Specific characteristics of the microbial community in the groundwater fluctuation zone

Microbial denitrification responses to elevated CO2 in lake-shore sediments under different flooding conditions

The lake-shore zone is a critical site for recognizing the ecological impacts of global climate change, such as persistent elevated CO2 (eCO2). However, the direct effect of eCO2 on functional microorganisms in lake-shore sediments is neglected or confused at previous studies. In this study, the short-term direct effects of eCO2 on microbial denitrification in lake-shore sediments were investigated by simulating four flooding conditions (non-flooding (NF), intermittent flooding (IF), alternating high and low water-level flooding (HLF), and constant water-level flooding (WF)) through incubation experiments in the laboratory. This work demonstrated eCO2 directly increased denitrification in lake-shore sediments under different water-level conditions by different regulatory mechanism within the short term of 24 days. The microbial pentose phosphate pathway (PPP) and glycolysis in HLF and WF sediments were enhanced, while glycolysis in NF and IF sediments were obviously suppressed. Additionally, eCO2 strengthened negative associations of nosZ-type and nirK-type denitrifiers communities in NF, while increased their positive associations in IF, HLF, and WF. The regulation of metabolic pathways in denitrifiers would become more important than their microbial community when denitrifiers adapting eCO2. And the rising CO2 levels shifted sediments hotspots for potential N2O emissions from dry-wet alternation areas in lake-shore zone to the near-lake regions and lake basins. Based on these results, we recommend adjusting water-level management strategies in the lake-shore zone to address the greenhouse effect. For example, increasing the size of intermittently flooded areas and decreasing the proportion of continuously flooded areas in the lake-shore zone to mitigate N2O emissions and optimize sediment denitrification.

Seasonal Dynamics of the Bacterial Community in Lake Urmia, a Hypersaline Ecosystem

Lake Urmia is one of the world's most unique and hypersaline aquatic ecosystems. The aim of this study was to investigate the diversity, abundance and frequency of these microorganisms in water samples from the eastern regions of the lake over four seasons. Amplicon sequencing for the 16S rRNA gene was performed to examine bacterial communities in the samples. The study revealed significant seasonal variations in water quality parameters and their influence on the microbial communities. Majority and rarity analyses showed that winter and spring had higher core abundance and higher Gini index values, indicating a greater dominance of certain genera, while autumn and summer had a more balanced distribution. Analysis of beta diversity using the Bray-Curtis dissimilarity index emphasized that bacterial communities diverge most strongly in summer and winter, reflecting the significant changes in the environment with the season. Overall, understanding the seasonal variation in water chemistry and bacterial communities is critical for effective ecosystem management and conservation efforts.

Evaluating radionuclide mobility in groundwater recharge areas of fractured natural barrier systems using multiple isotopes and microbial indicators

The distribution of uranium (U) concentrations, which reached up to 322 µg/L, was found to correlate with the pattern of fractures within the natural barrier system (NBS). Analysis of the vertical distribution of dissolved oxygen (DO), dissolved organic carbon (DOC), tritium (3H), microbial communities, and H2O and SO42- isotopes revealed insights into oxic water infiltration within the heterogeneous fractured system. Their distribution showed that the average infiltration depth at the KURT site is 200 m, while in external areas with a high frequency of fractures, oxic conditions extended down to 495 m. The SO42- isotopes suggested the potential for microbial sulfate reduction to play a role in regulating radionuclide mobility in the deep geological system. At approximately 500 m, genera capable of thriving under harsh conditions of low DO and high heavy metal concentrations, such as Novosphingobium, Comamonadaceae_uc, and Desulfuromonas_g2, were identified. These findings indicate hydrogeological variability and microbial adaptation within the deep NBS, highlighting the importance of understanding the deep geological environment for evaluating microbiome performance in regulating toxic radionuclides within repository systems. Overall, this study emphasizes the pivotal role of age tracers, stable isotopes, and microbiome in enhancing the assessment of the long-term stability of fractured granite barriers.

Marsh sediments chronically exposed to nitrogen enrichment contain degraded organic matter that is less vulnerable to decomposition via nitrate reduction

Blue carbon habitats, including salt marshes, can sequester carbon at rates that are an order of magnitude greater than terrestrial forests. This ecosystem service may be under threat from nitrate (NO3-) enrichment, which can shift the microbial community and stimulate decomposition of organic matter. Despite efforts to mitigate nitrogen loading, salt marshes continue to experience chronic NO3- enrichment, however, the long-term consequence of this enrichment on carbon storage remains unclear. To investigate the effect of chronic NO3- exposure on salt marsh organic matter decomposition, we collected sediments from three sites across a range of prior NO3- exposure: a relatively pristine marsh, a marsh enriched to ~70 μmol L-1 NO3- in the flooding seawater for 13 years, and a marsh enriched between 100 and 1000 μmol L-1 for 40 years from wastewater treatment effluent. We collected sediments from 20 to 25 cm depth and determined that sediments from the most chronically enriched site had less bioavailable organic matter and a distinct assemblage of active microbial taxa compared to the other two sites. We also performed a controlled anaerobic decomposition experiment to test whether the legacy of NO3- exposure influenced the functional response to additional NO3-. We found significant changes to microbial community composition resulting from experimental NO3- addition. Experimental NO3- addition also increased microbial respiration in sediments collected from all sites. However, sediments from the most chronically enriched site exhibited the smallest increase, the lowest rates of total NO3- reduction by dissimilatory nitrate reduction to ammonium (DNRA), and the highest DNF:DNRA ratios. Our results suggest that chronic exposure to elevated NO3- may lead to residual pools of organic matter that are less biologically available for decomposition. Thus, it is important to consider the legacy of nutrient exposure when examining the carbon cycle of salt marsh sediments.

Responses of microbial communities subjected to hydrodynamically induced disturbances in an organic contaminated site

Organic contaminated sites have gained significant attention as a prominent contributor to shallow groundwater contamination. However, limited knowledge exists regarding the impact of hydrodynamic effects on microbially mediated contaminant degradation at such sites. In this study, we investigated the distribution characteristics and community structure of prokaryotic microorganisms at the selected site during both wet and dry seasons, with a particular focus on their environmental adaptations. The results revealed significant seasonal variations (P < 0.05) in the α-diversity of prokaryotes within groundwater. The dry season showed more exclusive OTUs than the wet season. The response of prokaryotic metabolism to organic pollution pressure in different seasons was explored by PICRUSt2, and enzymes associated with the degradation of organic pollutants were identified based on the predicted functions. The results showed that hormesis was considered as an adaptive response of microbial communities under pollution stress. In addition, structural equation models demonstrated that groundwater level fluctuations can, directly and indirectly, affect the abundance and diversity of prokaryotes through other factors such as oxidation reduction potential (ORP), dissolved oxygen (DO), and naphthalene (Nap). Overall, our findings imply that the taxonomic composition and functional properties of prokaryotes in groundwater in organic contaminated sites is influenced by the interaction between seasonal variations and characteristics of organic pollution. The results provide new insights into microbiological processes in groundwater systems in organic contaminated sites.

A Critical Review of Groundwater Table Fluctuation: Formation, Effects on Multifields, and Contaminant Behaviors in a Soil and Aquifer System

The groundwater table fluctuation (GTF) zone is an important medium for the hydrologic cycle between unsaturated soil and saturated aquifers, which accelerates the migration, transformation, and redistribution of contaminants and further poses a potential environmental risk to humans. In this review, we clarify the key processes in the generation of the GTF zone and examine its links with the variation of the hydrodynamic and hydrochemistry field, colloid mobilization, and contaminant migration and transformation. Driven by groundwater recharge and discharge, GTF regulates water flow and the movement of the capillary fringe, which further control the advection and dispersion of contaminants in soil and groundwater. In addition, the formation and variation of the reactive oxygen species (ROS) waterfall are impacted by GTF. The changing ROS components partially determine the characteristic transformation of solutes and the dynamic redistribution of the microbial population. GTF facilitates the migration and transformation of contaminants (such as nitrogen, heavy metals, non-aqueous phase liquids, and volatile organic compounds) through colloid mobilization, the co-migration effect, and variation of the hydrodynamic and hydrochemistry fields. In conclusion, this review illustrates the limitations of the current literature on GTF, and the significance of GTF zones in the underground environment is underscored by expounding on the future directions and prospects.

Indirect regulation of topsoil nutrient cycling by groundwater depth: impacts on sand-fixing vegetation and rhizosphere bacterial communities

Introduction

The impact of groundwater table depth (GTD) on bacterial communities and soil nutrition in revegetated areas remains unclear.

Methods

We investigated the impacts of plant growth and soil physicochemical factors on rhizosphere bacterial communities under different GTD.

Results

The four plant growth indices (Pielou, Margalef, Simpson, and Shannon-Wiener indices) and soil water content (SWC) at the Artem and Salix sites all showed a decreasing trend with increasing GTD. Salix had a higher nutrient content than Artem. The response of plant rhizosphere bacterial communities to GTD changes were as follows. Rhizosphere bacteria at the Artem and Salix sites exhibited higher relative abundance and alpha diversity in SW (GTD < 5 m) compared than in DW (GTD > 5 m). Functional microbial predictions indicated that the rhizosphere bacterial communities of Artem and Salix promoted carbon metabolism in the SW. In contrast, Artem facilitated nitrogen cycling, whereas Salix enhanced both nitrogen cycling and phototrophic metabolism in the DW.

Discussion

Mantel test analysis revealed that in the SW of Artem sites, SWC primarily governed the diversity of rhizosphere and functional bacteria involved in the nitrogen cycle by affecting plant growth. In DW, functional bacteria increase soil organic carbon (SOC) to meet nutrient demands. However, higher carbon and nitrogen availability in the rhizosphere soil was observed in the SW of the Salix sites, whereas in DW, carbon nutrient availability correlated with keystone bacteria, and changes in nitrogen content could be attributed to nitrogen mineralization. This indicates that fluctuations in the groundwater table play a role in regulating microbes and the distribution of soil carbon and nitrogen nutrients in arid environments.

Deterministic factors modulating assembly of groundwater microbial community in a nitrogen-contaminated and hydraulically-connected river-lake-floodplain ecosystem

The river-lake-floodplain system (RLFS) undergoes intensive surface-groundwater mass and energy exchanges. Some freshwater lakes are groundwater flow-through systems, serving as sinks for nitrogen (N) entering the lake. Despite the threat of cross-nitrogen contamination, the assembly of the microbial communities in the RLFS was poorly understood. Herein, the distribution, co-occurrence, and assembly pattern of microbial community were investigated in a nitrogen-contaminated and hydraulically-connected RLFS. The results showed that nitrate was widely distributed with greater accumulation on the south than on the north side, and ammonia was accumulated in the groundwater discharge area (estuary and lakeshore). The heterotrophic nitrifying bacteria and aerobic denitrifying bacteria were distributed across the entire area. In estuary and lakeshore with low levels of oxidation-reduction potential (ORP) and high levels of total organic carbon (TOC) and ammonia, dissimilatory nitrate reduction to ammonium (DNRA) bacteria were enriched. The bacterial community had close cooperative relationships, and keystone taxa harbored nitrate reduction potentials. Combined with multivariable statistics and self-organizing map (SOM) results, ammonia, TOC, and ORP acted as drivers in the spatial evolution of the bacterial community, coincidence with the predominant deterministic processes and unique niche breadth for microbial assembly. This study provides novel insight into the traits and assembly of bacterial communities and potential nitrogen cycling capacities in RLFS groundwater.