Bacterial community structure in response to environmental impacts in the intertidal sediments along the Yangtze Estuary, China

Investigating organic sulfur in estuarine and offshore environments: A combined field and cultivation approach

Estuarine-offshore sediments accumulate substantial particulate organic matter, containing organic sulfur as a key component. However, the distribution and sources of organic sulfur in such environments remain poorly understood. This study investigated organic sulfur in the Yangtze River Estuary and adjacent East China Sea. Dissolved organic sulfur varied from 0.65 to 1.99 μmol/L (molar S:C 0.006-0.018), while particulate organic sulfur ranged from 0.42 to 2.69 μmol/L (molar S:C 0.007-0.082). Sedimentary organic sulfur exhibited a similar molar S:C ratio (0.014-0.071) to particulate organic sulfur in bottom water, implying that particulate matter deposition is a potential source. Furthermore, sediments exposed to frequent hypoxia harbored significantly higher organic sulfur and S:C values compared to non-hypoxic areas. Laboratory incubation experiments revealed the underlying mechanism: sustained activity of sulfate-reducing bacteria in hypoxic sediments led to a substantial increase in sedimentary organic sulfur (from 15 to 53 μmol/g) within 600 days. This microbially driven sulfurization rendered over 90 % of the organic sulfur resistant to acid hydrolysis. Therefore, this study demonstrates that, alongside particle deposition, microbial sulfurization significantly contributes to organic sulfur enrichment and likely promotes organic matter preservation in estuarine-offshore sediments, particularly under hypoxic conditions. This finding advances our understanding of organic sulfur sources in these vital ecosystems.

Typical antibiotic resistance genes and their association with driving factors in the coastal areas of Yangtze River Estuary

The massive use of antibiotics has led to the escalation of microbial resistance in aquatic environment, resulting in an increasing concern regarding antibiotic resistance genes (ARGs), posing a serious threat to ecological safety and human health. In this study, surface water samples were collected at eight sampling sites along the Yangtze River Estuary. The seasonal and spatial distribution patterns of 10 antibiotics and target genes in two major classes (sulfonamides and tetracyclines) were analyzed. The findings indicated a high prevalence of sulfonamide and tetracycline resistance genes along the Yangtze River Estuary. Kruskal-Wallis analysis revealed significant seasonal variations in the abundance of all target genes. The accumulation of antibiotic resistance genes in the coastal area of the Yangtze River Estuary can be attributed to the influence of urban instream runoff and the discharge of effluents from wastewater treatment plants. ANISOM analysis indicated significant seasonal differences in the microbial community structure. VPA showed that environmental factors contribute the most to ARG variation. PLS-PM demonstrate that environmental factors and microbial communities pose direct effect to ARG variation. Analysis of driving factors influencing ARGs in this study may shed new insights into the mechanism of the maintenance and propagation of ARGs.

Spatial heterogeneity of sedimentary organic matter sources in the Yangtze River estuary: Implications from fatty acid biomarkers

This study investigated the sources of sedimentary organic matter (OM) in the Yangtze River estuary (YRE), using multiple biomarkers. The results of stable carbon isotope (δ13C) and total organic carbon to nitrogen ratio (TOC/TN) suggests the contribution of marine-derived OM significantly increased seawards, while fatty acid (FA) composition provides more specific information on OM sources. In total, 30 components of FAs were identified at the studied 17 sites, which mainly composed of phytoplankton FA, followed by ubiquitous FA and bacterial FA, while terrestrial FA contributed less to the total FAs. Under the strong impacts of the large physicochemical gradients in the YRE, TOC, TN and FA components showed higher concentrations in the estuary mixing zone (especially within the turbidity maximum zone), attributing to their strong binding with OM-enriched fine particles. The spatial heterogeneity of sedimentary OM sources was highly impacted by salinity and Chl-a, as well as bacteria-mediated OM degradation.

Differences of bacterioplankton communities between the source and upstream regions of the Yangtze River: microbial structure, co-occurrence pattern, and environmental influencing factors

The source area of the Yangtze River is located in the hinterland of the Qinghai-Tibet Plateau, which is known as the "Earth's third pole." It is the water conservation area and the natural barrier of the ecosystem of the Yangtze River basin. It is also the most sensitive area of the natural ecosystem, and the ecological environment is very fragile. Microorganisms play key roles in the biogeochemical processes of water. In this paper, the bacterioplankton communities in the source and upstream regions of the Yangtze River were studied based on 16S rRNA high-throughput sequencing, and their environmental influencing factors were further analyzed. Results showed that the upstream region had higher richness and diversity than the source region. The predominant bacterial phyla in the source and upstream regions were Proteobacteria, Firmicutes, and Actinobacteriota. The bacterial phyla associated with municipal pollution and opportunistic pathogen, such as Firmicutes and Actinobacteriota, were more abundant in the upstream. By contrast, distinct planktonic bacterial genera associated with mining pollution, such as Acidiphilium and Acidithiobacillus, were more abundant in the source region. The co-occurrence network showed that the interaction of bacterioplankton community is more frequent in the upstream. The bacterioplankton community compositions, richness, and functional profiles were affected by the spatial heterogeneity. Moreover, variation partitioning analysis further confirmed that the amount of variation in the source region independently explained by variables of altitude was the largest, followed by water nutrient. This paper revealed the spatial distribution of planktonic bacterial communities in the source and upstream regions of the Yangtze River and its correlation with environmental factors, providing information support for ensuring the health and safety of aquatic ecosystems in the Yangtze River Basin.

Distinct Assembly Processes Structure Planktonic Bacterial Communities Among Near- and Offshore Ecosystems in the Yangtze River Estuary

The estuarine system functions as natural filters due to its ability to facilitate material transformation, planktonic bacteria play a crucial role in the cycling of complex nutrients and pollutants within estuaries, and understanding the community composition and assembly therein is crucial for comprehending bacterial ecology within estuaries. Despite extensive investigations into the composition and community assembly of two bacterial fractions (free-living, FLB; particle-attached, PAB), the process by which bacterioplankton communities in these two habitats assemble in the nearshore and offshore zones of estuarine ecosystems remains poorly understood. In this study, we conducted sampling in the Yangtze River Estuary (YRE) to investigate potential variations in the composition and community assembly of FLB and PAB in nearshore and offshore regions. We collected 90 samples of surface, middle, and bottom water from 16 sampling stations and performed 16S rRNA gene amplicon analysis along with environmental factor measurements. The results unveiled that the nearshore communities demonstrated significantly greater species richness and Chao1 indices compared to the offshore communities. In contrast, the nearshore communities had lower values of Shannon and Simpson indices. When compared to the FLB, the PAB exhibit a higher level of biodiversity and abundance. However, no distinct alpha and beta diversity differences were observed between the bottom, middle, and surface water layers. The community assembly analysis indicated that nearshore communities are predominantly shaped by deterministic processes, particularly due to heterogeneous selection of PAB; In contrast, offshore communities are governed more by stochastic processes, largely due to homogenizing dispersal of FLB. Consequently, the findings of this study demonstrate that nearshore and PAB communities exhibit higher levels of species diversity, while stochastic and deterministic processes exert distinct influences on communities among near- and offshore regions. This study further sheds new light on our understanding of the mechanisms governing bacterial communities in estuarine ecosystems.

Influences of hydrodynamics on microbial community assembly and organic carbon composition of resuspended sediments in shallow marginal seas

Hydrodynamic processes play a crucial role in driving the transmission of sediments, likely harboring diverse microbes and heterogeneous organic carbon (OC) in the ocean. Here we conducted continuous micro-erosion experiments on surface sediments retrieved from shallow marginal seas, and analyzed the microbial community structures, OC content, and isotope compositions (δ13C and Δ14C) of resuspended sediments to investigate the effects of hydrodynamics on microbial assembly and OC composition in marginal seas. Our results showed that gene abundance and major microbial compositions in resuspended sediments changed with varying benthic shear stresses, which evolved towards diversification after continuous hydrodynamic erosion. Aerobic bacteria were more likely to be eroded out from sediments under lower shear stresses compared with anaerobic bacteria. Our study provides evidence that hydrodynamic disturbances shape the assembly of microbial communities with different metabolic functions, especially for bacteria, which may spatially influence the microbial-mediated biogeochemical transformation in marginal seas. Isotopic results revealed that more terrestrial OC was resuspended under initial erosion, while more marine OC was eroded out with increasing shear stresses, suggesting that hydrodynamics may control the redistribution of different sourced OC and contribute to the dispersion and degradation of terrestrial OC during transport process. Our findings further suggest that the nature of resuspended OC may influence the assembly of sediment-attached microbes due to their metabolic preference for carbon sources, as evidenced by correlations between OC compositions and microbial diversity and abundance. We thus suggest that hydrodynamic disturbance is an extrinsic physical driver of OC redistribution and microbial reassembly, whereas OC may be an intrinsic factor influencing microbial colonization, helping to interpret the spatial heterogeneity of microbes and OC compositions observed in marginal sea sediments. Our study underscores the significant roles of hydrodynamic-driven sediment resuspension in shaping diverse microbial communities and redistributing OC in aquatic systems, and highlights the importance of this process in biogeochemical cycles and ecological environment evolution in shallow marginal sea systems.

Distribution heterogeneity of sediment bacterial community in the river-lake system impacted by nonferrous metal mines: Diversity, composition and co-occurrence patterns

Metal(loid) pollution caused by mining activities can affect microbial communities. However, knowledge of the diversity, composition, and co-occurrence patterns of bacterial communities in aquatic systems impacted by nonferrous metal mines. Here, the metal(loid) contents and bacterial communities in sediments from the Zijiang River (tributary to mainstream) to Dongting Lake were investigated by geochemical and molecular biology methods. The results indicated that the river sediments had lower pH and higher ecological risk of metal(loid)s than the lake sediment. The diversity and composition of bacterial communities in river sediments significantly (p < 0.05) differed from those in lake sediments, showing distributional heterogeneity. The biomarkers of tributary, mainstream, and lake sediments were mainly members of Deltaproteobacteria, Firmicutes, and Nitrospirae, respectively, reflecting species sorting in different habitats. Multivariate statistical analysis demonstrated that total and bioavailable Sb, As, and Zn were positively correlated with bacterial community richness. pH, TOC, TN, and Zn were crucial factors in shaping the distribution difference of bacterial communities. Environment-bacteria network analysis indicated that pH, SO42-, and total and bioavailable As and Sb greatly influenced the bacterial composition at the genus level. Bacteria-bacteria network analysis manifested that the co-occurrence network in mainstream sediments with a higher risk of metal(loid) pollution exhibited higher modularity and connectivity, which might be the survival mechanism for bacterial communities adapted to metal(loid) pollution. This study can provide a theoretical basis for understanding the ecological status of aquatic systems.

Functional redundancy in response to runoff input upholds microbial community in hydrocarbon-contaminated land-sea continuum

An experimental approach mimicking the land-sea continuum in microcosms was developed in order to determine the effect of the terrigenous inputs by soil runoff on the microbial functional potential in hydrocarbon (HC) contaminated marine coastal sediment. We hypothesized that the coalescent event increases the functional potential of microbial communities in marine coastal sediments, influencing the fate of HC in marine coastal ecosystems. The microbial functional potential including the HC degradation ability was assessed by DNA-array to compare the sediment receiving or not terrigenous inputs. The removal of HC and the functional gene richness in sediment was unchanged with the terrigenous inputs. However, the gene variants (GVs) composition was modified indicating functional redundancy. In addition, functional indicators including GVs related to sulfite reduction, denitrification and polyaromatic degradation were identified in higher proportion in sediment receiving terrigenous inputs. The terrigenous inputs modified the functional co-occurrence networks, showing a reorganization of the GVs associations with an increase of the network complexity. Different keystone GVs ensuring similar functions were identified in networks with or without terrigenous inputs, further confirming functional redundancy. We argue that functional redundancy maintains the structure of microbial community in hydrocarbon-contaminated land-sea continuum mixing zone. Our results provide helpful functional information for the monitoring and management of coastal environment affected by human land-based activities.

Planktonic and epilithic prokaryota community compositions in a large temperate river reflect climate change related seasonal shifts

In freshwaters, microbial communities are of outstanding importance both from ecological and public health perspectives, however, they are threatened by the impact of global warming. To reveal how different prokaryotic communities in a large temperate river respond to environment conditions related to climate change, the present study provides the first detailed insight into the composition and spatial and year-round temporal variations of planktonic and epilithic prokaryotic community. Microbial diversity was studied using high-throughput next generation amplicon sequencing. Sampling was carried out monthly in the midstream and the littoral zone of the Danube, upstream and downstream from a large urban area. Result demonstrated that river habitats predominantly determine the taxonomic composition of the microbiota; diverse and well-differentiated microbial communities developed in water and epilithon, with higher variance in the latter. The composition of bacterioplankton clearly followed the prolongation of the summer resulting from climate change, while the epilithon community was less responsive. Rising water temperatures was associated with increased abundances of many taxa (such as phylum Actinobacteria, class Gammaproteobacteria and orders Synechococcales, Alteromonadales, Chitinophagales, Pseudomonadales, Rhizobiales and Xanthomonadales), and the composition of the microbiota also reflected changes of several further environmental factors (such as turbidity, TOC, electric conductivity, pH and the concentration of phosphate, sulphate, nitrate, total nitrogen and the dissolved oxygen). The results indicate that shift in microbial community responding to changing environment may be of crucial importance in the decomposition of organic compounds (including pollutants and xenobiotics), the transformation and accumulation of heavy metals and the occurrence of pathogens or antimicrobial resistant organisms.

Spatiotemporal distributions of sulfonamide and tetracycline resistance genes and microbial communities in the coastal areas of the Yangtze River Estuary

In this paper, water and sediments were sampled at eight monitoring stations in the coastal areas of the Yangtze River Estuary in summer and autumn 2021. Two sulfonamide resistance genes (sul1 and sul2), six tetracycline resistance genes (tetM, tetC, tetX, tetA, tetO, and tetQ), one integrase gene (intI1), 16 S rRNA genes, and microbial communities were examined and analyzed. Most resistance genes showed relatively higher abundance in summer and lower abundance in autumn. One-way analysis of variance (ANOVA) showed significant seasonal variation of some ARGs (7 ARGs in water and 6 ARGs in sediment). River runoff and WWTPs are proven to be the major sources of resistance genes along the Yangtze River Estuary. Significant and positive correlations between intI1 and other ARGs were found in water samples (P < 0.05), implying that intI1 may influence the spread and propagation of resistance genes in aquatic environments. Proteobacteria was the dominant phylum along the Yangtze River Estuary, with an average proportion of 41.7%. Redundancy analysis indicated that the ARGs were greatly affected by temperature, dissolved oxygen, and pH in estuarine environments. Network analysis showed that Proteobacteria and Cyanobacteria were the potential host phyla for ARGs in the coastal areas of the Yangtze River Estuary.

Geographic patterns of microbial traits of river basins in China

River microbiotas contribute to critical geochemical processes and ecological functions of rivers but are sensitive to variations of environmental drivers. Understanding the geographic pattern of river microbial traits in biogeochemical processes can provide important insights into river health. Many studies have characterized river microbial traits in specific situations, but the geographic patterns of these traits and environmental drivers at a large scale are unknown. We reanalyzed 4505 raw 16S rRNA sequences samples for microbiota from river basins in China. The results indicated differences in the diversity, composition, and structure of microbiotas across diverse river basins. Microbial diversity and functional potential in the river basins decreased over time in northern China and increased in southern China due to niche differentiation, e.g., the Yangtze River basin was the healthiest ecosystem. River microbiotas were mainly involved in the cycling of carbon and nitrogen in the river ecosystems and participated in potential organic metabolic functions. Anthropogenic pollutants discharge was the most critical environmental driver for the microbial traits, e.g., antibiotic discharge, followed by climate change. The prediction by machine-learning models indicated that the continuous discharge of antibiotics and climate change led to high ecological risks for the rivers. Our study provides guidelines for improving the health of river ecosystems and for the formulation of strategies to restore the rivers.

Keystone microbial taxa organize micropollutant-related modules shaping the microbial community structure in estuarine sediments

The fluctuation of environmental conditions drives the structure of microbial communities in estuaries, highly dynamic ecosystems. Microorganisms inhabiting estuarine sediments play a key role in ecosystem functioning. They are well adapted to the changing conditions, also threatened by the presence of pollutants. In order to determine the environmental characteristics driving the organization of the microbial assemblages, we conducted a seasonal survey along the Adour Estuary (Bay of Biscay, France) using 16S rRNA gene Illumina sequencing. Microbial diversity data were combined with a set of chemical analyses targeting metals and pharmaceuticals. Microbial communities were largely dominated by Proteobacteria (41 %) and Bacteroidota (32 %), showing a strong organization according to season, with an important shift in winter. The composition of microbial communities showed spatial distribution according to three main areas (upstream, middle, and downstream estuary) revealing the influence of the Adour River. Further analyses indicated that the microbial community was influenced by biogeochemical parameters (C_org/N_org and δ¹³C) and micropollutants, including metals (As, Cu, Mn, Sn, Ti, and Zn) and pharmaceuticals (norfloxacin, oxolinic acid and trimethoprim). Network analysis revealed specific modules, organized around keystone taxa, linked to a pollutant type, providing information of paramount importance to understand the microbial ecology in estuarine ecosystems.

Composition of Particulate Matter and Bacterial Community in Gut Contents and Surrounding Sediments of Three Sipunculan Species (Siphonosoma australe, Phascolosoma arcuatum, and Sipunculus nudus)

Siphonosoma australe, Phascolosoma arcuatum, and Sipunculus nudus are three important sipunculan species in tropical intertidal zones. In this study, the particle size, organic matter content, and bacterial community composition in the gut contents of three different sipunculans and their surrounding sediments were analyzed. The grain size fractions of sipunculans' guts were significantly different from those of their surrounding sediments; particle size fractions < 500 μm were favored by the sipunculans. As for the total organic matter (TOM), higher contents of organic matter were observed in the guts than in the surrounding sediments in all three sipunculan species. The bacterial community composition of all the samples was investigated by 16S rRNA gene sequencing, in which a total of 8974 OTUs were obtained from 24 samples based on a 97% threshold. The predominant phylum identified from the gut contents of three sipunculans was Planctomycetota, while the predominant phylum in their surrounding sediments was Proteobacteria. At the genus level, the most abundant genus was Sulfurovum (average 4.36%) in the surrounding sediments, while the most abundant genus was Gplla (average 12.76%) in the gut contents. The UPGMA tree showed that the samples from the guts of three different sipunculans and their surrounding sediments were clustered separately into two groups, which showed that these three sipunculans had a different bacterial community composition with their surrounding sediments. The grain size and total organic matter (TOM) had the greatest impacts on the bacterial community composition at both the phylum and genus levels. In conclusion, the differences in particle size fractions, organic matter content, and bacterial community composition between the gut contents and surrounding sediments in these three sipunculan species might be caused by their selective ingestion.

Comparing diversity patterns and processes of microbial community assembly in water column and sediment in Lake Wuchang, China

The study compare the diversity patterns and processes of microbial community assembly in the water and sediment of Lake Wuchang (China) using high-throughput sequencing of 16S rRNA gene amplicons. A higher microbial α-diversity in the sediment was revealed (P < 0.01), and the most common bacterial phyla in water column were Proteobacteria, Cyanobacteria and Actinobacteria, while Proteobacteria, Acidobacteria, Chloroflexi and Nitrospirae were dominant in sediment. Functions related to phototrophy and nitrogen metabolism primarily occurred in the water column and sediment, respectively. The microbial communities in water column from different seasons were divided into three groups, while no such dispersion in sediment based on PCoA and ANOSIM. According to Pearson correlation analysis, water temperature, dissolved oxygen, water depth, total nitrogen, ammonium, and nitrite were key factors in determining microbial community structure in water column, while TN in sediment, conductivity, and organic matter were key factors in sediment. However, the stochastic processes (|βNTI| < 2) dominated community assembly in both the water column and sediment of Lake Wuchang. These data will provide a foundation for microbial development and utilization in lake water column and sediment under the circumstances of increasing tendency of lake ecological fishery in China.

Stochastic processes shape the aggregation of free-living and particle-attached bacterial communities in the Yangtze River Estuary, China

An estuary plays an important role in material and energy exchange between the land and sea, where complex physical, chemical, and biological processes occur. Here, we investigated the assembly processes of free-living (FL) and particle-associated (PA) bacterial communities in two seawater layers at five stations in the Yangtze River Estuary (YRE) by using 16S rRNA sequencing methods. The results indicated that Proteobacteria was the most abundant phylum in the YRE. The α-diversity of PA community was significantly higher than FL community, and analysis of similarity showed significantly different (Global R = 0.2809, p < 0.005). RDA revealed that phosphate (PO₄ ^3- ) was significantly correlated with PA bacterial community abundance (p < 0.05). An ecological null model showed that both PA and FL bacterial communities were mainly influenced by stochastic processes (PA: 100%, FL: 70%), which PA attached to nutrient particles and are less affected by environmental filtration. Dispersal limitation (50%) was the main assembly process of the PA community, while homogeneous selection (30%) and drift (30%) were important processes in the FL community assembly. The available substrate for colonization limits the transformation from FL to PA bacteria. This study would improve our understanding of FL and PA bacterial community structure and factors affecting assembly process in estuarine environments.

Biogeographic patterns of benthic microbial communities in metal(loid)-contaminated semi-enclosed bay

Anthropogenic activities can adversely impact biogeochemical processes essential for maintaining ecosystem health in semi-enclosed bays. However, the influence of anthropogenic contaminants such as potentially toxic elements on microbial communities that regulate biogeochemical cycles in semi-enclosed bays is poorly understood. We determined the concentrations of four potentially toxic elements (Cu, Zn, Pb, and As) in sediments from a typical tropical semi-enclosed bay in Guangdong, China. Source apportionment using Pearson's correlation analysis revealed that aquaculture activities were probably the primary source of Cu, Zn, and Pb. Using high-throughput sediment DNA sequencing, we found that Proteobacteria was the dominant phylum in sediments. There was no evidence suggesting site-specific variation in microbial function even though sediments adjacent to aquaculture discharge points had higher microbial diversity. In contrast, pollutant-specific variations were observed; for example, Zn and Pd showed potential adverse effects on the environmental information processing function, while As showed a negative correlation with metabolic function. Based on different environmental characteristics, future research should consider the impact of multiple factors on the bacterial community in aquaculture systems.

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.

Effects of a Furrow-Bed Seeding System on Stand Establishment, Soil Bacterial Diversity, and the Yield and Quality of Alfalfa Under Saline Condition

Salt stress account for large decreases in crop yield all over the world. Furrow-bed system is an efficient practice to promote plant growth in saline soil. However, the effects of Furrow-bed system on the soil environment and the growth of alfalfa (Medicago sativa L.) in salinity are not clear. For a wider and more detail evaluation, alfalfa were planted in saline sandy loam soil in fall, the effects of two plant systems (FU, furrow-bed seeding system; FL, flat-bed seeding system) on soil moisture, root zone salinity, soil microbial community structure, seedling emergence number in the early stage of the growth period and soil nutrient contents, alfalfa production characteristics in the second growth year were determined in a 2-year field experiment. The result showed that, compared with FL, FU resulted in increased soil moisture content and seedling emergence, and significantly reduced relative abundance of Actinobacteria and Choroflexi in soil, but it did not affect root zone salinity at the seedling stage. In April of second growth year, the soil salinity was lower, and the soil available phosphorus, potassium, nitrogen, and soil organic matter contents of the root zone were higher in FU than in FL. Compared with FL, FU resulted in increased yield (by 37.5%), protein content (by 3.6%), and potassium concentration (by 33.2%), and decreased ash content (by 7.7%), and sodium concentration (by 19.0%) in alfalfa plants. Pearson's correlation analysis indicated that the increased yield was positively correlated with seedling emergence, soil available potassium, total nitrogen, and organic matter contents, and shoot potassium content and negatively correlated with shoot sodium content. The relative abundance of Actinobacteria was negatively correlated with alfalfa ash, calcium, and sodium concentrations, and positively correlated with shoot potassium content. Taken together, the results indicate that Furrow-bed seeding in early fall alleviated salt stress of alfalfa and have the potential to enhance the yield and quality of alfalfa cultivated in saline soils by improving the soil environment and regulating the growth and physiology of alfalfa. Graphical Abstract.

Tracking the impacts of nutrient inputs on estuary ecosystem function

Estuaries are one of the most impacted coastal environments globally, subjected to multiple stressors from urban, industry and coastal development. With increasing anthropogenic activity surrounding estuarine systems, sewage inputs have become a common concern. Stable isotope analysis provides a well-established tool to investigate the incorporation of nitrogen into marine organisms and identify major nutrient sources. Benthic macroinvertebrate communities are often used as bioindicators in ecological studies as they typically display predictable responses to anthropogenic pressures, however have a suite of limitations and costs associated with their use. 16S rDNA amplicon sequencing techniques allow for investigation of the microbial communities inhabiting complex environmental samples, with potential as a tool in the ecological assessment of pollution. These communities have not yet been adequately considered for ecological studies and biomonitoring, with a need to better understand interactions with environmental stressors and implications for ecosystem function. This study used a combination of stable isotope analysis to trace the uptake of anthropogenic nitrogen in biota, traditional assessment of benthic macroinvertebrate communities, and 16S rDNA genotyping of benthic microbial communities. Stable isotope analysis of seagrass and epiphytes identified multiple treated and untreated sewage inputs, ranges of 5.2-7.2‰ and 1.9-4.0‰ for δ¹⁵N respectively, as the dominant nitrogen source at specific locations. The benthic macroinvertebrate community reflected these inputs with shifts in dominant taxa and high abundances of polychaetes at some sites. Microbial communities provided a sensitive indication of impact with a breadth of information not available using traditional techniques. Composition and predicted function reflected sewage inputs, particularly within sediments, with the relative abundance of specific taxa and putative pathogens linked to these inputs. This research supports the growing body of evidence that benthic microbial communities respond rapidly to anthropogenic stressors and have potential as a monitoring tool in urban estuarine systems.

Human activities can drive sulfate-reducing bacteria community in Chinese intertidal sediments by affecting metal distribution

Sulfate-reducing bacteria (SRB), which are ubiquitous in intertidal sediments, play an important role in global sulfur and carbon cycles, and in the bioremediation of toxic metalloids/metals. Pollution from human activities is now a major challenge to the sustainable development of the intertidal zone, but little is known about how and to what extent various anthropic and/or natural factors affect the SRB community. In the current study, based on the dsrB gene, we investigated the SRB community in intertidal sediment along China's coastline. The results showed that dsrB gene abundances varied among different sampling sites, with the highest average abundance of SRB at XHR (near the Bohai Sea). The SRB community structures showed obvious spatial distribution patterns with latitude along the coastal areas of China, with Desulfobulbus generally being the dominant genus. Correlation analysis and redundancy discriminant analysis revealed that total organic carbon (TOC) and pH were significantly correlated with the richness of the SRB community, and salinity, pH, sulfate and climatic parameters could be the important natural factors influencing the composition of the SRB community. Moreover, metals, especially bioavailable metals, could regulate the diversity and composition of the SRB communities. Importantly, according to structural equation model (SEM) analysis, anthropic factors (e.g., population, economy and industrial activities) could drive SRB community diversity directly or by significantly affecting the concentrations of metals. This study provides the first comprehensive investigation of the direct and indirect anthropic factors on the SRB community in intertidal sediments on a continental scale.

Characteristics of bacterial community structure and function associated with nutrients and heavy metals in coastal aquaculture area

Coastal aquaculture area has become one of the critical zones that are more susceptible to the influence of human activity. Many aquaculture operations invariably result in the accumulation of nutrients and heavy metals in the coastal ecosystem. Our study investigated sediment bacterial community structure and function across 23 sites under the influence of nutrients and heavy metals in the coastal aquaculture area. The habitat environment of the sediment was described by analyzing physicochemical characteristics. Sediment bacterial community structure and diversity were investigated by 16S rRNA sequencing. The sequencing data presented that Proteobacteria, Bacteroidetes, Planctomycetes, Acidobacteria and Chloroflexi were predominant at phylum level. Variations in the bacterial community composition and diversity were significant (P < 0.01) among different groups (according to the distance from the bank side) which indicated that specific environmental conditions had shaped distinct bacterial community. Specifically, bacterial diversity and composition were significantly influenced by the temperature, salinity, pH, dissolved oxygen (DO), TOC, TON, nitrite, nitrate and heavy metals (P < 0.05). Results related to functional prediction demonstrated that carbon, nitrogen and sulfur metabolism were the dominant processes in the coastal aquaculture area. In the meantime, the potential pathogens such as Arcobacter was found in site S3, which indicated the possible threat to the cultured species in this area. Overall, variations in bacterial communities caused by nutrients and heavy metals can affect biogeochemical cycles, which may provide an indication for the protection of coastal aquaculture environments.

Full-length 16S rRNA gene sequencing reveals spatiotemporal dynamics of bacterial community in a heavily polluted estuary, China

Understanding the bacterial community structure of the river estuary could provide insights into the resident microorganisms in response to environmental pollution. In this study, the bacterial community structure of Liaohe Estuary was investigated using single-molecule real-time sequencing (SMRT). A total of 57 samples were collected and grouped according to habitat, space, season, and lifestyle. In seawater, regardless of whether it is particle-attached (PA) or free-living (FL) bacteria, the area with higher alpha diversity is the nearshore area in the dry season, while it is the midstream area in the wet season. The bacterial communities in sediment and seawater samples were different at the genus level in the nearshore area, and habitat type was the main factor. A marked difference in the bacterial community was observed in the dry season between different lifestyles but not in the wet season, which resulted from lifestyle transitions of bacterioplankton. Bacterial community varied spatially but not seasonally in sediment samples. In seawater, both FL and PA bacterial communities varied spatially during the wet season. Seasonal differences were only observed in FL bacterial community. Zn and sand were the principal determining factors of the bacterial community in the sediment, Cu and salinity were the main environmental factors for FL bacteria, and Cu, salinity, Zn and temperature were the main environmental factors for PA bacteria. Besides, the tide and nutrients were also the main drivers of the bacterial community in seawater. The indicative taxa, related to Cyanobium_PCC-6307, Pseudomonas and Vibrio, further evidenced the presence of possible bloom, crude oil and pathogen contamination. Overall, our results can contribute to the knowledge of the bacterial community and anthropogenic impacts on the Liaohe Estuary.

Biogeography of ammonia oxidizers in New England and Gulf of Mexico salt marshes and the potential importance of comammox

Few studies have focused on broad scale biogeographic patterns of ammonia oxidizers in coastal systems, yet understanding the processes that govern them is paramount to understanding the mechanisms that drive biodiversity, and ultimately impact ecosystem processes. Here we present a meta-analysis of 16 years of data of ammonia oxidizer abundance, diversity, and activity in New England (NE) salt marshes and 5 years of data from marshes in the Gulf of Mexico (GoM). Potential nitrification rates were more than 80x higher in GoM compared to NE marshes. However, nitrifier abundances varied between regions, with ammonia-oxidizing archaea (AOA) and comammox bacteria significantly greater in GoM, while ammonia-oxidizing bacteria (AOB) were more than 20x higher in NE than GoM. Total bacterial 16S rRNA genes were also significantly greater in GoM marshes. Correlation analyses of rates and abundance suggest that AOA and comammox are more important in GoM marshes, whereas AOB are more important in NE marshes. Furthermore, ratios of nitrifiers to total bacteria in NE were as much as 80x higher than in the GoM, suggesting differences in the relative importance of nitrifiers between these systems. Communities of AOA and AOB were also significantly different between the two regions, based on amoA sequences and DNA fingerprints (terminal restriction fragment length polymorphism). Differences in rates and abundances may be due to differences in salinity, temperature, and N loading between the regions, and suggest significantly different N cycling dynamics in GoM and NE marshes that are likely driven by strong environmental differences between the regions.

Compositional changes of sedimentary microbes in the Yangtze River Estuary and their roles in the biochemical cycle

Due to the geographical circumstances, the Yangtze River Estuary (YRE) and the adjacent East China Sea are extensively influenced by both anthropogenic activities and environmental factors. To reveal the responses of microbes in surface sediment to environmental factors and their contributions to the biogeochemical cycle in this area, surface sediment and overlying water samples were collected at 21 stations from the estuary to the coastal region. Water and sediment parameters were determined, and 16S rRNA genes of microbes in sediment samples were sequenced using high throughput sequencing technology. The results indicated that ocean currents, sediment density (SD), nutrients, sulfate (SO₄^2-), and salinity were the key factors shaping the microbial communities. Coastal microbes were affected mainly by SD, whereas anthropogenic discharge might have been responsible for a decrease in indigenous microbial diversity in the ocean. Due to the anthropogenic discharge, the most representative bacteria in the nearshore were aerobic and chemoheterotrophic bacteria, including ammonia-oxidizing bacteria, nitrite-oxidizing bacteria, denitrifying bacteria, and polyphosphate accumulating organisms. In the offshore, anaerobic bacteria, thermophilic bacteria, halophilic bacteria, sulfate-reducing bacteria, and sulfide oxidizing bacteria were the dominant bacteria, and these were characterized by strong solidarity and cooperative properties within the malnourished environment. In summary, these results provide a new perspective for revealing the biogeochemical significance of the bacterial lineages in the YRE, as well as constructive guidance for the management of the marginal sea ecosystems in distinct regions.

Influences of Seasonal Monsoons on the Taxonomic Composition and Diversity of Bacterial Community in the Eastern Tropical Indian Ocean

The Indian Ocean is characterized by its complex physical systems and strong seasonal monsoons. To better understand effects of seasonal monsoon-driven circulation on the bacterioplanktonic community structure in surface waters and the bacterial distribution response to vertical stratification, patterns of seasonal, and vertical distribution of bacterial communities in the Eastern Tropical Indian Ocean were investigated using 16S rRNA gene profiling. Water samples were collected during the Southwest monsoon (from June to August), the fall inter-monsoon (from October and November) and the Northeast monsoon (from December to January), respectively, onboard during three cruises from July 2016 to January 2018. Surface bacterioplankton communities in these three seasons and in the upper water (3-300 m with six depths) during the Northeast monsoon contained a diverse group of taxa, mainly Proteobacteria, Cyanobacteria, Actinobacteria, Bacteroidetes, and Chloroflexi. Redundancy discriminant analysis (RDA) uncovered that temperature, salinity, and dissolved oxygen (DO) were crucial environmental parameters that affected the structure of bacterial community in overall surface samples. However, significant differences in the composition of the bacterial community are likely due to changes in concentrations of salinity during the fall inter-monsoon, while phosphate for both the Southwest monsoon and the Northeast monsoon. Pearson's analysis revealed that the seasonal variation rather than the vertical variation of environmental factors had a more significant impact on the composition of bacterial community. In addition, a clear seasonal pattern of bacterial co-occurrence showed that inter-taxa associations during the fall inter-monsoon were closer than during the Northeast monsoon and the Southwest monsoon. Overall, our results implied clear differences in the composition of bacterial community, with more pronounced seasonal variation compared to the vertical variation in response to environmental changes.

Antibiotic resistance genes in sediments of the Yangtze Estuary: From 2007 to 2019

To better understand the occurrence and succession of antibiotic resistance genes (ARGs) in the environment, the investigation of ARGs in sediment for a long time scale is urgently needed. In this study, sediment samples were taken in the Yangtze Estuarine area from 2007 to 2019, and the interannual variations in ARGs and their possible physicochemical and socioeconomic influencing factors were analyzed. The results showed that the abundance of ARGs, including sul1, sul2, tetM, tetW, aac(6')-Ib and qnrS, was higher in recent years (from 2015 to 2019) than that in earlier years (from 2007 to 2011), and heavier ARG pollution was found in Wusongkou (WSK) samples than in Liuhekou (LHK) samples. According to the redundancy discriminant analysis (RDA) and correlation analysis, the antibiotics (especially individual antibiotic categories, including oxytetracycline, doxycycline hyclate and norfloxacin), metals and a metal resistance gene (zntA) and total organic carbon (TOC) showed significant correlations to ARGs. In addition, antibiotics, metals, TOC and ARGs were also significantly correlated with several socioeconomic indices. Furthermore, the extended STIRPAT model analysis revealed that the second industry product and the first industry product were the major socioeconomic driver factors for the ARG distribution at WSK and LHK, respectively. Overall, with socioeconomic development, antibiotics, metals, TOC and ARGs increased in sediment. In addition, antibiotics, metals and TOC may participate in the regulation of the occurrence and distribution of ARGs in the Yangtze Estuary for the long time scale.

Bacterial community composition and diversity in the ballast water of container ships arriving at Yangshan Port, Shanghai, China

Ballast water is a major vector of invasion by protozoans and metazoans. Bacterial invasion is less-well understood. We surveyed the bacterial diversity of ballast water from 26 container ships arriving at the Yangshan Deepwater Port, Shanghai, China during 2015-2016. We characterized the ballast microbiome using high-throughput sequencing (HTS) based on V4-V5 region of 16S rRNA genes. We simultaneously monitored physicochemical parameters of the ballast water, including temperature, pH, dissolved oxygen (DO), salinity, turbidity, total suspended solid (TSS), particulate organic carbon (POC), NO₂, NH₄, PO₄. Proteobacteria was the dominant phylum, comprising more than 50% of the OTUs of almost all vessels, followed by Bacteroidetes (12.08%), Actinobacteria (4.86%) Planctomycetes (3.24%) and Cyanobacteria (1.95%). The relative abundance of Cyanobacteria differed among vessels. It was negatively correlated with temperature, NO₃, pH, TSS, PO₄, and turbidity and positively correlated with NH₄, POC. The genus Synechococcus was the most common Cyanobacteria in our results. Escherichia coli were relatively rare; they are indicator-species of D-2 standards published by the IMO. The relative abundance of the genus Vibrio ranged from 0.003% to 24.88% among different vessels. Our results showed that HTS was able to profile the bacterial communities in ballast-waters, even when the approach was restricted by technical and other obstacles.

Sediment Bacterial Community Structure Under the Influence of Different Domestic Sewage Types

Sediment bacterial communities are critical to the biogeochemical cycle in river ecosystems, but our understanding of the relationship between sediment bacterial communities and their specific input streams in rivers remains insufficient. In this study, we analyzed the sediment bacterial community structure in a local river receiving discharge of urban domestic sewage by applying Illumina MiSeq high-throughput sequencing. The results showed that the bacterial communities of sediments samples of different pollution types had similar dominant phyla, mainly Proteobacteria, Actinobacteria, Chloroflexi and Firmicutes, but their relative abundances were different. Moreover, there were great differences at the genus level. For example, the genus Bacillus showed statistically significant differences in the hotel site. The clustering of bacterial communities at various sites and the dominant families (i.e., Nocardioidaceae, and Sphingomonadaceae) observed in the residential quarter differed from other sites. This result suggested that environmentally induced species sorting greatly influenced the sediment bacterial community composition. The bacterial cooccurrence patterns showed that the river bacteria had a nonrandom modular structure. Microbial taxonomy from the same module had strong ecological links (such as the nitrogenium cycle and degradation of organic pollutants). Additionally, PICRUSt metabolic inference analysis showed the most important function of river bacterial communities under the influence of different types of domestic sewage was metabolism (e.g., genes related to xenobiotic degradation predominated in residential quarter samples). In general, our results emphasize that the adaptive changes and interactions in the bacterial community structure of river sediment represent responses to different exogenous pollution sources.

Impact of long-term industrial contamination on the bacterial communities in urban river sediments

BACKGROUND

The contamination of the aquatic environment of urban rivers with industrial wastewater has affected the abiotic conditions and biological activities of the trophic levels of the ecosystem, particularly sediments. However, most current research about microorganism in urban aquatic environments has focused on indicator bacteria related to feces and organic pollution. Meanwhile, they ignored the interactions among microorganisms. To deeply understand the impact of industrial contamination on microbial community, we study the bacterial community structure and diversity in river sediments under the influence of different types of industrial pollution by Illumina MiSeq high-throughput sequencing technology and conduct a more detailed analysis of microbial community structure through co-occurrence networks.

RESULTS

The overall community composition and abundance of individual bacterial groups differed between samples. In addition, redundancy analysis indicated that the structure of the bacterial community in river sediments was influenced by a variety of environmental factors. TN, TP, TOC and metals (Cu, Zn and Cd) were the most important driving factors that determined the bacterial community in urban river sediments (P < 0.01). According to PICRUSt analysis, the bacterial communities in different locations had similar overall functional profiles. It is worth noting that the 15 functional genes related to xenobiotics biodegradation and metabolism were the most abundant in the same location. The non-random assembly patterns of bacterial composition in different types of industrially polluted sediments were determined by a co-occurrence network. Environmental conditions resulting from different industrial pollutants may play an important role in determining their co-occurrence patterns of these bacterial taxa. Among them, the bacterial taxa involved in carbon and nitrogen cycles in module I were relatively abundant, and the bacterial taxa in module II were involved in the repair of metal pollution.

CONCLUSIONS

Our data indicate that long-term potential interactions between different types of industrial pollution and taxa collectively affect the structure of the bacterial community in urban river sediments.

Anaerobic microbe mediated arsenic reduction and redistribution in coastal wetland soil

Arsenic (As) pollution in coastal wetland soil has attracted attention. However, how anaerobic microbes impact the fate of As in coastal wetland environments remains poorly understood. To elucidate underlying mechanisms of anaerobic microbes mediated As mobilization, incubation experiments were performed in this study. The results demonstrate that the concentrations of total dissolved As and As(III) were higher in biotic incubations compared with abiotic controls. The dissolved As(III) concentrations increased and reached maximum values of 11.0 ± 1.2 and 12.0 ± 1.1 μg/L for biotic incubations with and without additional sulfate, respectively. Sulfate and Fe reduction induced by anaerobic microbes were evidenced by the detection of sulfide and Fe(II) in biotic incubations. The sequential extraction results indicated that the content of crystalline Fe mineral fraction of As (As_cry) increased and that of amorphous Fe mineral fraction of As (As_amo) decreased in the solid phase. Therefore, the released As was attributed to microbially mediated reductive dissolution of amorphous Fe mineral matter and, after 40 days of incubation, the decreased As might be immobilized via re-adsorption onto, or co-precipitation with, the newly formed crystalline Fe minerals. The 16S rRNA results indicated that Proteobacteria, Chloroflexi, Actinobacteria, and Firmicutes constituted the majority of the bacterial community in biotic incubations. The sulfate-reducing bacterium Desulfocapsa induced sulfate reduction and further promoted the reduction and release of As in soils. This study provides insights into the mechanism for As mobilization and redistribution in coastal wetland soils.

Bacterial community composition in soils covered by different vegetation types in the Yancheng tidal marsh

Coastal wetland vegetation plays an important role in maintaining ecological function and is a key factor affecting the soil bacterial community. Spartina alterniflora was introduced to the Yancheng tidal marsh to stabilize the sediments and gradually replaced the native plants. However, the changes in the soil bacterial community profile caused by S. alterniflora invasion are poorly characterized. Here, we used MiSeq sequencing to compare the composition of the bacterial community in soil at different depths under exotic S. alterniflora (SA), native Phragmites australis (PA), and native Suaeda salsa (SS). The results showed that the pH value was lower, but the salinity, soil organic carbon, total nitrogen, and number of 16S rRNA genes were higher in SA soils than in PA and SS soils. Overall, Proteobacteria was the dominant bacterial phylum, followed by Chloroflexi, Acidobacteria, Planctomycetes, Gemmatimonadetes, and Nitrospirae. Anaerolineae in the Chloroflexi phylum showed the greatest difference based on vegetation, accounting for 14.4% of the overall bacterial community in SA soils but only about 3.8% of those in PA and SS soils. The composition, interaction, and predicted functional profiles of the bacterial community in SA soils were significantly different from those in PA and SS soils, especially for functions related to the sulfur and nitrogen cycles. Salinity was negatively correlated with the Shannon index and accounted for 37.7% of the total variation in the bacterial community, making it the most important environmental factor. Our results showed the differences in bacterial community composition among different vegetation types and soil depths in the Yancheng tidal marsh, which provides a microbial basis for a better understanding of the ecological functions in this ecosystem.

Estuarine sediments are key hotspots of intracellular and extracellular antibiotic resistance genes: A high-throughput analysis in Haihe Estuary in China

Estuaries lie between terrestrial/freshwater and marine ecosystems, receive considerable pollutant input from land-based sources, and are considerably influenced by human activities. However, little attention has been paid to combined research on extracellular antibiotic resistance genes (eARGs) and intracellular ARGs (iARGs) in the estuarine environment. In this study, we profiled eARGs and iARGs in sediments from Haihe Estuary, China by adopting high-throughput quantitative PCR and investigated their relationship with mobile genetic elements (MGEs), the bacterial community and environmental factors. The results showed that the abundance of eARGs ranged from 9.06 × 10⁶ to 1.32 × 10⁸ copies/g and that of iARGs ranged from 3.31 × 10⁷ to 2.93 × 10⁸ copies/g, indicating that estuarine sediments were key hotspots of eARGs and iARGs. Additionally, multidrug resistance genes were both highly diverse and abundant in Haihe Estuary, especially in coastal samples. The high abundance of vancomycin and carbapenemase resistance genes may pose a potential health risk to human. Salinity altered the composition and structure of the bacterial community. Partial redundancy analysis showed that the bacterial community and MGEs appeared to be the major drivers of ARG variance in estuarine sediment. This study provides an overview of the distribution of eARG and iARG along the Haihe Estuary and draws attention to the need to control pollutants in estuary ecosystems.

Distribution of microbial communities in metal-contaminated nearshore sediment from Eastern Guangdong, China

Nearshore environments are a critical transitional zone that connects the marine and terrestrial/freshwater ecosystems. The release of anthropogenic chemicals into nearshore ecosystems pose a human and environmental health risk. We investigated the microbial diversity, abundance and function in metal-contaminated sediments collected from the Rongjiang, Hanjiang and Lianjiang River estuaries and adjacent coastal areas using high throughput sequencing. The concentration of nutrients (NO₃-N, NO₂-N, NH₄-N, PO₄-P) and metal (Cu, Zn, Cd, Pb, As, Hg) contaminants were higher at the mouth of the rivers compared to the coastal lines, and this was confirmed using cluster analysis. Estimates obtained using geoaccumulation index showed that about 38.9% of the sites were contaminated with Pb and the pollution load index showed that sediment from the mouth of Hanjiang River Estuary was moderately polluted with metals. In the nearshore sediment samples collected, Proteobacteria, Bacteroidetes, Planctomycetes, Chloroflexi, Acidobacteria were the dominant phylum with relative abundances of 46.6%, 8.05%, 6.47%, 5.26%, and 4.59%, respectively. There was no significant correlation between environmental variables and microbial abundance and diversity except for total organic carbon (TOC) (diversity; r = 0.569, p < 0.05) and Cr (diversity; r = 0.581, p < 0.05). At phyla level, Nitrospirae had a significant negative correlation with all metals except Cr, while OD1 had a significant positive correlation with all the metals. Overall, changes in nearshore sediment microbial communities by environmental factors were observed, and these may affect biogeochemical cycling.

Indigenous PAH degraders along the gradient of the Yangtze Estuary of China: Relationships with pollutants and their bioremediation implications

This study investigated the network of polycyclic aromatic hydrocarbon (PAH) degraders in the Yangtze estuarine and coastal areas. Along the estuarine gradients, Proteobacteria and Bacteroidetes were the dominant bacterial phyla, and forty-six potential PAH degraders were identified. The abundance of genes encoding the alpha subunit of the PAH-ring hydroxylating dioxygenases (PAH-RHDα) of gram-negative bacteria ranged from 5.5 × 10⁵ to 5.8 × 10⁷ copies g^-1, while that of gram-positive bacteria ranged from 1.3 × 10⁵ to 2.0 × 10⁷ copies g^-1. The PAH-degraders could represent up to 0.2% of the total bacterial community and mainly respond to PAHs and Cu concentrations, which indicate anthropogenic activities. Salinity and pH showed negative regulating effects on the PAH-degrading potential and the tolerance of bacteria to pollutants. PAH degraders such as Novosphingobium and Mycobacterium exhibit heavy-metal tolerance and core roles in the network of PAH degraders. These outcomes have important implications for bioremediation.

Bacterial Community Assembly in a Typical Estuarine Marsh with Multiple Environmental Gradients

Bacterial communities play essential roles in estuarine marsh ecosystems, but the interplay of ecological processes underlying their community assembly is poorly understood. Here, we studied the sediment bacterial communities along a linear gradient extending from the water-land junction toward a high marsh, using 16S rRNA gene amplicon sequencing. Bacterial community compositions differed significantly between sediment transects. Physicochemical properties, particularly sediment nutrient levels (i.e., total nitrogen [TN] and available phosphorus [AP]), as well as sediment physical structure and pH (P < 0.05), were strongly associated with the overall community variations. In addition, the topological properties of bacterial cooccurrence networks varied with distance to the water-land junction. Both node- and network-level topological features revealed that the bacterial network of sediments farthest from the junction was less intense in complexity and interactions than other sediments. Phylogenetic null modeling analysis showed a progressive transition from stochastic to deterministic community assembly for the water-land junction sites toward the emerging terrestrial system. Taken together, data from this study provide a detailed outline of the distribution pattern of the sediment bacterial community across an estuarine marsh and inform the mechanisms and processes mediating bacterial community assembly in marsh soils.IMPORTANCE Salt marshes represent highly dynamic ecosystems where the atmosphere, continents, and the ocean interact. The bacterial distribution in this ecosystem is of great ecological concern, as it provides essential functions acting on ecosystem services. However, ecological processes mediating bacterial assembly are poorly understood for salt marshes, especially the ones located in estuaries. In this study, the distribution and assembly of bacterial communities in an estuarine marsh located in south Hangzhou Bay were investigated. The results revealed an intricate interplay between stochastic and deterministic processes mediating the assembly of bacterial communities in the studied gradient system. Collectively, our findings illustrate the main drivers of community assembly, taking into consideration changes in sediment abiotic variables and potential biotic interactions. Thus, we offer new insights into estuarine bacterial communities and illustrate the interplay of ecological processes shaping the assembly of bacterial communities in estuarine marsh ecosystems.

Depth-related change of sulfate-reducing bacteria community in mangrove sediments: The influence of heavy metal contamination

This study provides new insight towards the effects of heavy metal contamination on sulfate-reducing bacteria (SRB) in mangrove ecosystem. We investigated SRB communities in mangrove sediments (0-30 cm depth) from Futian, Xixiang and Shajing mangrove wetlands in Shenzhen, China, with different heavy metal contamination levels. The results showed that SRB community abundance (1.71 × 10⁷-3.04 × 10⁸ dsrB gene copies g^-1 wet weight sediment) was depth-related and significantly correlated with Cd and Ni concentrations. The α-diversity indices of SRB community (Chao1 = 21.25-84.50, Shannon = 2.31-2.96) were significantly correlated with Cd level in mangrove sediments. Desulfobacteraceae, Desulfobulbaceae and Syntrophobacteraceae acted as major SRB groups in mangrove sediments, and Syntrophobacteraceae was most sensitive to metal contamination. UniFrace clustering analysis revealed that SRB community structure was influenced by the heavy metal concentrations. Moreover, redundancy analysis indicated that Cd and total phosphorus were the major environmental factors affecting the SRB structure in mangrove sediments.

Characteristics of microbial community indicate anthropogenic impact on the sediments along the Yangtze Estuary and its coastal area, China

In the contaminated coastal sediments, variations of microbial community can reflect the impact of anthropogenic activities. The identification, evaluation and monitoring of the potential bio-indicator species and biomarker communities are vital for the ecological studies in sedimentary environments. Based on the high-throughput sequencing, the microbial communities were characterized in the sediments along the Yangtze Estuary and its coastal area. The results showed that the structure and composition of microbial communities varied greatly among different sampling sites at the phyla level, especially for Euryarchaeota. Metabolic pathway and quantitative PCR analyses suggested that the methane metabolism-related microbes were mainly included in the phylum of Euryarchaeota. Elevated abundances of methane metabolism-related microbes were found at Shidongkou (SDK) and Wusongkou (WSK), where microbes were seriously impacted by the wastewater treatment plant (WWTP) effluent and urban runoff. By comparing with the Euryarchaeota in WWTP sludge, the relatively high abundance of Euryarchaeota in sediment at SDK may be mainly related to the massive growth of indigenous species, promoted by anthropogenic nutrients. Moreover, redundancy discriminant analysis and correlation analysis revealed that methanogens and methanotrophs mainly respond to the nutrients and metals, such as total organic carbon, total phosphorus, total nitrogen, SO₄^2-, NO₂^-, NH₄⁺, Cr, and Zn, which were often related to human activities. Network analyses showed that the species related to the metabolism of methane may play a vital role in the interassociation among different microbial communities. Therefore, methanogens, methanotrophs and their community compositions could be considered as potential bio-indicator species and biomarker communities, indicating anthropogenic activities in the sediments along the Yangtze Estuary and its coastal area.