Spatiotemporal variations of halogenated polycyclic aromatic hydrocarbons in sediments of Pearl River Estuary: Occurrence, sources, and potential ecological risks.

Labile organic matter favors a low N2O yield during nitrogen removal in estuarine sediments

A Study on the Mechanical Behaviour of Pearl River Estuary Sediments

River estuaries are dynamic and complex ecosystems influenced by various natural processes, including climatic fluctuations and anthropogenic activities. The Pearl River Estuary (PRE), one of the largest in China, receives significant land-based pollutants due to its proximity to densely populated areas and urban development. This study aimed to characterize the composition, diversity, and distribution patterns of sediment microbial communities (bacteria, archaea, and eukaryotes) and investigated the connection with environmental parameters within the PRE and adjacent shelf. Physicochemical conditions, such as oxygen levels, nitrogen compounds, and carbon content, were analyzed. The study found that the microbial community structure was mainly influenced by site location and core depth, which explained approximately 67% of the variation in each kingdom. Sites and core depths varied in sediment properties such as organic matter content and redox conditions, leading to distinct microbial groups associated with specific chemical properties of the sediment, notably C/N ratio and NH4+ concentration. Despite these differences, certain dominant taxonomic groups were consistently present across all sites: Gammaproteobacteria in bacteria; Bathyarchaeia, Nitrososphaeria, and Thermoplasmata in archaea; and SAR in Eukaryota. The community diversity index was the highest in the bacteria kingdom, while the lowest values were observed at site P03 across the three kingdoms and were significantly different from all other sites. Overall, this study highlights the effect of depth, core depth, and chemical properties on sediment microbiota composition. The sensitivity and dynamism of the microbiota, along with the possibility of identifying specific markers for changes in environmental conditions, is valuable for managing and preserving the health of estuaries and coastal ecosystems.

Characterizing sedimentary black carbon in the Pearl River Estuary, Southern China

Spatial gradients and molecular transformations of DOM, DON and DOS in human-impacted estuarine sediments

Assessing the impacts of cumulative anthropogenic disturbances on estuarine ecosystem health is challenging. Using spatially distributed sediments from the Pearl River Estuary (PRE) in southern China, which are significantly influenced by anthropogenic activities, we demonstrated that metagenomics-based surveillance of benthic microbial communities is a robust approach to assess anthropogenic impacts on estuarine benthic ecosystems. Correlational and threshold analyses between microbial compositions and environmental conditions indicated that anthropogenic disturbances in the PRE sediments drove the taxonomic and functional variations in the benthic microbial communities. An ecological community threshold of anthropogenic disturbances was identified, which delineated the PRE sediments into two groups (H and L) with distinct taxa and functional traits. Group H, located nearshore and subjected to a higher level of anthropogenic disturbances, was enriched with pollutant degraders, putative human pathogens, fecal pollution indicators, and functional traits related to stress tolerance. In contrast, Group L, located offshore and subjected to a lower level of anthropogenic disturbances, was enriched with halotolerant and oligotrophic taxa and functional traits related to growth and resource acquisition. The machine learning random forest model identified a number of taxonomic and functional indicators that could differentiate PRE sediments between Groups H and L. The identified ecological community threshold and microbial indicators highlight the utility of metagenomics-based microbial surveillance in assessing the adverse impacts of anthropogenic disturbances in estuarine sediments, which can assist environmental management to better protect ecosystem health.

Evaluation of heterocyte glycolipids with a hexose sugar moiety for tracing terrestrial organic matter in the South China Sea

ABSTRACTComplete ammonia oxidizers (comammox) have been widely detected in riverine and estuarine ecosystems. However, knowledge about the process of comammox community assembly from freshwater to marine environments is still limited. Here, based on deep sequencing, we investigated the community composition of comammox along a salinity gradient in the Pearl River Estuary (PRE), South China. Our results showed that comammox microorganisms in the PRE sediments were extremely diverse and displayed distinct distributional patterns between upstream and downstream habitats. Quantitative PCR demonstrated that comammox was the dominant ammonia-oxidizing microorganism (AOM) in the PRE upstream sediments, and ammonia-oxidizing archaea (AOA) dominated the PRE downstream sediments, while ammonia-oxidizing bacteria (AOB) were not dominant in any section of the PRE. Neutral modeling revealed that stochastic processes explained a limited part of the variation in the comammox community. The majority of beta nearest-taxon index values were higher than 2, indicating that comammox community assembly in the PRE sediments was better explained through a deterministic process than through a stochastic process. Salinity and total nitrogen were the most important contributing factors that shaped the comammox community. This study expanded the current knowledge of the diversity and niche preference of comammox in the estuarine ecosystem, and further enhances our understanding of the assembly of comammox community from freshwater to marine environments.IMPORTANCE Microbial communities are shaped by stochastic (emigration, immigration, birth, death, and genetic drift of species) and deterministic (e.g., environmental factors) processes. However, it remains unknown as to which type of process is more important in influencing the comammox community assembly from freshwater to marine environments. In this study, we compared the relative importance of stochastic and deterministic processes in shaping the assembly of the comammox community, which demonstrated that the deterministic process was more important in determining the community assembly patterns in the PRE ecosystem.

Unraveling microbe-mediated degradation of lignin and lignin-derived aromatic fragments in the Pearl River Estuary sediments

Heavy metals and Pb isotopes in a marine sediment core record environmental changes and anthropogenic activities in the Pearl River Delta over a century

Dissimilatory nitrate reduction to ammonia (DNRA) process, competing with denitrification and anaerobic ammonia oxidation (anammox) for nitrate, is an important nitrogen retention pathway in the environment. Previous studies on DNRA bacterial diversity and composition focused on the surface sediments in estuaries, but studies on the deep sediments are limited, and the linkage between DNRA community structure and complex estuarine environment remains unclear. In this study, through high-throughput sequencing of nrfA gene followed by high-resolution sample inference, we examined spatially and temporally the composition and diversity of DNRA bacteria along a salinity gradient in five sediment cores of the Pearl River Estuary (PRE). We found a higher diversity and richness of DNRA bacteria in sediments with lower organic carbon, where sea water intersects fresh water. Moreover, the DNRA bacterial communities had the specific spatially distribution coupling with their metabolic difference along the salinity gradient of the Pearl River Estuary, but no obvious difference along the sediment depth. The distribution of DNRA bacteria in the PRE was largely driven by various environmental factors, including salinity, Oxidation-Reduction Potential (ORP), ammonium, nitrate and Corg/NO3−. Furthermore, dominant DNRA bacteria were found to be the key populations of DNRA communities in the PRE sediments by network analysis. Collectively, our results showed that niche difference of DNRA bacteria indeed occurs in the Pearl River Estuary.

Dissimilatory nitrate reduction to ammonia (DNRA) process, competing with denitrification and anaerobic ammonia oxidation (anammox) for nitrate, is an important nitrogen retention pathway in the environment. Previous studies on DNRA bacterial diversity and composition focused on the surface sediments in estuaries, but studies on the deep sediments are limited, and the linkage between DNRA community structure and complex estuarine environment remains unclear. In this study, through high-throughput sequencing of nrfA gene followed by high-resolution sample inference, we examined spatially and temporally the composition and diversity of DNRA bacteria along a salinity gradient in five sediment cores of the Pearl River Estuary (PRE). We found a higher diversity and richness of DNRA bacteria in sediments with lower organic carbon, where sea water intersects fresh water. Moreover, the DNRA bacterial communities had the specific spatially distribution coupling with their metabolic difference along the salinity gradient of the Pearl River Estuary, but no obvious difference along the sediment depth. The distribution of DNRA bacteria in the PRE was largely driven by various environmental factors, including salinity, Oxidation-Reduction Potential (ORP), ammonium, nitrate and Corg/NO3-. Furthermore, dominant DNRA bacteria were found to be the key populations of DNRA communities in the PRE sediments by network analysis. Collectively, our results showed that niche difference of DNRA bacteria indeed occurs in the Pearl River Estuary.

Niche specificity and potential terrestrial organic carbon utilization of benthic Bathyarchaeota in a eutrophic subtropic estuarine system

Tetrabromobisphenol A and hexabromocyclododecanes in sediments and biota from two typical mangrove wetlands of South China: Distribution, bioaccumulation and biomagnification

Denitrification, an important process in microbial mediated nitrogen cycle, plays important roles in nitrogen loss in estuarine sediments. However, the function of denitrifiers in the estuarine subsurface sediments remained poorly understood. In this study, we analyzed the potential activity, abundance and community structure of nirS-type denitrifiers using 15N-labeled incubation quantitative-PCR and high throughput sequencing techniques in sediment cores from Pearl River Estuary (PRE). Results showed that subsurface sediments had nearly same level denitrification potential activity compare to surface sediments, although the abundance of nirS gene decreased sharply from surface to bottom in sediment cores. Meanwhile, nirS gene abundance exhibit significant temporal variations, which is consistent with denitrification potential activity. Moreover, the community structure and diversity of nirS-type denitrifiers in sediment cores exhibited remarkable temporal shift pattern. For spatial variation, no significant difference was observed of denitrifiers community structure in each sediment core from the surface to the subsurface, while there were significant different diversity characteristic among different cores. Redundancy analysis (RDA) showed that multiple environmental factors including salinity, pH, oxidation-reduction potential, nutrient content and organic substances synergistically shaped the diversity and distribution of nirS-type denitrifers in PRE sediments. Our results showed that nirS-type denitrifers played important roles in the nitrogen removal in subsurface sediments of PRE.

Anaerobic ammonium oxidation (anammox) is an important pathway for the removal of fixed nitrogen from aquatic and terrestrial ecosystems. Previous studies on anammox were focused on the surface sediments in estuaries, but the activity and community composition of anammox bacteria in the estuarine subsurface sediments remained unknown. In this study, we used high-throughput sequencing of 16S rRNA gene combined with 15N isotope tracing method to investigate the activity, diversity, and spatio-temporal distribution of anammox bacteria in sediment cores of the Pearl River Estuary (PRE). Our results indicated that anammox in the subsurface sediments has significant potential activity, contributing to approximately 17.49% of the total microbial nitrogen loss. A variety of anammox bacteria, including Candidatus Scalindua, Ca. Brocadia, Ca. Jettenia, and Ca. Kuenenia, were all detected in the subsurface sediments. Moreover, the anammox bacterial community had a significant specific geographic distribution but no obvious difference along the sediment depth. Multiple environmental factors including salinity, and NH4+ and NO3− contents, synergistically shaped the diversity and distribution of anammox bacteria in PRE sediments.

210Pb dating to investigate the historical variations and identification of different sources of heavy metal pollution in sediments of the Pearl River Estuary, Southern China

Mercury methylation-related microbes and genes in the sediments of the Pearl River Estuary and the South China Sea

Viruses are an abundant and active component of marine sediments and play a significant role in microbial ecology and biogeochemical cycling at local and global scales. To obtain a better understanding of the ecological characteristics of the viriobenthos, the abundance and morphology of viruses and the diversity and community structure of T4-type phages were systematically investigated in the surface sediments of the subtropical Pearl River Estuary (PRE). Viral abundances ranged from 4.49 × 108 to 11.7 × 108 viruses/g and prokaryotic abundances ranged from 2.63 × 108 to 9.55 × 108 cells/g, and both decreased from freshwater to saltwater. Diverse viral morphotypes, including tailed, spherical, filamentous, and rod-shaped viruses, were observed using transmission electron microscopy. Analysis of the major capsid gene (g23) indicated that the sediment T4-type phages were highly diverse and, similar to the trend in viral abundances, their diversity decreased as the salinity increased. Phylogenetic analysis suggested that most of the g23 operational taxonomic units were affiliated with marine, paddy soil, and lake groups. The T4-type phage communities in freshwater and saltwater sediments showed obvious differences, which were related to changes in the Pearl River discharge. The results of this study demonstrated both allochthonous and autochthonous sources of the viral community in the PRE sediments and the movement of certain T4-type viral groups between the freshwater and saline water biomes.