Exploring the Environmental Influences and Community Assembly Processes of Bacterioplankton in a Subtropical Coastal System: Insights from the Beibu Gulf in China.

Due to rapid urbanization, the Beibu Gulf, a semi-closed gulf in the northwestern South China Sea, faces escalating ecological and environmental threats. Understanding the assembly mechanisms and driving factors of bacterioplankton in the Beibu Gulf is crucial for preserving its ecological functions and services. In the present study, we investigated the spatiotemporal dynamics of bacterioplankton communities and their assembly mechanisms in the Beibu Gulf based on the high-throughput sequencing of the bacterial 16S rRNA gene. Results showed significantly higher bacterioplankton diversity during the wet season compared to the dry season. Additionally, distinct seasonal variations in bacterioplankton composition were observed, characterized by an increase in Cyanobacteria and Thermoplasmatota and a decrease in Proteobacteria and Bacteroidota during the wet season. Null model analysis revealed that stochastic processes governed bacterioplankton community assembly in the Beibu Gulf, with drift and homogenizing dispersal dominating during the dry and wet seasons, respectively. Enhanced deterministic assembly of bacterioplankton was also observed during the wet season. Redundancy and random forest model analyses identified the physical properties (e.g., salinity and temperature) and nutrient content (e.g., nitrate) of water as primary environmental drivers influencing bacterioplankton dynamics. Moreover, variation partitioning and distance-decay of similarity revealed that environmental filtering played a significant role in shaping bacterioplankton variations in this rapidly developed coastal ecosystem. These findings advance our understanding of bacterioplankton assembly in coastal ecosystems and establish a theoretical basis for effective ecological health management amidst ongoing global changes.

Spatiotemporal distribution patterns and coupling effects of aquatic environmental factors in the dry-wet season over a decade from the Beibu Gulf, South China Sea.

Since the 21st century, the Beibu Gulf area has been affected by increasing anthropogenic activities, which makes the coastal aquatic ecosystem extremely concerning. However, the comprehensive exploration and analysis of the long-term scale behavior change characteristics of various water quality environment factors is still limited. Through comprehensively detecting coastal surface water environmental behavior information from 33 locations in the Beibu Gulf from 2005 to 2015, we revealed and quantified mutual response characteristics and patterns of various environmental indicators. The main environmental pollution indicators (e.g., COD, NH4+, NO3-, and DIP) showed a gradual decrease in concentration from the coast to the offshore sea area, and significantly increases during the wet season. The semi-enclosed Maowei Sea exhibited the most prominent performance with significant differences compared to other regions in Beibu Gulf. The average Chlorophyll-a (Chla) content in the coastal area of the Beibu Gulf during the wet season was more than twice that of the dry season, yet the interaction pattern between Chla and environmental factors in the two seasons was opposite to its concentration behavior, accompanied by a closely significant relationship with thermohaline structure and the input of nitrogen and phosphorous nutrients. The multivariate statistical analysis results of total nutrient dynamics suggested that the Beibu Gulf was clearly divided into different regions in both dry and wet season clusters. The present study can provide a comprehensive perspective for the spatial and temporal migration patterns and transformation laws of coastal water environmental factor, which should contribute to improve the prevention countermeasure of nutrient pollution in coastal environment.

Seasonal dynamics of antibiotic resistance genes and mobile genetic elements in a subtropical coastal ecosystem: Implications for environmental health risks.

Antibiotic resistance poses a considerable global public health concern, leading to heightened rates of illness and mortality. However, the impact of seasonal variations and environmental factors on the health risks associated with antibiotic resistance genes (ARGs) and their assembly mechanisms is not fully understood. Based on metagenomic sequencing, this study investigated the antibiotic resistome, mobile genetic elements (MGEs), and microbiomes in a subtropical coastal ecosystem of the Beibu Gulf, China, over autumn and winter, and explored the factors influencing seasonal changes in ARG and MGE abundance and diversity. Results indicated that ARG abundance and diversity were higher in winter than in autumn, with beta-lactam and multidrug resistance genes being the most diverse and abundant, respectively. Similarly, MGE abundance and diversity increased in winter and were strongly correlated with ARGs. In contrast, more pronounced associations between microbial communities, especially archaea, and the antibiotic resistome were observed in autumn than in winter. The co-occurrence network identified multiple interactions between MGEs and various multidrug efflux pumps in winter, suggesting a potential for ARG dissemination. Multivariate correlation analyses and path modeling indicated that environmental factors driving microbial community changes predominantly influenced antibiotic resistome assembly in autumn, while the relative importance of MGEs increased significantly in winter. These findings suggest an elevated health risk associated with antimicrobial resistance in the Beibu Gulf during winter, attributed to the dissemination of ARGs by horizontal gene transfer. The observed seasonal variations highlight the dynamic nature of antibiotic resistance dissemination in coastal ecosystems, emphasizing the need for comprehensive surveillance and management measures to address the growing threat of antimicrobial resistance in vulnerable environments.

The adsorption and release mechanism of different aged microplastics toward Hg(II) via batch experiment and the deep learning method.

Aged microplastics are ubiquitous in the aquatic environment, which inevitably accumulate metals, and then alter their migration. Whereas, the synergistic behavior and effect of microplastics and Hg(II) were rarely reported. In this context, the adsorptive behavior of Hg(II) by pristine/aged microplastics involving polystyrene, polyethylene, polylactic acid, and tire microplastics were investigated via kinetic (pseudo-first and second-order dynamics, the internal diffusion model), Langmuir, and Freundlich isothermal models; the adsorption and desorption behavior was also explored under different conditions. Microplastics aged by ozone exhibited a rougher surface attached with abundant oxygen-containing groups to enhance hydrophilicity and negative surface charge, those promoted adsorption capacity of 4-20 times increment compared with the pristine microplastics. The process (except for aged tire microplastics) was dominated by a monolayer chemical reaction, which was significantly impacted by pH, salinity, fulvic acid, and co-existing ions. Furthermore, the adsorbed Hg(II) could be effectively eluted in 0.04% HCl, simulated gastric liquids, and seawater with a maximum desorption amount of 23.26 mg/g. An artificial neural network model was used to predict the performance of microplastics in complex media and accurately capture the main influencing factors and their contributions. This finding revealed that aged microplastics had the affinity to trap Hg(II) from freshwater, whereafter it released the Hg(II) once transported into the acidic medium, the organism's gastrointestinal system, or the estuary area. These indicated that aged microplastics could be the sink or the source of Hg(II) depending on the surrounding environment, meaning that aged microplastics could be the vital carrier to Hg(II).

Coupling isotopic signatures and partial extraction method to examine lead pollution in mangrove sediments.

Elevated lead (Pb) has been widely observed in mangrove sediments due to human activities, yet understanding the sources of Pb in these sediments and the factors influencing Pb accumulation is challenging. Here, we combined Pb isotopes with partial extraction methods to study Pb contamination levels in mangrove sediments from the eastern and western parts of the Maowei Sea, China. Our results showed that the Pb in the leachate and residual fraction was mainly from anthropogenic and natural sources, respectively. The use of 204Pb isotope analysis can reveal some overlooked differences between anthropogenic and natural sources. Calculation by Bayesian mixing model showed no significant difference in the total anthropogenic contribution between the two sites, but the relative contribution of each end member differed. The contribution of Pb/Zn ores was much higher in the eastern sites (30.9 ± 5.1%) than in the west (18.4 ± 5.5%), while that of agricultural activities was much lower in the east (5.2 ± 3.1%) than in the west (13.5 ± 4.6%). The elevated anthropogenic Pb accumulation in mangrove sediments was ascribed to organic matter. This study provides more data on Pb isotopic composition and new insights into Pb biogeochemistry in the mangrove environment.

Determining effect of seagrass-mediated CO2 flux on the atmospheric cooling potential of a subtropical intertidal seagrass meadow.

Atmospheric greenhouse gas (GHG) emissions from seagrass meadows that determine the ecosystem atmospheric cooling effect have rarely been quantified. This study measured the simultaneous fluxes direct to the atmosphere of three GHGs (CO2, CH4 and N2O) within a Halophila beccarii seagrass meadow and an adjacent unvegetated bare intertidal flat, and their relationships to seagrass abundance and relevant soil parameters. The results showed that seasonal variation in seagrass abundance was strongly linked with the CO2 exchange rate. The CH4 and N2O fluxes were similarly low at both sites and comparable between winter and summer. The global warming potential of CH4 and N2O reduced the ecosystem CO2 uptake by only 5 % at the seagrass site. The results indicated that the H. beccarii meadow had a stronger atmospheric cooling effect than the bare flat and that the seagrass-mediated CO2 flux in this oligotrophic seagrass meadow primarily determined the atmospheric cooling effect.

Short-Term Exposure to MPs and DEHP Disrupted Gill Functions in Marine Bivalves.

The synergistic impact of microplastics (MPs) and organic pollutants remains poorly understood in the marine environment. This study aimed to assess the toxicity of polypropylene microplastics (PS) and/or di-(2-ethylhexyl) phthalate (DEHP) on marine clams. Both Ruditapes philippinarum and Tegillarca granosa were exposed to PS and DEHP individually and combined at environmentally relevant concentrations for 48 h. The filtration rate, antioxidant enzymes activity, lipid peroxidation, reactive oxygen species accumulation, and histological alterations were evaluated. Our results show that single or co-exposure to MPs and DEHP significantly decreases the filtration rate in both type of clams, but the latter exhibited stronger inhibition effect. Close examination of accumulation of reactive oxygen species and related biomarkers revealed that combined exposure exerts greater oxidative stress in the cells, which causes more serious histopathological damage in the gills of the bivalves. Our study implies that MPs, in synergy with organic pollutants, can be more harmful for marine organisms.

Development and validation of the DGT technique using the novel cryogel for measuring dissolved Hg(II) in the estuary.

The complex seawater matrix has significantly influenced the determination of estuarine dissolved Hg(II), hindering its monitoring and risk assessment in maricultural areas. In this work, SiO2-SH-DGT assembled by the sulfhydryl-modified silica cryogel (SiO2-SH cryogel) as the novel binding phase was developed to tackle this problem. The uniform dispersion of the cryogel into binding gel was advantageous for achieving remarkable and comparable capacity, which endowed the estimated diffusion coefficient (D) to be 1.39-3.08 times of the existing research. The SiO2-SH-DGT performance was independent of pH (3-9), ionic strength (10-800 mM), fulvic acid at low content, and seawater matrix (Na+, K+, Ca2+, Cl-), but the high content of Mg2+ did interfere with the Hg(II) accumulation, which manifested as competitive adsorption and diffusion. Therefore, the calibrated model was established by calibrating accumulated mass (M') and diffusion coefficient (D') based on the Mg2+ concentration, its high accuracy was further verified in the lab. Finally, SiO2-SH-DGT was deployed in the three typical aquaculture areas in Beibu Gulf, field trials achieved the actual Hg(II) level to be 1.52-5.38 ng/L with consideration of the diffusion boundary layer. The finding could provide new thought and technical support for metal pollution monitoring in estuary maricultural areas.

Adsorption of di (2-ethylhexyl) phthalate (DEHP) to microplastics in seawater: a comparison between pristine and aged particles.

Microplastics (MPs) are a widely distributed pollutant and have been attracting global attention. The increasing abundance of MPs in marine environments has raised concern about their adverse effects on marine organisms and influence on the fate of contaminants in seawater. In this study, we investigated the effects of natural aging on the adsorption of di (2-ethylhexyl) phthalate (DEHP), one of the most widely used phthalic acid esters (PAEs), in two types of MPs (polyethylene and polystyrene). Biofilm was observed on the surface of MPs after 3-month exposure in seawater. Atomic force microscopy revealed there were significant physical changes in the MPs after aging. Aging in coastal seawater for 3 months significantly reduced the MPs' surface roughness and adhesion, and increased the Young's modulus at the same time. Adsorption isotherms of DEHP indicated that aged MPs had stronger binding capacity of the organic contaminant than pristine MPs. Our data shed some light on the biogeochemical role of MPs in marine environments.

Mechanisms of Metal Tolerance in Halophytes: A Mini Review.

Industrialization and urbanization of coastal wetlands have caused metal pollution worldwide. Phytoremediation has been widely used for treating soil and water pollution, and halophytes are considered a promising remediation method to address metal pollution. However, application of halophytes in phytoremediation is still in its infancy. To increase awareness of halophytes, the metal accumulation, tolerance, and mechanisms of metal detoxification in halophytes are reviewed here. Several halophytes are considered as potential candidates for phytoremediation because they have strong accumulation capacity of metals.

Prevalence and distribution of domoic acid and cyclic imines in bivalve mollusks from Beibu Gulf, China.

Beibu Gulf is an important shellfish aquaculture area in the northwest of the South China Sea, China. In this study, the toxin profile and spatial-temporal distribution of domoic acid (DA) and 10 lipophilic phycotoxins were systematically analyzed in the bivalve mollusks collected in Beibu Gulf from October 2018 to October 2020. Neurotoxin DA was first detected in the mollusks from the investigative regions with a prevalence of 17.7%, peaking at 401 µg kg-1. Cyclic imines (CIs) including gymnodimine-A (GYM-A, 46.6%) and 13-desmethyl-spirolide-C (SPX1, 15.8%) predominated the lipophilic phycotoxins in shellfish, peaking at 10.1 µg kg-1 and 19.6 µg kg-1, respectively. Gymnodimine-A partially accompanied by SPX1 was detected in all batches of shellfish samples, suggesting that Alexandrium ostenfeldii and Karenia selliformis were possible sources of CIs-group toxins in Beibu Gulf. During the investigative period, relatively higher levels of DA occurred in shellfishes from March to August, while slightly higher contents of CIs in mollusks appeared in October and December. Spatial distribution of the targeted phycotoxins demonstrated that shellfishes tended to accumulate relatively higher contents of toxins in Lianzhou, Qinzhou and Tieshan bays.

The distribution of microplastics in water, sediment, and fish of the Dafeng River, a remote river in China.

Although rivers are one of the dominant pathways by which microplastics reach the oceans, reports on remote rivers are rare. Dafeng River is located in Guangxi Province, China, is an important water source and a habitat of a coastal dolphin, Sousa chinensis, which is a first-class national protected animal in China. In this study, we determined the distribution and characteristics of microplastics in the surface water, sediment, and fish of the river. During the dry and rainy seasons, the microplastics content of the surface water ranged from 3 × 10-4-2.5 × 10-3 items/L (7 × 10-4-0.12 items/m2) and 4 × 10-5-9 × 10-4 items/L (2 × 10-3-2.8 × 10-2 items/m2), while those in the sediment samples ranged from 9.4 to 50.3 items/kg (dry weight) and 0.0-21.3 items/kg, respectively. The pollution level during the dry season was approximately two to three times higher than that during the rainy season (P < 0.05). The estimated annual load of microplastics carried by the Dafeng River flow was 8.3 × 108 particles. The microplastics pollution in the Dafeng River was closely related with residential activities. The contents of microplastics in the digestive tracts and gills of fish ranged from 8 × 102 to 5.7 × 103 items/kg (0.3-6.7 items/individual) and 2 × 102 to 1.7 × 103 items/kg (0.1-3.0 items/individual), respectively. The proportion of blue particles in fish was significantly higher than that in the water and sediment, which may indicate that they prefer blue-colored items. The microplastics pollution levels in the water, sediment, and fish of the Dafeng River decreased in the following order: fish > sediment > surface water in terms of items/kg. The level of microplastics pollution in the Dafeng River was relatively low; however, that in aquatic organisms was more severe. Our work highlights the requirement for concern towards microplastics pollution in the organisms of remote rivers.

Control of phytoplankton by oysters and the consequent impact on nitrogen cycling in a Subtropical Bay.

Although there has been growing interest in using oysters to remediate estuarine eutrophication since the 1980s, the role of oysters in remediation remains controversial. In this study, we took advantage of the intensive oyster farming in a nutrient-enriched estuary to evaluate the control of phytoplankton blooms by oysters. Observing the phytoplankton density variations in March and October, we found that despite doubled nutrient levels in March, oysters kept the phytoplankton density low. This is the first estuary-scale evidence that oysters can effectively suppress phytoplankton blooms. Measuring the isotopic signals in seawater revealed the significant impact of pelagic nitrogen cycling in the estuary. In March, oysters facilitated NO3- removal in the water column by enhancing the coupled phytoplankton assimilation-oyster filtration and denitrification in the sediments. While in October, the oysters-related nitrification could outcompete the removal processes, adding a significant amount of NO3- to the water column. Our study illustrates the influence of massive oysters on pelagic nitrogen cycling in an "oyster-remediated" estuary, providing implications for restoring oyster reefs to mitigate the symptoms of estuarine eutrophication.

Pollution and ecological risk assessment, and source identification of heavy metals in sediment from the Beibu Gulf, South China Sea.

The Beibu Gulf is an important passageway between China and the Association of Southeast Asian Nations, where there has been an increase in pollution of heavy metals (HMs). High concentrations of Pb, Cr, Cd, Cu, Zn, As, and Hg in surface sediment were found in Qinzhou Bay, Fangchenggang, and other coastal areas. Stochastic geo-accumulation analyses identified the pollution to be "uncontaminated"; however, it had an 18% probability of deterioration. The Cd, Hg, and As pollution were relatively serious. Principal component analysis, positive matrix factor model, and mercury isotopes demonstrated that the HMs could mainly be attributed to industrial sources including petrochemical, coal-fired, metal and metalloid processing, leather tanning, and human activities: anthropogenic sources accounted for approximately 70% of all the contaminations. This study demonstrates the contribution of terrigenous input to HMs even at a low level and provides basic data for the coordinated development of land and marine resources.

Long-term trends of microplastics in seawater and farmed oysters in the Maowei Sea, China.

Microplastic pollution in marine environments and organisms has received a great deal of international attention. However, the long-term field studies of microplastics are rare. Here, we assessed annual variation in microplastic abundance in the Maowei Sea, a classic mariculture bay in southern China, and analyzed the long-term accumulation in oyster tissues. U-shaped time trends of microplastics in water were observed from January to December in 2018 in the estuarine region, inner bay, and mouth bay sites, representing an inverse relationship with the local rainfall patterns. The common microplastic particles in Maowei Sea are PET/PE fibers, and polystyrene foams, which are mainly related to textile pollution and fishery activities. After one year of continuous monitoring, we did not find accumulation of microplastics in the whole soft tissues of oyster after 10% KOH digestion. No significant correlation of microplastic abundances between water and oysters was observed. The microplastic abundance in oyster was correlated with some environmental variables (i.e. salinity, pH, nutrients and total organic carbon) of the surrounding water following Spearman correlation analysis. The microplastic levels in oysters could probably be influenced by the environmental variables.

Litterfall-derived organic matter enhances mercury methylation in mangrove sediments of South China.

Mercury (Hg) contamination in mangrove ecosystems has received increasing attention in recent years. Although many studies have investigated methylmercury (MeHg) contamination and its relationship to a number of environmental factors in mangrove sediments, the production of MeHg in this carbon-rich ecosystem has not been fully evaluated. In this study, we measured the total mercury (THg) and MeHg concentrations in the sediments collected from seven mangrove forests in China. In addition, we examined the origin and quality of sedimentary organic matter (OM), trying to evaluate their influence on the MeHg accumulation in mangrove sediments. We found that litterfall played an important role in regulating THg and MeHg contents in mangrove sediments. THg and MeHg concentrations in the mangrove sediments were positively correlated to OM content and the labile fraction of the OM. Multiple evidence (stable carbon isotopes, monosaccharide compositions, and biogenic silica) suggested that OM in mangrove sediments was dominated by input from litterfall. THg and MeHg concentrations were elevated at the sediments with higher input of mangrove OM. We observed that addition of mangrove litter stimulated the production of MeHg under anaerobic conditions. Overall, our results suggested that litterfall acted as a source of inorganic Hg, labile carbon, and low-molecular-weight OM which greatly favor the Hg methylation. Our study provides new insights into the MeHg production in mangrove sediments.

Isotopic Composition Reveals the Impact of Oyster Aquaculture on Pelagic Nitrogen Cycling in a Subtropical Estuary.

To offset estuarine eutrophication, interest is increasing in restoring oyster reefs and expanding oyster aquaculture. However, ecosystem-scale evidence is lacking on oyster assemblages' impacts on estuarine pelagic nitrogen (N) cycling. Using a multiple-isotope approach and isotope-mixing model, we examined the sources, transformations, and influence of intensive oyster aquaculture on N pollution in a subtropical estuary. The salinity-dependent NO3- and NH4+ concentrations and their correlations with isotopic signals (δ15N-NO3-, δ18O-NO3-, δ15N-NH4+) indicated the nutrient spatial distribution in low-salinity areas was largely regulated by mixing between freshwater and seawater. However, the intensive oyster aquaculture greatly increased nitrification in the estuary. In high-salinity areas where oyster assemblages were absent, the assimilation of NO3- by phytoplankton became dominant and sharply increased the δ15N-NO3- and δ18O-NO3-. Soil organic nitrogen and fertilizer, domestic sewage, and wastewater treatment plants were the major NO3- sources in the estuary, while internal nitrification contributed 20.6% to the NO3- pool. Oyster biodeposits comprised up to one-third of the particulate organic matter in the water column, and as much as 47.3% of the NH4+ pool could be from the oysters. Our study shows that oysters significantly contribute to the pelagic nutrient pools and N transformations, adding an important dimension to our understanding of oyster assemblages' impacts on estuarine N cycling.

Bulk sedimentary phosphorus in relation to organic carbon, sediment textural properties and hydrodynamics in the northern Beibu Gulf, South China Sea.

Bulk sedimentary phosphorus (P) is studied to evaluate its source, distribution, preservation and enrichment in relation with organic carbon (OC), sediment textures and moisture contents in the northern Beibu Gulf. Approximately 80% of surface sediments in the investigated sites were composed of coarse sandy texture (>63 µm). Total P (TP), inorganic P (IP) and organic P (OP) contents were lower to medium range compared to the levels reported for other marginal seas. Sedimentary OC and P were derived from mixed sources, with high terrestrial influence in the coastal areas (molar OC/OP ratios >250:1). The distribution of P corroborated with the variation tendency of fine-grained sediments, moisture contents and OC. Both IP and OP may significantly influence the trophic state of seawater if released from surface sediments. Influenced by hydrodynamics, frequent resuspension and high abundance of sand, TP is less preserved, and shows low to moderate enrichment in surface sediments.

Neutral monosaccharides and their relationship to metal contamination in mangrove sediments.

Mangrove sediments act as an important natural sink and a secondary source for trace metals. The main objective of this study was to investigate metal contamination and its relationship to mangrove-derived carbohydrates in mangrove sediments. Sixteen metals (Be, V, Cr, Co, Ni, Cu, Zn, Ga, As, Sr, Cd, Sn, Sb, Ba, Tl, and Pb）were analyzed in the surface sediments from four sites at different latitudes on the southeast coastline of China. The sedimentary organic matter was characterized by Rock-Eval pyrolysis, and the neutral sugars were examined by gas chromatograph mass spectrometry. Our results from the enrichment factors indicated that the mangrove sediments were no enriched by Ga, Sr, and Ba, minor enriched by Be, V, Cr, Co, Ni, Cu, Zn, As, Sn, Sb, Tl, and Pb, and moderate enriched by Cd. Litterfall was a major source of organic matter in the mangrove sediments, and the neutral sugars were mainly derived from this litterfall. Significant correlations were detected between the total organic carbon, pyrolytic parameters, neutral sugars, and enrichment factors of V, Cr, Co, Ni, Zn, and Cd, suggesting the input of neutral carbohydrates played an important role in enhancing the metal accumulation in the mangrove sediments. The mangrove litterfall itself was a major source of metals for the sediments, and the mangrove-derived organic matter enhanced the sediment's metal accumulation.

Phosphorus chemical speciation and seasonal variations in surface sediments of the Maowei Sea, northern Beibu Gulf.

This study presents the distribution, seasonal variations and factors influencing phosphorus (P) forms in surface sediments from the Maowei Sea. P forms were measured using the sequential extraction (SEDEX) procedures. Inorganic P (IP) was the predominant chemical form of total P (TP). Fe-bound P (FeP) was the main IP form. Sediment particle sizes, organic matter distribution, terrestrial input and aquaculture activity were responsible for the seasonal variations of different forms of P in sediment. In summer, the average proportions of P fractions in TP followed the order of organic P (OP) > Fe-P > authigenic P (CaP) > detrital P (De-P) > exchangeable P (Ex-P); in winter, the corresponding order was OP > Fe-P > De-P > Ca-P > Ex-P. The potential bio-available P accounted for 71.1 ±â€¯4.9% and 70.6 ±â€¯6.3% of TP in summer and winter, respectively. Sedimentary organic matter mainly came from land-based sources in winter.