardation processes

Geogenic arsenic (As) contamination of groundwater poses a major threat to global health, particularly in Asia. To mitigate this exposure, groundwater is increasingly extracted from low-As Pleistocene aquifers. This, however, disturbs groundwater flow and potentially draws high-As groundwater into low-As aquifers. Here we report a detailed characterisation of the Van Phuc aquifer in the Red River Delta region, Vietnam, where high-As groundwater from a Holocene aquifer is being drawn into a low-As Pleistocene aquifer. This study includes data from eight years (2010–2017) of groundwater observations to develop an understanding of the spatial and temporal evolution of the redox status and groundwater hydrochemistry. Arsenic concentrations were highly variable (0.5–510 μg/L) over spatial scales of <200 m. Five hydro(geo)chemical zones (indicated as A to E) were identified in the aquifer, each associated with specific As mobilisation and retardation processes. At the riverbank (zone A), As is mobilised from freshly deposited sediments where Fe(III)-reducing conditions occur. Arsenic is then transported across the Holocene aquifer (zone B), where the vertical intrusion of evaporative water, likely enriched in dissolved organic matter, promotes methanogenic conditions and further release of As (zone C). In the redox transition zone at the boundary of the two aquifers (zone D), groundwater arsenic concentrations decrease by sorption and incorporations onto Fe(II) carbonates and Fe(II)/Fe(III) (oxyhydr)oxides under reducing conditions. The sorption/incorporation of As onto Fe(III) minerals at the redox transition and in the Mn(IV)-reducing Pleistocene aquifer (zone E) has consistently kept As concentrations below 10 μg/L for the studied period of 2010–2017, and the location of the redox transition zone does not appear to have propagated significantly. Yet, the largest temporal hydrochemical changes were found in the Pleistocene aquifer caused by groundwater advection from the Holocene aquifer. This is critical and calls for detailed investigations

Freezing nucleation apparatus puts new slant on study of biological ice nucleators in precipitation

For decades, drop-freezing instruments have contributed to a better understanding of biological ice nucleation and its likely implications for cloud and precipitation development. Yet, current instruments have limitations. Drops analysed on a cold stage are subject to evaporation and potential contamination. The use of closed tubes provides a partial solution to these problems, but freezing events are still difficult to be clearly detected. Here, we present a new apparatus where freezing in closed tubes is detected automatically by a change in light transmission upon ice development, caused by the formation of air bubbles and crystal facets that scatter light. Risks of contamination and introduction of biases linked to detecting the freezing temperature of a sample are then minimized. To illustrate the performance of the new apparatus we show initial results of two assays with snow samples. In one, we repeatedly analysed the sample (208 tubes) over the course of a month with storage at +4 °C, during which evidence for biological ice nucleation activity emerged through an increase in the number of ice nucleators active around −4 °C. In the second assay, we indicate the possibility of increasingly isolating a single ice nucleator from a precipitation sample, potentially determining the nature of a particle responsible for a nucleation activity measured directly in the sample. These two seminal approaches highlight the relevance of this handy apparatus for providing new points of view in biological ice nucleation research

Clues that decaying leaves enrich Arctic air with ice nucleating particles

Decaying leaves from Arctic regions have previously been reported to produce large numbers of ice nucleating particles (IN). Their atmospheric relevance is unclear. Our initial observations at a coastal mountain observatory in northern Norway reveal a tripling in concentrations of IN active at −15 °C (IN-15) in oceanic air after about one day of passage over land (from 1.7 and 4.9 IN-15 m−3, to 9.6 and 12.2 IN-15 m−3). Analysis of leaf litter collected near the observatory supports the earlier report of numerous IN associated with leaf litter on the ground (2 ⋅ 102 IN-15 μg−1 litter particles < 5 μm). Together, both findings suggest that decaying leaves are a strong emission source of IN to the Arctic boundary layer

Coupling flow analysis with geochemical and microbiological analyses to assess the bioremediation feasibility of a contaminated aquifer

An integrated approach combining classic and molecular microbiological methods, “in vitro” bioremediation assays and groundwater numerical modeling, has been established to identify optimized solutions for remediating aquifers contaminated with organic pollutants. Bacteria have been isolated from an aquifer contaminated with toluene and methyl tert-butyl ether (MTBE), selected for their growth with contaminants as a sole carbon source and identified through 16S rDNA partial sequencing. Successive biodegradation laboratory tests have been performed to determine which chemical conditions were more appropriate for the isolated bacteria to more efficiently oxidize toluene and MTBE. A groundwater model was created using FEFLOW code first to determine the movement of the plume front and second to simulate the impact of the biodegradation processes along the groundwater flow directions based on the bioremediation rates obtained in the laboratory. The results show that this innovative and interdisciplinary model can be used to assist in developing monitoring and remediation plans for cleaning up complex contaminated groundwater sites. This approach successfully combines the identification of the optimum biogeochemical conditions for bacterial biodegradation to occur with the predictability of the development of the process over time, ensuring decisive support in the management of contaminated sites

Noble gases in aquitard provide insight into underlying subsurface stratigraphy and free gas formation

Abstract Biogeochemical gas production resulting in free gas phase formation can severely affect groundwater and solute transport in aquifers. Such gas–water interactions are important in aquifers affected by geogenic As, which are commonly associated with biogeochemical CH4 production. Additionally, the influence of aquitards on As concentrations in contaminated aquifers has recently been challenged. These observations prompted the analysis through a heterogeneous aquitard overlying a high CH4−gas‐producing zone of an As‐contaminated aquifer. A sediment core taken through the aquitard was analyzed for noble gases to assess how the aquitard physically contributes to the underlying gas production. Results reveal that the aquitard pore space is unsaturated in two separate layers resulting in hanging pore water constrained by an air‐like gas phase. This interlayering of unsaturated and saturated zones identifies the aquitard's stratigraphy as key in determining hydrostatic pressure—a main control of free gas formation (i.e., CH4) in the underlying aquifer. The partly unsaturated conditions reduce the hydrostatic pressure by 30% compared with fully saturated conditions. To our knowledge, this is the first study applying noble gases to examine the influence of an aquitards physical state on gas production in an underlying aquifer. Further, such partly unsaturated sediment layers of low conductivity might provide preferential pathways for periodic water flow, fostering aquitard–aquifer solute transport. Groundwater samples additionally collected throughout the study site confirm more widespread degassing than previously reported. Up to 90% of the expected atmospheric noble gas concentrations is lost from groundwater immediately below the investigated sediment core

Noble gases in aquitard provide insight into underlying subsurface stratigraphy and free gas formation

Biogeochemical gas production resulting in free gas phase formation can severely affect groundwater and solute transport in aquifers. Such gas-water interactions are important in aquifers affected by geogenic As, which are commonly associated with biogeochemical CH4 production. Additionally, the influence of aquitards on As concentrations in contaminated aquifers has recently been challenged. These observations prompted the analysis through a heterogeneous aquitard overlying a high CH4-gas-producing zone of an As-contaminated aquifer. A sediment core taken through the aquitard was analyzed for noble gases to assess how the aquitard physically contributes to the underlying gas production. Results reveal that the aquitard pore space is unsaturated in two separate layers resulting in hanging pore water constrained by an air-like gas phase. This interlayering of unsaturated and saturated zones identifies the aquitard's stratigraphy as key in determining hydrostatic pressure-a main control of free gas formation (i.e., CH4) in the underlying aquifer. The partly unsaturated conditions reduce the hydrostatic pressure by 30% compared with fully saturated conditions. To our knowledge, this is the first study applying noble gases to examine the influence of an aquitards physical state on gas production in an underlying aquifer. Further, such partly unsaturated sediment layers of low conductivity might provide preferential pathways for periodic water flow, fostering aquitard-aquifer solute transport. Groundwater samples additionally collected throughout the study site confirm more widespread degassing than previously reported. Up to 90% of the expected atmospheric noble gas concentrations is lost from groundwater immediately below the investigated sediment core.ISSN:1539-166

Ice nucleators, bacterial cells and Pseudomonas syringae in precipitation at Jungfraujoch

Ice nucleation is a means by which the deposition of an airborne microorganism can be accelerated under favourable meteorological conditions. Analysis of 56 snow samples collected at the high-altitude observatory Jungfraujoch (3580 m a.s.l.) revealed an order-of-magnitude-larger dynamic range of ice-nucleating particles active at −8 °C (INPs−8) compared to the total number of bacterial cells (of which on average 60 % was alive). This indicates a shorter atmospheric residence time for INPs−8. Furthermore, concentrations of INPs−8 decreased much faster, with an increasing fraction of water precipitated from the air mass prior to sampling, than the number of total bacterial cells. Nevertheless, at high wind speeds (> 50 km h−1) the ratio of INPs−8 to total bacterial cells largely remained in a range between 10−2 and 10−3, independent of prior precipitation, likely because of recent injections of particles in regions upwind. Based on our field observations, we conclude that ice nucleators travel shorter legs of distance with the atmospheric water cycle than the majority of bacterial cells. A prominent ice-nucleating bacterium, Pseudomonas syringae, has been previously supposed to benefit from this behaviour as a means to spread via the atmosphere and to colonise new host plants. Therefore, we targeted this bacterium with a selective cultivation approach. P. syringae was successfully isolated for the first time at such an altitude in 3 of 13 samples analysed. Colony-forming units of this species constituted a minor fraction (10−4) of the numbers of INPs−8 in these samples. Overall, our findings expand the geographic range of habitats where this bacterium has been found and corroborate theories on its robustness in the atmosphere and its propensity to spread to colonise new habitats

Wood Species of Armenia and Scientific Bases of Introduction thereof

Available from VNTIC / VNTIC - Scientific & Technical Information Centre of RussiaSIGLERURussian Federatio