Changes in bumblebee queen gut microbiotas during and after overwintering diapause.

Bumblebees are key pollinators with gut microbiotas that support host health. After bumblebee queens undergo winter diapause, which occurs before spring colony establishment, their gut microbiotas are disturbed, but little is known about community dynamics during diapause itself. Queen gut microbiotas also help seed worker microbiotas, so it is important that they recover post-diapause to a typical community structure, a process that may be impeded by pesticide exposure. We examined how bumblebee queen gut microbiota community structure and metabolic potential shift during and after winter diapause, and whether post-diapause recovery is affected by pesticide exposure. To do so, we placed commercial Bombus impatiens queens into diapause, euthanizing them at 0, 2 and 4 months of diapause. Additionally, we allowed some queens to recover from diapause for 1 week before euthanasia, exposing half to the common herbicide glyphosate. Using whole-community, shotgun metagenomic sequencing, we found that core bee gut phylotypes dominated queen gut microbiotas before, during and after diapause, but that two phylotypes, Schmidhempelia and Snodgrassella, ceased to be detected during late diapause and recovery. Despite fluctuations in taxonomic community structure, metabolic potential remained constant through diapause and recovery. Also, glyphosate exposure did not affect post-diapause microbiota recovery. However, metagenomic assembly quality and our ability to detect microbial taxa and metabolic pathways declined alongside microbial abundance, which was substantially reduced during diapause. Our study offers new insights into how bumblebee queen gut microbiotas change taxonomically and functionally during a key life stage and provides guidance for future microbiota studies in diapausing bumblebees.

Exposure to a fungicide for a field-realistic duration does not alter bumble bee fecal microbiota structure.

Social bees are frequently exposed to pesticides when foraging on nectar and pollen. Recent research has shown that pesticide exposure not only impacts social bee host health but can also alter the community structure of social bee gut microbiotas. However, most research on pesticide-bee gut microbiota interactions has been conducted in honey bees; bumble bees, native North American pollinators, have received less attention and, due to differences in their ecology, may be exposed to certain pesticides for shorter durations than honey bees. Here, we examine how exposure to the fungicide chlorothalonil for a short, field-realistic duration alters bumble bee fecal microbiotas (used as a proxy for gut microbiotas) and host performance. We expose small groups of Bombus impatiens workers (microcolonies) to field-realistic chlorothalonil concentrations for 5 days, track changes in fecal microbiotas during the exposure period and a recovery period, and compare microcolony offspring production between treatments at the end of the experiment. We also assess the use of fecal microbiotas as a gut microbiota proxy by comparing community structures of fecal and gut microbiotas. We find that chlorothalonil exposure for a short duration does not alter bumble bee fecal microbiota structure or affect microcolony production at any concentration but that fecal and gut microbiotas differ significantly in community structure. Our results show that, at least when exposure durations are brief and unaccompanied by other stressors, bumble bee microbiotas are resilient to fungicide exposure. Additionally, our work highlights the importance of sampling gut microbiotas directly, when possible.IMPORTANCEWith global pesticide use expected to increase in the coming decades, studies on how pesticides affect the health and performance of animals, including and perhaps especially pollinators, will be crucial to minimize negative environmental impacts of pesticides in agriculture. Here, we find no effect of exposure to chlorothalonil for a short, field-realistic period on bumble bee fecal microbiota community structure or microcolony production regardless of pesticide concentration. Our results can help inform pesticide use practices to minimize negative environmental impacts on the health and fitness of bumble bees, which are key native, commercial pollinators in North America. We also find that concurrently sampled bumble bee fecal and gut microbiotas contain similar microbes but differ from one another in community structure and consequently suggest that using fecal microbiotas as a proxy for gut microbiotas be done cautiously; this result contributes to our understanding of proxy use in gut microbiota research.

The divergent effects of nitrate and ammonium application on mercury methylation, demethylation, and reduction in flooded paddy slurries.

The production of methylmercury (MeHg) in flooded paddy fields determines its accumulation in rice grains; this, in turn, results in MeHg exposure risks for not only rice-eating humans but also wildlife. Nitrogen (N) fertilizers have been widely applied in rice cultivation fields to supply essential nutrients. However, the effects of N fertilizer addition on mercury (Hg) transformations are not unclear. This limits our understanding of MeHg formation in rice paddy ecosystems. In this study, we spiked three Hg tracers (200HgII, Me198Hg, and 202Hg0) in paddy slurries fertilized with urea, ammonium, and nitrate. The influences of N fertilization on Hg methylation, demethylation, and reduction and the underlying mechanisms were elucidated. The results revealed that dissimilatory nitrate reduction was the dominant process in the incubated paddy slurries. Nitrate addition inhibited HgII reduction, HgII methylation, and MeHg demethylation. Competition between nitrates and other electron acceptors (e.g., HgII, sulfate, or carbon dioxide) under dark conditions was the mechanism underlying nitrate-regulated Hg transformation. Ammonium and urea additions promoted HgII reduction, and anaerobic ammonium oxidation coupled with HgII reduction (Hgammox) was likely the reason. This work highlighted that nitrate addition not only inhibited HgII methylation but also reduced the demethylation of MeHg and therefore may generate more accumulation of MeHg in the incubated paddy slurries. Findings from this study link the biogeochemical cycling of N and Hg and provide crucial knowledge for assessing Hg risks in intermittently flooded wetland ecosystems.

Mining-impacted rice paddies select for Archaeal methylators and reveal a putative (Archaeal) regulator of mercury methylation.

Methylmercury (MeHg) is a microbially produced neurotoxin derived from inorganic mercury (Hg), which accumulation in rice represents a major health concern to humans. However, the microbial control of MeHg dynamics in the environment remains elusive. Here, leveraging three rice paddy fields with distinct concentrations of Hg (Total Hg (THg): 0.21-513 mg kg-1 dry wt. soil; MeHg: 1.21-6.82 ng g-1 dry wt. soil), we resorted to metagenomics to determine the microbial determinants involved in MeHg production under contrasted contamination settings. We show that Hg methylating Archaea, along with methane-cycling genes, were enriched in severely contaminated paddy soils. Metagenome-resolved Genomes of novel putative Hg methylators belonging to Nitrospinota (UBA7883), with poorly resolved taxonomy despite high completeness, showed evidence of facultative anaerobic metabolism and adaptations to fluctuating redox potential. Furthermore, we found evidence of environmental filtering effects that influenced the phylogenies of not only hgcA genes under different THg concentrations, but also of two housekeeping genes, rpoB and glnA, highlighting the need for further experimental validation of whether THg drives the evolution of hgcAB. Finally, assessment of the genomic environment surrounding hgcAB suggests that this gene pair may be regulated by an archaeal toxin-antitoxin (TA) system, instead of the more frequently found arsR-like genes in bacterial methylators. This suggests the presence of distinct hgcAB regulation systems in bacteria and archaea. Our results support the emerging role of Archaea in MeHg cycling under mining-impacted environments and shed light on the differential control of the expression of genes involved in MeHg formation between Archaea and Bacteria.

Mercury and Sulfur Redox Cycling Affect Methylmercury Levels in Rice Paddy Soils across a Contamination Gradient.

Methylmercury (MeHg) contamination in rice via paddy soils is an emerging global environmental issue. An understanding of mercury (Hg) transformation processes in paddy soils is urgently needed in order to control Hg contamination of human food and related health impacts. Sulfur (S)-regulated Hg transformation is one important process that controls Hg cycling in agricultural fields. In this study, Hg transformation processes, such as methylation, demethylation, oxidation, and reduction, and their responses to S input (sulfate and thiosulfate) in paddy soils with a Hg contamination gradient were elucidated simultaneously using a multi-compound-specific isotope labeling technique (200HgII, Me198Hg, and 202Hg0). In addition to HgII methylation and MeHg demethylation, this study revealed that microbially mediated reduction of HgII, methylation of Hg0, and oxidative demethylation-reduction of MeHg occurred under dark conditions; these processes served to transform Hg between different species (Hg0, HgII, and MeHg) in flooded paddy soils. Rapid redox recycling of Hg species contributed to Hg speciation resetting, which promoted the transformation between Hg0 and MeHg by generating bioavailable HgII for fuel methylation. Sulfur input also likely affected the microbial community structure and functional profile of HgII methylators and, therefore, influenced HgII methylation. The findings of this study contribute to our understanding of Hg transformation processes in paddy soils and provide much-needed knowledge for assessing Hg risks in hydrological fluctuation-regulated ecosystems.

Global change effects on biogeochemical mercury cycling.

Past and present anthropogenic mercury (Hg) release to ecosystems causes neurotoxicity and cardiovascular disease in humans with an estimated economic cost of $117 billion USD annually. Humans are primarily exposed to Hg via the consumption of contaminated freshwater and marine fish. The UNEP Minamata Convention on Hg aims to curb Hg release to the environment and is accompanied by global Hg monitoring efforts to track its success. The biogeochemical Hg cycle is a complex cascade of release, dispersal, transformation and bio-uptake processes that link Hg sources to Hg exposure. Global change interacts with the Hg cycle by impacting the physical, biogeochemical and ecological factors that control these processes. In this review we examine how global change such as biome shifts, deforestation, permafrost thaw or ocean stratification will alter Hg cycling and exposure. Based on past declines in Hg release and environmental levels, we expect that future policy impacts should be distinguishable from global change effects at the regional and global scales.

Multi-compartmental model of glymphatic clearance of solutes in brain tissue.

The glymphatic system is the subject of numerous pieces of research in biology. Mathematical modelling plays a considerable role in this field since it can indicate the possible physical effects of this system and validate the biologists' hypotheses. The available mathematical models that describe the system at the scale of the brain (i.e. the macroscopic scale) are often solely based on the diffusion equation and do not consider the fine structures formed by the perivascular spaces. We therefore propose a mathematical model representing the time and space evolution of a mixture flowing through multiple compartments of the brain. We adopt a macroscopic point of view in which the compartments are all present at any point in space. The equations system is composed of two coupled equations for each compartment: One equation for the pressure of a fluid and one for the mass concentration of a solute. The fluid and solute can move from one compartment to another according to certain membrane conditions modelled by transfer functions. We propose to apply this new modelling framework to the clearance of 14C-inulin from the rat brain.

How arsenic contamination influences downslope wetland plant and microbial community structure and function.

Mine tailings are prevalent worldwide and can adversely impact adjacent ecosystems, including wetlands. This study investigated the impact of gold (Au) mine tailings contamination on peatland soil and pore water geochemistry, vegetation and microbial communities, and microbial carbon (C) cycling. Maximum arsenic (As) concentrations in peat and pore water reached 20,137 mg kg-1 and 16,730 µg L-1, respectively, but decreased by two orders of magnitude along a 128 m gradient extending from the tailings into the wetland. Carbon and other macronutrient (N, P, K) concentrations in peat and pore water significantly increased with distance from contamination. Relative percent cover and species richness of vascular and non-vascular plants significantly increased with distance into the wetland, with higher non-vascular richness being found at intermediate distances before transitioning to a vascular plant dominated community. Bacterial and archaeal community composition exhibited a decreased proportion of members of the phylum Acidobacteria (notably of the order Acidobacteriales) and increased diversity and richness of methanogens across a larger range of orders farther from the tailings source, an indication of microbial C-cycling potential. Consistent with changes in microbial communities, in vitro microbial CH4 production potential significantly increased with distance from the contaminant source. This study demonstrates both the profound negative impact that metalliferous tailings contamination can have on above and belowground communities in peatlands, and the value of wetland preservation and restoration.

Viral spillover risk increases with climate change in High Arctic lake sediments.

The host spectrum of viruses is quite diverse, as they can sustainedly infect a few species to several phyla. When confronted with a new host, a virus may even infect it and transmit sustainably in this new host, a process called 'viral spillover'. However, the risk of such events is difficult to quantify. As climate change is rapidly transforming environments, it is becoming critical to quantify the potential for spillovers. To address this issue, we resorted to a metagenomics approach and focused on two environments, soil and lake sediments from Lake Hazen, the largest High Arctic freshwater lake in the world. We used DNA and RNA sequencing to reconstruct the lake's virosphere in both its sediments and soils, as well as its range of eukaryotic hosts. We then estimated the spillover risk by measuring the congruence between the viral and the eukaryotic host phylogenetic trees, and show that spillover risk increases with runoff from glacier melt, a proxy for climate change. Should climate change also shift species range of potential viral vectors and reservoirs northwards, the High Arctic could become fertile ground for emerging pandemics.

Time-Dependent Biosensor Fluorescence as a Measure of Bacterial Arsenic Uptake Kinetics and Its Inhibition by Dissolved Organic Matter.

Microbe-mediated transformations of arsenic (As) often require As to be taken up into cells prior to enzymatic reaction. Despite the importance of these microbial reactions for As speciation and toxicity, understanding of how As bioavailability and uptake are regulated by aspects of extracellular water chemistry, notably dissolved organic matter (DOM), remains limited. Whole-cell biosensors utilizing fluorescent proteins are increasingly used for high-throughput quantification of the bioavailable fraction of As in water. Here, we present a mathematical framework for interpreting the time series of biosensor fluorescence as a measure of As uptake kinetics, which we used to evaluate the effects of different forms of DOM on uptake of trivalent arsenite. We found that thiol-containing organic compounds significantly inhibited uptake of arsenite into cells, possibly through the formation of aqueous complexes between arsenite and thiol ligands. While there was no evidence for competitive interactions between arsenite and low-molecular-weight neutral molecules (urea, glycine, and glyceraldehyde) for uptake through the aquaglyceroporin channel GlpF, which mediates transport of arsenite across cell membranes, there was evidence that labile DOM fractions may inhibit arsenite uptake through a catabolite repression-like mechanism. The observation of significant inhibition of arsenite uptake at DOM/As ratios commonly encountered in wetland pore waters suggests that DOM may be an important control on the microbial uptake of arsenite in the environment, with aspects of DOM quality playing an important role in the extent of inhibition. IMPORTANCE The speciation and toxicity of arsenic in environments like rice paddy soils and groundwater aquifers are controlled by microbe-mediated reactions. These reactions often require As to be taken up into cells prior to enzymatic reaction, but there is limited understanding of how microbial arsenic uptake is affected by variations in water chemistry. In this study, we explored the effect of dissolved organic matter (DOM) quantity and quality on microbial As uptake, with a focus on the role of thiol functional groups that are well known to form aqueous complexes with arsenic. We developed a quantitative framework for interpreting fluorescence time series from whole-cell biosensors and used this technique to evaluate effects of DOM on the rates of microbial arsenic uptake. We show that thiol-containing compounds significantly decrease rates of As uptake into microbial cells at environmentally relevant DOM/As ratios, revealing the importance of DOM quality in regulating arsenic uptake, and subsequent biotransformation, in the environment.

Paleoecotoxicology: Developing methods to assess the toxicity of lake sediment records influenced by legacy gold mining.

The contamination of lakes by industrial emissions is an issue of international concern. Traditional paleolimnology examines sedimentary micro-fossils to infer the biological response to natural and anthropogenic stressors over time. Here, we calculate a theoretical biological effect for historic sediment sections using Probable Effect Concentration Quotient (PEC-Q) and arsenic specific quotient methods and develop novel time-constrained sediment toxicity test methods using a cultured Daphnia sp. combined with a whole cell microbial biosensor to assess the toxicity of past industrial contamination with modern testing methods. These methods were developed using sediments collected from Pocket Lake (Northwest Territories, Canada), a lake known to have exhibited a significant ecological shift following input from nearby gold smelter emissions during the mid 20th century. We then applied these methods to near-, mid-, and far-field sites to assess the response of Daphnia sp. to varying contaminant load. Daphnia sp. mortality exposed to dated sediments indicated a strong concordance with the timing of mining activities, and a strong concordance with PEC-Q and arsenic specific toxicity quotients. In contrast, a decrease in Daphnia mortality was observed during pre-, and post-mining periods when the contaminant burden was lower. Initial assessments of bioavailability using a microbial biosensor indicated that arsenic in porewater is 72-96% bioavailable, and limited evidence that oxidative stress may contribute to the Daphnia sp. toxic response. These results indicate that lake sediment archives can be used to infer missing biomonitoring data in sites of legacy anthropogenic influence, which will be useful for those seeking to conduct cost-effective and efficient preliminary environmental risk assessments.

Impacts of Mercury Exposure Levels and Sources on the Demethylation of Methylmercury Through Human Gut Microbiota.

This study aims to investigate methylmercury (MeHg) demethylation processes in human gut. Here, we determined the compositions and MeHg demethylation rates of gut microbiota in residents from different Hg exposure levels (Wanshan (WS) town and Yangtou (YT) town) and different Hg exposure sources (Zhuchang (ZC) town and YT town) regions. MeHg and inorganic Hg exposure levels in residents of WS town were significantly higher than those of YT and ZC town. Desulfovibrio and Methanogens, which related to Hg methylation/demethylation, showed significantly higher abundance in WS and ZC, comparing with YT. In vitro experiments demonstrated that human intestinal microbiota could degrade MeHg directly. Besides, gut microbiota in WS and ZC exhibited significantly higher demethylation rates than YT, suggesting Desulfovibrio and Methanogens may play important roles in intestinal MeHg demethylation. This study highlights Hg exposure levels and sources may affect demethylation efficiency of gut microbiota, which provides new insights for MeHg demethylation processes in human body.

Mercury drives microbial community assembly and ecosystem multifunctionality across a Hg contamination gradient in rice paddies.

Soil microbial communities are critical for maintaining terrestrial ecosystems and fundamental ecological processes. Mercury (Hg) is a heavy metal that is toxic to microorganisms, but its effects on microbial community assembly and ecosystem multifunctionality in rice paddy ecosystems remain largely unknown. In the current study, we analyzed the microbial community structure and ecosystem multifunctionality of paddy soils across a Hg contamination gradient. The results demonstrated that Hg contamination significantly altered the microbial community structure. The microbial communities were predominantly driven by deterministic selection rather than stochastic processes. The random forest model and variation partition analysis demonstrated that the Hg level was the most important predictor of microbial profiles. Ecosystem multifunctionality decreased across the Hg concentration gradient, and multifunctionality was significantly correlated with soil biodiversity, suggesting that Hg-induced reductions in soil biodiversity led to reduced ecosystem services. A structural equation model showed that Hg contamination directly and indirectly affected ecosystem multifunctionality. The present work broadens our knowledge of the assembly of the microbiome in rice paddies across a Hg contamination gradient and highlights the significance of soil biodiversity in regulating ecosystem functions, especially in Hg-polluted rice paddies.

Diffusion of H2 S from anaerobic thiolated ligand biodegradation rapidly generates bioavailable mercury.

Methylmercury is a potent neurotoxin that biomagnifies through food webs and which production depends on anaerobic microbial uptake of inorganic mercury (Hg) species. One outstanding knowledge gap in understanding Hg methylation is the nature of bioavailable Hg species. It has become increasingly obvious that Hg bioavailability is spatially diverse and temporally dynamic but current models are mostly built on single thiolated ligand systems, omitting ligand exchanges and interactions, or the inclusion of dissolved gaseous phases. In this study, we used a whole-cell anaerobic biosensor to determine the role of a mixture of thiolated ligands on Hg bioavailability. Serendipitously, we discovered how the diffusion of trace amounts of exogenous biogenic H2 S, originating from anaerobic microbial ligand degradation, can alter Hg speciation - away from H2 S production site - to form bioavailable species. Regardless of its origins, H2 S stands as a mobile mediator of microbial Hg metabolism, connecting spatially separated microbial communities. At a larger scale, global planetary changes are expected to accelerate the production and mobilization of H2 S and Hg, possibly leading to increased production of the potent neurotoxin; this work provides mechanistic insights into the importance of co-managing biogeochemical cycle disruptions.

Pesticide-induced disturbances of bee gut microbiotas.

Social bee gut microbiotas play key roles in host health and performance. Worryingly, a growing body of literature shows that pesticide exposure can disturb these microbiotas. Most studies examine changes in taxonomic composition in Western honey bee (Apis mellifera) gut microbiotas caused by insecticide exposure. Core bee gut microbiota taxa shift in abundance after exposure but are rarely eliminated, with declines in Bifidobacteriales and Lactobacillus near melliventris abundance being the most common shifts. Pesticide concentration, exposure duration, season and concurrent stressors all influence whether and how bee gut microbiotas are disturbed. Also, the mechanism of disturbance-i.e. whether a pesticide directly affects microbial growth or indirectly affects the microbiota by altering host health-likely affects disturbance consistency. Despite growing interest in this topic, important questions remain unanswered. Specifically, metabolic shifts in bee gut microbiotas remain largely uninvestigated, as do effects of pesticide-disturbed gut microbiotas on bee host performance. Furthermore, few bee species have been studied other than A. mellifera, and few herbicides and fungicides have been examined. We call for these knowledge gaps to be addressed so that we may obtain a comprehensive picture of how pesticides alter bee gut microbiotas, and of the functional consequences of these changes.

The underappreciated role of natural organic matter bond Hg(II) and nanoparticulate HgS as substrates for methylation in paddy soils across a Hg concentration gradient.

Rice consumption is the major pathway for human methylmercury (MeHg) exposure in inland China, especially in mercury (Hg) contaminated regions. MeHg production, a microbially driven process, depends on both the chemical speciation of inorganic divalent mercury, Hg(II), that determines Hg bioavailability for methylation. Studies have shown that Hg(II) speciation in contaminated paddy soils is mostly controlled by natural organic matter and sulfide levels, which are typically thought to limit Hg mobility and bioavailability. Yet, high levels of MeHg are found in rice, calling for reconsideration of the nature of Hg species bioavailable to methylators in paddy soils. Here, we conducted incubation experiments using a multi-isotope tracer technique including 198Hg(NO3)2, natural organic matter bond Hg(II) (NOM-199Hg(II)), ferrous sulfide sorbed Hg(II) (≡FeS-200Hg(II)), and nanoparticulate mercuric sulfide (nano-202HgS), to investigate the relative importance of geochemically diverse yet relevant Hg(II) species on Hg methylation in paddy soils across a Hg concentration gradient. We show that methylation rates for all Hg(II) species tested decreased with increasing Hg concentrations, and that methylation rates using NOM-199Hg(II) and nano-202HgS as substrates were similar or greater than rates obtained using the labile 198Hg(NO3)2 substrate. ≡FeS-200Hg(II) yielded the lowest methylation rate in all sites, and thus the formation of FeS is likely a sink for labile 198Hg(NO3)2 in sulfide-rich paddy soils. Moreover, the variability in the methylation data for a given site (1 to 5-fold variation depending on the Hg species) was smaller than what was observed across the Hg concentration gradient (103-104 fold variation between sites). These findings emphasize that at broad spatial scales, site-specific characteristics, such as microbial community structure, need to be taken into consideration, alongside the nature of the Hg substrate available for methylation, to determine net MeHg production. This study highlights the importance of developing site-specific strategies for remediating Hg pollution.

Golden Gate Assembly of Aerobic and Anaerobic Microbial Bioreporters.

Microbial bioreporters provide direct insight into cellular processes by producing a quantifiable signal dictated by reporter gene expression. The core of a bioreporter is a genetic circuit in which a reporter gene (or operon) is fused to promoter and regulatory sequences that govern its expression. In this study, we develop a system for constructing novel Escherichia coli bioreporters based on Golden Gate assembly, a synthetic biology approach for the rapid and seamless fusion of DNA fragments. Gene circuits are generated by fusing promoter and reporter sequences encoding yellow fluorescent protein, mCherry, bacterial luciferase, and an anaerobically active flavin-based fluorescent protein. We address a barrier to the implementation of Golden Gate assembly by designing a series of compatible destination vectors that can accommodate the assemblies. We validate the approach by measuring the activity of constitutive bioreporters and mercury and arsenic biosensors in quantitative exposure assays. We also demonstrate anaerobic quantification of mercury and arsenic in biosensors that produce flavin-based fluorescent protein, highlighting the expanding range of redox conditions that can be examined by microbial bioreporters. IMPORTANCE Microbial bioreporters are versatile genetic tools with wide-ranging applications, particularly in the field of environmental toxicology. For example, biosensors that produce a signal output in the presence of a specific analyte offer less costly alternatives to analytical methods for the detection of environmental toxins such as mercury and arsenic. Biosensors of specific toxins can also be used to test hypotheses regarding mechanisms of uptake, toxicity, and biotransformation. In this study, we develop an assembly platform that uses a synthetic biology technique to streamline construction of novel Escherichia coli bioreporters that produce fluorescent or luminescent signals either constitutively or in response to mercury and arsenic exposure. Beyond the synthesis of novel biosensors, our assembly platform can be adapted for numerous applications, including labeling bacteria for fluorescence microscopy, developing gene expression systems, and modifying bacterial genomes.

Treatment-induced shrinking of tumour aggregates: a nonlinear volume-filling chemotactic approach.

Motivated by experimental observations in 3D/organoid cultures derived from glioblastoma, we propose a novel mechano-transduction mechanism where the introduction of a chemotherapeutic treatment induces mechanical changes at the cell level. We analyse the influence of these individual mechanical changes on the properties of the aggregates obtained at the population level. We employ a nonlinear volume-filling chemotactic system of partial differential equations, where the elastic properties of the cells are taken into account through the so-called squeezing probability, which depends on the concentration of the treatment in the extracellular microenvironment. We explore two scenarios for the effect of the treatment: first, we suppose that the treatment acts only on the mechanical properties of the cells and, in the second one, we assume it also prevents cell proliferation. We perform a linear stability analysis which enables us to identify the ability of the system to create patterns and fully characterize their size. Moreover, we provide numerical simulations in 1D and 2D that illustrate the shrinking of the aggregates due to the presence of the treatment.

The effect of legacy gold mining on methylmercury cycling and microbial community structure in northern freshwater lakes.

Smelting activities at Giant Mine (Yellowknife, NWT, Canada) have resulted in high sulfate and arsenic concentrations in nearby lakes. Here we tested whether historic smelting affects current mercury (Hg) cycling in 35 freshwater lakes over a 2800 km2 area around the former gold mine. We sampled lake water and sediment over three consecutive years (2015-2017) using a factorial sampling design that accounted for different environmental variables known to affect the net methylmercury (MeHg) levels in water. Stable Hg(ii) and MeHg isotope tracers were used to quantify Hg methylation and demethylation rate constants in sediments, and 16S rRNA gene amplicon sequencing was used to characterize microbial community structure. This study reveals that the fraction of methylated total Hg (% MeHg) found in surface water is positively correlated to the sulfate gradient, while the rate at which Hg is methylated (Km) in sediments is negatively correlated with total arsenic, and positively correlated with dissolved organic carbon, total phosphorous, and % MeHg in the water. Furthermore, 6 of the 28 lakes that had detectable demethylation rate constants (Kd) also had significantly lower DOC concentrations than lakes with non-detectable Kd. Our results also show that legacy pollution from smelting activities is affecting the structure of microbial communities in lake sediments. This study reveals the complex dynamics of Hg cycling in this northern environment, highlighting the importance of large-scale studies in which the effect of multiple pollution gradients (e.g. arsenic and sulfate) must be taken into consideration.

Stable Isotope Fractionation Reveals Similar Atomic-Level Controls during Aerobic and Anaerobic Microbial Hg Transformation Pathways.

Mercury (Hg) is a global pollutant and potent neurotoxin that bioaccumulates in food webs as monomethylmercury (MeHg). The production of MeHg is driven by anaerobic and Hg redox cycling pathways, such as Hg reduction, which control the availability of Hg to methylators. Anaerobes play an important role in Hg reduction in methylation hot spots, yet their contributions remain underappreciated due to how challenging these pathways are to study in the absence of dedicated genetic targets and low levels of Hg0 in anoxic environments. In this study, we used Hg stable isotope fractionation to explore Hg reduction during anoxygenic photosynthesis and fermentation in the model anaerobe Heliobacterium modesticaldum Ice1. We show that cells preferentially reduce lighter Hg isotopes in both metabolisms, leading to mass-dependent fractionation, but mass-independent fractionation commonly induced by UV-visible light is absent. Due to the variability associated with replicate experiments, we could not discern whether dedicated physiological processes drive Hg reduction during photosynthesis and fermentation. However, we demonstrate that fractionation is affected by the interplay between pathways controlling Hg recruitment, accessibility, and availability alongside metabolic redox reactions. The combined contributions of these processes lead to isotopic enrichment during anoxygenic photosynthesis that is in between the values reported for anaerobic respiratory microbial Hg reduction and abiotic photoreduction. Isotope enrichment during fermentation is closer to what has been observed in aerobic bacteria that reduce Hg through dedicated detoxification pathways. Our work suggests that similar controls likely underpin diverse microbe-mediated Hg transformations that affect Hg's fate in oxic and anoxic habitats. IMPORTANCE Anaerobic and photosynthetic bacteria that reduce mercury affect mercury delivery to microbes in methylation sites that drive bioaccumulation in food webs. Anaerobic mercury reduction pathways remain underappreciated in the current view of the global mercury cycle because they are challenging to study, bearing no dedicated genetic targets to establish physiological mechanisms. In this study, we used stable isotopes to characterize the physiological processes that control mercury reduction during photosynthesis and fermentation in the model anaerobe Heliobacterium modesticaldum Ice1. The sensitivity of isotope analyses highlighted the subtle contribution of mercury uptake to the isotope signature associated with anaerobic mercury reduction. When considered alongside the isotope signatures associated with microbial pathways for which genetic determinants have been identified, our findings underscore the narrow range of isotope enrichment that is characteristic of microbial mercury transformations. This suggests that there are common atomic-level controls for biological mercury transformations across a broad range of geochemical conditions.