Diversity and predictive metabolic pathways of the prokaryotic microbial community along a groundwater salinity gradient of the Pearl River Delta, China

Saline mine water influences eukaryote life in shallow groundwater of a tropical sandy stream

Eukaryotic communities in groundwater may be particularly sensitive to disturbance because they are adapted to stable environmental conditions and often have narrow spatial distributions. Traditional methods for characterising these communities, focussing on groundwater-inhabiting macro- and meiofauna (stygofauna), are challenging because of limited taxonomic knowledge and expertise (particularly in less-explored regions), and the time and expense of morphological identification. The primary objective of this study was to evaluate the vulnerability of eukaryote communities in shallow groundwater to mine water discharge containing elevated concentrations of magnesium (Mg) and sulfate (SO4). The study was undertaken in a shallow sand bed aquifer within a wet-dry tropical setting. The aquifer, featuring a saline mine water gradient primarily composed of elevated Mg and SO4, was sampled from piezometers in the creek channel upstream and downstream of the mine water influence during the dry season when only subsurface water flow was present. Groundwater communities were characterised using both morphological assessments of stygofauna from net samples and environmental DNA (eDNA) targeting the 18S rDNA and COI mtDNA genes. eDNA data revealed significant shifts in community composition in response to mine waters, contrasting with findings from traditional morphological composition data. Changes in communities determined using eDNA data were notably associated with concentrations of SO42-, Mg2+ and Na+, and water levels in the piezometers. This underscores the importance of incorporating molecular approaches in impact assessments, as relying solely on traditional stygofauna sampling methods in similar environments may lead to inaccurate conclusions about the responses of the assemblage to studied impacts.

Higher diversity of sulfur-oxidizing bacteria based on soxB gene sequencing in surface water than in spring in Wudalianchi volcanic group, NE China

INTRODUCTION

Sulfur-oxidizing bacteria (SOB) play a key role in the biogeochemical cycling of sulfur.

OBJECTIVES

To explore SOB diversity, distribution, and physicochemical drivers in five volcanic lakes and two springs in the Wudalianchi volcanic field, China.

METHODS

This study analyzed microbial communities in samples via high-throughput sequencing of the soxB gene. Physical-chemical parameters were measured, and QIIME 2 (v2019.4), R, Vsearch, MEGA7, and Mothur processed the data. Alpha diversity indices and UPGMA clustering assessed community differences, while heat maps visualized intra-sample variations. Canoco 5.0 analyzed community-environment correlations, and NMDS, Adonis, and PcoA explored sample dissimilarities and environmental factor correlations. SPSS v.18.0 tested for statistical significance.

RESULTS

The diversity of SOB in surface water was higher than in springs (more than 7.27 times). We detected SOB affiliated to β-proteobacteria (72.3 %), α-proteobacteria (22.8 %), and γ-proteobacteria (4.2 %) distributed widely in these lakes and springs. Rhodoferax and Cupriavidus were most frequent in all water samples, while Rhodoferax and Bradyrhizobium are dominant in surface waters but rare in springs. SOB genera in both habitats were positively correlated. Co-occurrence analysis identified Bradyrhizobium, Blastochloris, Methylibium, and Metyhlobacterium as potential keystone taxa. Redundancy analysis (RDA) revealed positive correlations between SOB diversity and total carbon (TC), Fe2+, and total nitrogen (TN) in all water samples.

CONCLUSION

The diversity and community structure of SOB in volcanic lakes and springs in the Wudalianchi volcanic group were clarified. Moreover, the diversity and abundance of SOB decreased with the variation of water openness, from open lakes to semi-enclosed lakes and enclosed lakes.

Environmental stressors altered the groundwater microbiome and nitrogen cycling: A focus on influencing mechanisms and pathways

Nitrogen cycling, as an important biogeochemical process in groundwater, strongly impacts the energy and matter flow of groundwater ecology. Phthalate esters (PAEs) were screened as key environmental stressors in the groundwater of Beijing, contributing to the alteration of microbial community structure and functions; thus, it could be deduced that these stressors might influence nitrogen cycling that is almost exclusively mediated by microorganisms. Identification of the influences of PAEs on groundwater nitrogen cycling and exploration of the potential influence mechanisms and pathways are vital but still challenging. This study explored the influence mechanisms and pathways of the environmental stressor PAE on nitrogen cycling in groundwater collected from a typical monitoring station in Beijing based on high-throughput sequencing and bioinformatics analysis combined with mediation analysis methods. The results suggested that among the 5 detected PAEs, dimethyl phthalate and diethyl phthalate significantly negatively impacted nitrogen cycling processes, especially nitrogen fixation and denitrification processes (p < 0.05), in groundwater. Their influences were fully or partially mediated by functional microorganisms, particularly assigned keystone genera (such as Dechloromonas, Aeromonas and Noviherbaspirillum), whose abundance was significantly inhibited by these PAEs via dysregulation of carbohydrate metabolism and activation of defense mechanisms. These findings confirmed that the influences of environmental stressors PAEs on nitrogen cycling in groundwater might be mediated by the "PAE stress-groundwater microbiome-nitrogen cycling alteration" pathway. This study may advance the understanding of the consequences of environmental stressors on groundwater ecology and support the ecological hazard assessment of groundwater stressors.

Bioenergetics of aerobic and anaerobic growth of Shewanella putrefaciens CN32

Shewanella putrefaciens is a model dissimilatory iron-reducing bacterium that can use Fe(III) and O2 as terminal electron acceptors. Consequently, it has the ability to influence both aerobic and anaerobic groundwater systems, making it an ideal microorganism for improving our understanding of facultative anaerobes with iron-based metabolism. In this work, we examine the bioenergetics of O2 and Fe(III) reduction coupled to lactate oxidation in Shewanella putrefaciens CN32. Bioenergetics were measured directly via isothermal calorimetry and by changes to the chemically defined growth medium. We performed these measurements from 25 to 36°C. Modeling metabolism with macrochemical equations allowed us to define a theoretical growth stoichiometry for the catabolic reaction of 1.00 O2:lactate and 1.33 Fe(III):lactate that was consistent with the observed ratios of O2:lactate (1.20 ± 0.23) and Fe(III):lactate (1.46 ± 0.15) consumption. Aerobic growth showed minimal variation with temperature and minimal variation in thermodynamic potentials of incubation. Fe(III)-based growth showed a strong temperature dependence. The Gibbs energy and enthalpy of incubation was minimized at ≥30°C. Energy partitioning modeling of Fe(III)-based calorimetric incubation data predicted that energy consumption for non-growth associate maintenance increases substantially above 30°C. This prediction agrees with the data at 33 and 35°C. These results suggest that the effects of temperature on Shewanella putrefaciens CN32 are metabolism dependent. Gibbs energy of incubation above 30°C was 3-5 times more exergonic with Fe(III)-based growth than with aerobic growth. We compared data gathered in this study with predictions of microbial growth based on standard-state conditions and based on the thermodynamic efficiency of microbial growth. Quantifying the growth requirements of Shewanella putrefaciens CN32 has advanced our understanding of the thermodynamic constraints of this dissimilatory iron-reducing bacterium.

Functional characterization of an efficient ibuprofen-mineralizing bacterial consortium

Ibuprofen (IBU) is a widely used non-steroidal anti-inflammatory drug (NSAID), which has attracted widespread attention due to its high frequency of environmental detection, non-degradability and potential ecological risks. However, little is known about the functional characterization of the highly efficient IBU-mineralizing consortium. In this study, an IBU-mineralizing consortium C6 was obtained by continuous enrichment of the original consortium C1 accumulated the metabolite of 2-Hydroxyibuprofen (2HIBU). Methylobacter, Pseudomonas, and Dokdonella spp. were significantly enriched in the consortium C6. Streptomyces sp. had a relative abundance of about 0.01 % in the consortium C1 but extremely low (< 0.001 %) in the consortium C6. Subsequently, two IBU degraders, Streptomyces sp. D218 and Pseudomonas sp. M20 with detection of 2HIBU or not, were isolated from the consortia C1 and C6, respectively. These results imply that the degradation of IBU in the consortia C1 and C6 may be mainly mediated by key players of Streptomyces and Pseudomonas, respectively. This study showed that the composition of the core functional strains of the bacterial community structure was changed by continuous enrichment, which affected the degradation process of IBU. These findings provide new insights into our understanding of the biotransformation process of NSAIDs and provide valuable strain resources for bioremediation.

Microbial community structural response to variations in physicochemical features of different aquifers

Introduction

The community structure of groundwater microorganisms has a significant impact on groundwater quality. However, the relationships between the microbial communities and environmental variables in groundwater of different recharge and disturbance types are not fully understood.

Methods

In this study, measurements of groundwater physicochemical parameters and 16S rDNA high-throughput sequencing technology were used to assess the interactions between hydrogeochemical conditions and microbial diversity in Longkou coastal aquifer (LK), Cele arid zone aquifer (CL), and Wuhan riverside hyporheic zone aquifer (WH). Redundancy analysis indicated that the primary chemical parameters affecting the microbial community composition were NO3 -, Cl-, and HCO3 -.

Results

The species and quantity of microorganisms in the river-groundwater interaction area were considerably higher than those in areas with high salinity [Shannon: WH (6.28) > LK (4.11) > CL (3.96); Chao1: WH (4,868) > CL (1510) > LK (1,222)]. Molecular ecological network analysis demonstrated that the change in microbial interactions caused by evaporation was less than that caused by seawater invasion under high-salinity conditions [(nodes, links): LK (71,192) > CL (51,198)], whereas the scale and nodes of the microbial network were greatly expanded under low-salinity conditions [(nodes, links): WH (279,694)]. Microbial community analysis revealed that distinct differences existed in the classification levels of the different dominant microorganism species in the three aquifers.

Discussion

Environmental physical and chemical conditions selected the dominant species according to microbial functions. Gallionellaceae, which is associated with iron oxidation, dominated in the arid zones, while Rhodocyclaceae, which is related to denitrification, led in the coastal zones, and Desulfurivibrio, which is related to sulfur conversion, prevailed in the hyporheic zones. Therefore, dominant local bacterial communities can be used as indicators of local environmental conditions.

High spatial heterogeneity and low connectivity of bacterial communities along a Mediterranean subterranean estuary

Subterranean estuaries are biogeochemically active coastal sites resulting from the underground mixing of fresh aquifer groundwater and seawater. In these systems, microbial activity can largely transform the chemical elements that may reach the sea through submarine groundwater discharge (SGD), but little is known about the microorganisms thriving in these land-sea transition zones. We present the first spatially-resolved characterization of the bacterial assemblages along a coastal aquifer in the NW Mediterranean, considering the entire subsurface salinity gradient. Combining bulk heterotrophic activity measurements, flow cytometry, microscopy and 16S rRNA gene sequencing we find large variations in prokaryotic abundances, cell size, activity and diversity at both the horizontal and vertical scales that reflect the pronounced physicochemical gradients. The parts of the transect most influenced by freshwater were characterized by smaller cells and lower prokaryotic abundances and heterotrophic production, but some activity hotspots were found at deep low-oxygen saline groundwater sites enriched in nitrite and ammonium. Diverse, heterogeneous and highly endemic communities dominated by Proteobacteria, Patescibacteria, Desulfobacterota and Bacteroidota were observed throughout the aquifer, pointing to clearly differentiated prokaryotic niches across these transition zones and little microbial connectivity between groundwater and Mediterranean seawater habitats. Finally, experimental manipulations unveiled large increases in community heterotrophic activity driven by fast growth of some rare and site-specific groundwater Proteobacteria. Our results indicate that prokaryotic communities within subterranean estuaries are highly heterogeneous in terms of biomass, activity and diversity, suggesting that their role in transforming nutrients will also vary spatially within these terrestrial-marine transition zones.

Microbial Diversity in Groundwater and Its Response to Seawater Intrusion in Beihai City, Southern China

Seawater intrusion is a major concern commonly found in coastal aquifers worldwide. Because of the intense aquifer exploitation and land-based marine aquaculture in the coastal area of Beihai City, Guangxi Zhuang Autonomous Region, China, numerous underground aquifers in this area have been affected by seawater intrusion. However, the microbial communities in freshwater aquifers and their response to seawater intrusion are still unclear. In this study, groundwater from three aquifers was collected from three monitoring sites at different distances from the coastline in the coastal area of Beihai City, and the hydrochemical characteristics of these groundwater samples and the structure of the associated microbial communities were analyzed. The Cl- concentration of the samples indicated that seawater intrusion had occurred in the research area up to 1.5 km away from the coastline, but the monitoring site 2 km away from the coastline had yet to be affected. Statistical analysis showed that the bacterial communities in different groundwater aquifers were significantly correlated with the Cl- concentration, thereby suggesting that the extent of seawater intrusion might be one of the primary factors shaping bacterial composition in groundwater of this area, but the composition and distribution of archaea did not show a significant response to seawater intrusion and presented no apparent correlation with the Cl- concentration. α-, γ-Proteobacteria and Bacteroidota were the dominant bacterial lineages, accounting for about 58-95% of the bacterial communities. Meanwhile, the predominant archaeal taxa were mainly composed of Crenarchaeota, Nanoarchaeota, and Thermoplasmatota, as accounting for 83-100%. Moreover, there was significant spatial heterogeneity of microbial communities in the aquifers affected by varying degrees of seawater intrusion. The microbial communities inhabiting the unconfined aquifer were influenced by the geochemical fluctuation caused by seawater infiltration from land-based marine aquaculture ponds and the diffusion of eutrophic surface water. In contrast, changes in microbial community structure in the confined aquifers were closely related to the environmental gradient caused by different degrees of seawater intrusion. In addition, we also found that the tidal cycle did not significantly affect the structure of microbial communities inhabiting confined aquifers that had been long affected by seawater intrusion.

Saltwater intrusion affecting NO2- accumulation in demersal fishery species by bacterially mediated N-cycling

To investigate the underlying effects of saltwater intrusion (SWI) on bottom aquatic ecosystems, a set of environmental parameters and the bacterial community were determined and analyzed by sampling bottom water and surface sediments at the Modaomen waterway of the Pearl River Estuary. Biodiversity of fishery species and their relationship with the environment variables were analyzed together. NO₃^- and NO₂^- concentration down-regulation and NH₄⁺ concentration up-regulation in water and sediment were observed along the resulting salinity gradient, indicating that SWI affected N-cycling. Further investigation via 16 s sequencing revealed that taxonomic and functional composition of the bacterial community in the sediment displayed greater discretization than in water, implying that SWI exerted a greater impact on the sedimentary bacterial community. Metagenomic sequencing showed that the sedimentary bacterial community was associated with NO₃^-, NO₂^-, and NH₄⁺ transformation under SWI, and that this was driven by salinity and conductivity. Nitrogen metabolism and denitrification related genes were expressed at higher levels in high salinity than in low salinity, consistent with the increased enzymatic activities of NiR and NR. The NO₂^- concentration in the muscle of six selected fishery species was significantly decreased by 11.15-65.74% (P < 0.05) along the salinity gradient, indicating that SWI reduced NO₂^- accumulation. The results suggest that SWI alleviates NO₂^- accumulation in demersal fishery species via bacterial mediation of N-cycling.

Different spatiotemporal dynamics, ecological drivers and assembly processes of bacterial, archaeal and fungal communities in brackish-saline groundwater

The presence of brackish-saline groundwater (BSG) poses great harms for human health, agricultural and industrial activity. Understanding how the major environmental features in BSG determine microbiota coalescence is crucial for groundwater monitoring optimization. Based on metabarcoding analysis of 242 PCR-amplified samples, we provided the first blueprints about distinct spatiotemporal distributions, ecological drivers and assembly processes of bacterial, archaeal and fungal communities in BSG obtained from new-constructed wells at Xiong'an New Area, China. Our study demonstrated that bacterial and archaeal communities exhibited significant spatial turnovers, while fungal community displayed the most obvious seasonal variation. Environmental filtering drove bacterial compositions more than those of archaea and fungi. Total dissolved solids (TDS), one of the most critical hydrochemical factors for salinization, had a stronger effect on bacterial spatiotemporal turnover than on those of the other two taxonomic groups, while chemical oxygen demand (CODMn) was more significantly associated with prokaryotic community variations. Bacterial and archaeal taxa dominated the metacommunity network and connected closely, and TDS was mostly related to archaeal subnetwork topological features, suggesting a significant influence of TDS on species association patterns within archaea. Specific functional guilds like bacterial nitrite oxidation, anammox, and archaeal methanogenesis were enriched in lower-TDS habitats, while higher TDS favored bacterial communities involved in dark oxidation of sulfur compounds, fumarate respiration, and cellulolysis. Finally, we confirmed that bacterial and archaeal assembly processes were governed by determinism in each season, and that of fungi was more regulated by stochasticity. Higher TDS was speculated to lead bacterial assembly more deterministic and that of fungi more random. Together, these findings provided an integrate theoretical framework about the unique responses of the three life domains to brackish-saline stress, and had important implications for microbial ecological prediction in groundwater.

Identification of groundwater microbial communities and their connection to the hydrochemical environment in southern Laizhou Bay, China

The microbial community plays an important role in the biogeochemical cycle in coastal groundwater ecosystems. However, the composition and controlling factors of the microbial community in coastal closed groundwater systems (CCGSs) with high salinity have rarely been studied. Here, we investigated and analyzed the hydrochemical characteristics and microbial community composition of seven brine samples with high total dissolved solid (TDS) values ranging from 74.5 to 132.3 g/L within and across three coastal saltworks (Yangkou, Hanting, and Changyi) in southern Laizhou Bay (SLB). The bacterial diversity was independent of salinity. Compared with those of low-salinity groundwater, the diversity of the microbial community in brine was lower, but the richness was slightly higher. There was a significant correlation between the microbial community diversity and groundwater sources, which indicated that the microbial communities were affected by groundwater sources. A comparison of the microbial community compositions of the three saltworks showed that the Hanting and Changyi saltworks had similar microbial communities due to their similar sampling depths. In addition, the main force shaping the differences in the microbial communities in both coastal open groundwater systems (COGSs) and CCGSs was identified as the hydraulic connection with the seawater controlled by hydrogeological conditions formed throughout geological history. This study can help to elucidate the biogeochemical processes in coastal aquifers.

Sustainable Urban Water Management in China: A Case Study from Guangzhou and Kunming

In China, the notion of a water sensitive city has gained popularity in urban water management as a result of the detrimental effects of flooding and pollution caused by developmental activities. Urban systems and their interrelationships are critical for long-term urban water management and water sensitivity. This article is a case study considering how a strength, weakness, opportunities, and threat (SWOT) analysis-based approach to urban water management interventions in Guangzhou and Kunming cities (China) enables decision makers to identify solutions for cities to become more water-sensitive and resilient. The similar difficulties and rewards with respect to the contexts of both cities were synthesized using SWOT analysis. The contextual SWOT analysis, in conjunction with the comprehensive inclusion of Sustainable Development Goals (SDGs) in intervention planning in these cities, revealed that a water-sensitive-cities approach requires the establishment of a comprehensively multi-objective rainwater management system; this approach would have the goals of reducing rainwater draining sources, controlling processes and adaptive measures, and governing the system to make it more resilient. The water strategy should be holistic and adaptive, capable of providing a broad range of ecological services and other social benefits consistent with the fulfilment of the Sustainable Development Goals, and adaptable to other Chinese cities seeking to achieve water sensitivity.

Saline mine-water alters the structure and function of prokaryote communities in shallow groundwater below a tropical stream

Bacteria and archaea (prokaryotes) are vital components for maintaining healthy function of groundwater ecosystems. The prokaryotic community composition and associated putative functional processes were examined in a shallow sandy aquifer in a wet-dry tropical environment. The aquifer had a contaminated gradient of saline mine-water, which primarily consisted of elevated magnesium (Mg²⁺) and sulfate (SO₄^2-), although other major ions and trace metals were also present. Groundwaters were sampled from piezometers, approximately 2 m in depth, located in the creek channel upstream and downstream of the mine-water influence. Sampling occurred during the dry-season when only subsurface water flow was present. Next generation sequencing was used to analyse the prokaryote assemblages using 16S rDNA and metabolic functions were predicted with FAPROTAX. Significant changes in community composition and functional processes were observed with exposure to mine-waters. Communities in the exposed sites had significantly lower relative abundance of methanotrophs such as Methylococcaceae and methanogens (Methanobacteriaceae), but higher abundance in Nitrososphaeraceae, associated with nitrification, indicating potentially important changes in the biogeochemistry of the exposed sites. The changes were most strongly correlated with concentrations of SO₄^2-, Mg²⁺ and Na⁺. This knowledge allows an assessment of the risk of mine-water contamination to groundwater ecosystem function and aids mine-water management.

Natural Background Level and Contamination of Shallow Groundwater Salinity in Various Aquifers in a Coastal Urbanized Area, South China

Assessing natural background levels (NBLs) of chemical components in groundwater is useful for the evaluation of groundwater contamination in urbanized areas. The present study assessed the NBL of total dissolved solids (TDS) in various groundwater units in the Pearl River Delta (PRD) where urbanization is a large scale and discussed factors controlling groundwater salinity contamination in the PRD. Results showed that the NBL of TDS in groundwater in the coastal-alluvial plain was more than 1.5 times that in other groundwater units because of the seawater intrusion in this groundwater unit. By contrast, interactions of water and soils/rocks were the main factors controlling the NBLs of TDS in other groundwater units. Groundwater salinity contamination in the PRD was positively correlated with the urbanization level. Wastewater from township-village enterprises and industrial wastewater were likely to be the main sources for groundwater salinity contamination in the PRD. Moreover, the wastewater leakage from sewer systems was one of the main pathways for groundwater salinity contamination in urbanized areas, because the proportion of groundwater salinity contamination in urbanized areas formed in 1988–1998 was more than 1.5 times that in urbanized areas formed in 1998–2006 regardless of groundwater units. Besides, sewage irrigation and leakage of landfill leachate were also important sources for groundwater salinity contamination in the PRD.

The Role of Ophiopogonin D in Atherosclerosis: Impact on Lipid Metabolism and Gut Microbiota

Gut microbiota has been proven to play an important role in many metabolic diseases and cardiovascular disease, particularly atherosclerosis. Ophiopogonin D (OPD), one of the effective compounds in Ophiopogon japonicus, is considered beneficial to metabolic syndrome and cardiovascular diseases. In this study, we have illuminated the effect of OPD in ApoE knockout (ApoE[Formula: see text] mice on the development of atherosclerosis and gut microbiota. To investigate the potential ability of OPD to alleviate atherosclerosis, 24 eight-week-old male ApoE[Formula: see text] mice (C57BL/6 background) were fed a high-fat diet (HFD) for 12 weeks, and 8 male C57BL/6 mice were fed a normal diet, serving as the control group. ApoE[Formula: see text] mice were randomly divided into the model group, OPD group, and simvastatin group ([Formula: see text]= 8). After treatment for 12 consecutive weeks, the results showed that OPD treatment significantly decreased the plaque formation and levels of serum lipid compared with those in the model group. In addition, OPD improved oral glucose tolerance and insulin resistance as well as reducing hepatocyte steatosis. Further analysis revealed that OPD might attenuate atherosclerosis through inhibiting mTOR phosphorylation and the consequent lipid metabolism signaling pathways mediated by SREBP1 and SCD1 in vivo and in vitro. Furthermore, OPD treatment led to significant structural changes in gut microbiota and fecal metabolites in HFD-fed mice and reduced the relative abundance of Erysipelotrichaceae genera associated with cholesterol metabolism. Collectively, these findings illustrate that OPD could significantly protect against atherosclerosis, which might be associated with the moderation of lipid metabolism and alterations in gut microbiota composition and fecal metabolites.

Susceptibility mapping of groundwater salinity using machine learning models

Increasing groundwater salinity has recently raised severe environmental and health concerns around the world. Advancement of the novel methods for spatial salinity modeling and prediction would be essential for effective management of the resources and planning mitigation policies. The current research presents the application of machine learning (ML) models in groundwater salinity mapping based on the dichotomous predictions. The groundwater salinity is predicted using the essential factors (i.e., identified by the simulated annealing feature selection methodology) through k-fold cross-validation methodology. Six ML models, namely, flexible discriminant analysis (FDA), mixture discriminant analysis (MAD), boosted regression tree (BRT), multivariate adaptive regression spline (MARS), random forest (RF), support vector machine (SVM), were employed to groundwater salinity mapping. The results of the modeling indicated that the SVM model had superior performance than other models. Variables of soil order, groundwater withdrawal, precipitation, land use, and elevation had the most contribute to groundwater salinity mapping. Results highlighted that the southern parts of the region and some parts in the north, northeast, and west have a high groundwater salinity, in which these areas are mostly matched with soil order of Entisols, bareland areas, and low elevations.

Bacterial community variations with salinity in the saltwater-intruded estuarine aquifer

Bacterial community has been significantly enrolled in the biogeochemical cycling of the coastal subsurface ecosystem. The bacterial community variations with salinity have been extensively investigated in the surface environment, such as lake, soil, and estuary, but not in the subsurface environment. Here we explore the responses of bacterial populations to the salinity and other environmental factors (EFs) by considering both the abundant and rare sub-community in a coastal Holocene groundwater system. Our study results indicate that the bacterial diversity was independent of the salinity in both the abundance and rare sub-community. Besides diversity, no flourishing of abundant bacteria relative abundance is observed with increasing or decreasing salinity. Yet the rare taxa exhibit a bio-growth with salinity, which has a significant correlation (p < 0.001) with sulfate concentration. The responses of the abundant sub-community taxa to nutrients, temperature, pH, and dissolved oxygen are insensitive. However, the correlation between δ¹⁸O, δD and the entire community diversity is significant, which demonstrates the bacterial community is affected by the groundwater origin. Besides, not all the species in one class or order are necessarily shaped by the same factor. To quantify the impact of EFs on the community properties, analyses in different taxonomic levels is suggested. These findings imply that the spatial organization of microbial communities is complicated and influenced by multiple factors on a regional scale. The investigated results are useful for understanding biogeochemical processes in the coastal groundwater.

The microbial dimension of submarine groundwater discharge: current challenges and future directions

Despite the relevance of submarine groundwater discharge (SGD) for ocean biogeochemistry, the microbial dimension of SGD remains poorly understood. SGD can influence marine microbial communities through supplying chemical compounds and microorganisms, and in turn, microbes at the land–ocean transition zone determine the chemistry of the groundwater reaching the ocean. However, compared with inland groundwater, little is known about microbial communities in coastal aquifers. Here, we review the state of the art of the microbial dimension of SGD, with emphasis on prokaryotes, and identify current challenges and future directions. Main challenges include improving the diversity description of groundwater microbiota, characterized by ultrasmall, inactive and novel taxa, and by high ratios of sediment-attached versus free-living cells. Studies should explore microbial dynamics and their role in chemical cycles in coastal aquifers, the bidirectional dispersal of groundwater and seawater microorganisms, and marine bacterioplankton responses to SGD. This will require not only combining sequencing methods, visualization and linking taxonomy to activity but also considering the entire groundwater–marine continuum. Interactions between traditionally independent disciplines (e.g. hydrogeology, microbial ecology) are needed to frame the study of terrestrial and aquatic microorganisms beyond the limits of their presumed habitats, and to foster our understanding of SGD processes and their influence in coastal biogeochemical cycles.

Groundwater microbial communities and their connection to hydrochemical environment in Golmud, Northwest China

Groundwater microbial community normally co-varies with the associated geochemical transect in some hydrogeological sections along flowpath. However, in hydrogeological section with similar geochemical transect (e.g., salinity, ion compositions) how microbial community in groundwater varies are poorly understood. In this study, groundwater samples were collected at six boreholes vertically and horizontally along a generalized groundwater flowpath in the Golmud area, Qaidam Basin, northwest China. High-throughput sequencing and multivariate statistical analysis were applied to explore the underlying relationships between microbial community structure and hydrogeochemical environment. The result showed that microbial communities changed considerably at both horizontal and vertical scales, although the groundwater samples were of relatively stable ionic compositions and hydrochemical types. The dominant bacterial phyla in groundwater varied from Alphaproteobacteria, Betaproteobacteria and Flavobacteriia in 'phreatic and phreatic-like' groundwater in the recharge area to Gammaproteobacteria in the confined groundwater in the lacustrine plain. At both vertical and horizontal scale, Gammaproteobacteria increased while Alpha- and Betaproteobacteria decreased as the function of distance. Genera Roseateles, Aquabacterium, Sphingomonas, Acinetobacter, Acidovorax and Flavobacterium presented in phreatic groundwater, while Pseudomonas, Hydrogenophaga and Perlucidibaca presented in confined groundwater. Spatial distribution of microbial community was highly affected by the pH (for 'phreatic and phreatic-like' groundwater) and ORP (for confined groundwater) of groundwater that had similar salinity or ion compositions. This research extends our knowledge about microbial communities' variation along groundwater flowpath in studied area and similar arid or semi-arid areas.

Hydro-biogeochemical impacts of fugitive methane on a shallow unconfined aquifer

Oil and gas development can result in natural gas migration into shallow groundwater. Methane (CH₄), the primary component of natural gas, can subsequently react with solutes and minerals in the aquifer to create byproducts that affect groundwater chemistry. Hydro-biogeochemical processes induced by fugitive gas from leaky oil and gas wells are currently not well understood. We monitored the hydro-biogeochemical responses of a controlled natural gas release into a well-studied Pleistocene beach sand aquifer (Canadian Forces Base Borden, Ontario, Canada). Groundwater samples were collected before, during, and up to 700 days after gas injection and analyzed for pH, major and minor ions, alkalinity, dissolved gases, stable carbon isotope ratios of CO₂ and CH₄, and microbial community composition. Gas injection resulted in a dispersed plume of free and dissolved phase natural gas, affecting groundwater chemistry in two distinct temporal phases. Initially (i.e. during and immediately after gas injection), pH declined and major ions and trace elements fluctuated; at times increasing above baseline concentrations. Changes in the short-term were due to invasion of deep groundwater with elevated total dissolved solids entrained with the upward migration of free phase gas and, reactions that were instigated through the introduction of constituents other than CH₄ present in the injected gas (e.g. CO₂). At later times, more pronounced aerobic and anaerobic CH₄ oxidation led to subtle increases in major ions (e.g. Ca²⁺, H₄SiO₄) and trace elements (e.g. As, Cr). Microbial community profiling indicated a persistent perturbation to community composition with a conspicuous ingrowth of taxa implicated in aerobic CH₄ oxidation as well anaerobic S, N and Fe species metabolism.