Mangrove restoration promotes the anti-scouribility of the sediments by modifying inherent microbial community and extracellular polymeric substance

Why the disinfection efficiency of ultraviolet radiation may become unsatisfactory at low suspended solid concentrations: The mechanism of extracellular polymeric substances secretion induced by different particles

Due to the global outbreaks caused by pathogens, disinfection has attracted widespread attention, especially as the final inactivation step in wastewater treatment plants (WWTPs). Ultraviolet (UV) radiation is regarded as one of low carbon disinfection methods without chemical agents, but in practice, the effects are sometimes unsatisfactory, e.g., Escherichia coli (E. coli) still stay alive excessively at low concentrations of suspended solids (SS) that meets the discharge standards (<10 mg l-1). WWTPs focus on each process's efficiency, while the particle influence of different upstream processes on UV radiation is ignored. This study investigated the changes in extracellular polymeric substances (EPS) structure and biofilm from the perspective of single cell and cell-to-cell interactions at low SS concentrations before and after UV radiation. We disclosed that the effective characteristics of effluent SS particles were mainly related to some specific operation units, like coagulation sedimentation or sand filtration, which induced EPS secretion and biofilm formation, and subsequently caused the different responses of cells within the biofilm to UV radiation. Compared with filtrating sand particles, the magnetic coagulant induced more polysaccharide secretion and expanded the EPS structure, leading to a thicker and more effective protective layer to weaken UV intensity and reduce reactive oxygen species levels inside cells. The microbial community analysis of a real WWTP confirmed the survival of microorganisms with abilities of EPS secretion and sheltering others. Therefore, considering the potential positive effect of particles (such as metal particles) in the upstream process on the microbial aggregation in the subsequent process, it is recommended that the UV disinfection requires a lower SS concentration standard or a higher UV radiation dose. This study provides a basis for effective UV disinfection at the present application standards.

New insights into rhizosphere bacterial community shaped by lettuce genotypes for divergent degradation efficiencies of phthalates

Rhizosphere dissipation of organic pollutants benefits safe utilization of the polluted agricultural soil. Nevertheless, dissipation variation of phthalates (PAEs) in rhizosphere among different vegetable genotypes and the related microbial mechanisms remain unknown. Here, twelve lettuce cultivars with different genetic relationships identified by 18S rRNA gene sequencing were cultivated in soil spiked with di-(2-ethylhexyl) phthalate (DEHP). Bacterial communities and function genes in rhizosphere of lettuce were analyzed by 16S rRNA gene and metagenomic sequencing. Results showed significant variations in DEHP concentrations of roots (2.8-15.3 mg/kg) and shoots (0.70-1.8 mg/kg) among 12 cultivars. Notably, cultivars L11 and L12 showed the lowest DEHP accumulation in roots and shoots, being lower by 82 % and 58 % than the highest accumulators (cultivars L5 and L6), respectively. This accumulation variation was closely connected with their genetic relationships and exhibited genotype-dependent trait. The significantly different bacterial community diversities and structures were recorded in rhizosphere among 12 cultivars. Especially, bacterial communities in rhizosphere of cultivars L11 and L12 (low-DEHP accumulators with high DEHP dissipation) strengthened their adaptation by enriching pollutant-resistant taxa, increasing extracellular polymeric substance contents and biofilm formation, as well as constructing complex ecological networks under DEHP pollution. Moreover, PAE-degrading bacteria and genes (e.g., hydrolase65, phtAb, and pcaI) in rhizosphere were enriched by low-DEHP accumulators, which benefited DEHP removal and subsequently safe agricultural products. This study provides new insights into microbial mechanisms on rhizosphere DEHP degradation and its correlation with accumulation variation among different crop genotypes.

Mangroves increased the mercury methylation potential in the sediment by producing organic matters and altering microbial methylators community

Mangrove ecosystem has attracted global attention as a hotspot for mercury (Hg) methylation. Although numerous biotic and abiotic parameters have been reported to influence methylmercury (MeHg) production in sediments, the key factors determining the elevated MeHg levels in mangrove wetlands have not been well addressed. In this study, Hg levels in the sediments from different habitats (mudflats, mangrove fringe, and mangrove interior) in the Futian mangrove wetland were investigated, aiming to characterize the predominant factors affecting the MeHg production and distinguish the key microbial taxa responsible for Hg methylation. MeHg concentrations in the sediments from the mangrove interior (1.03 ± 0.34 ng g-1 dw) were significantly higher than those in mudflats (0.26 ± 0.08 ng g-1 dw) and mangrove fringe (0.45 ± 0.10 ng g-1 dw). Mangrove vegetation also promoted the accumulation of organic matters in sediments, which stimulated the growth of methylators, ultimately leading to an elevated MeHg level in the sediment. The data from 16S sequencing and random forest analysis further indicated that the increased abundances of Desulfococcus and Desulfosarcina, which belong to complete-oxidizing microbes with acetyl-CoA pathway and are favored by mangrove vegetation, were the primary contributors to MeHg production. Besides, syntrophic partners of methylators (e.g. Syntrophus) also play a considerable role in MeHg production. The present findings provide a deep understanding of Hg-methylation in mangrove wetlands, and offers valuable insights into of the interactions between mangrove plants and soil microbiome in the presence of Hg contamination.

The combined effects of microplastics and their additives on mangrove system: From the sinks to the sources of carbon

Mangrove ecosystems, a type of blue carbon ecosystems (BCEs), are vital to the global carbon cycle. However, the combined effects of microplastics (MPs) and plastic additives on carbon sequestration (CS) in mangroves remain unclear. Here, we comprehensively review the sources, occurrence, and environmental behaviors of MPs and representative plastic additives in mangrove ecosystems, including flame retardants, such as polybrominated diphenyl ethers (PBDEs), and plasticizers, such as phthalate esters (PAEs). Mangrove ecosystems have a complex influence on the behaviors of MPs and additives. Under the action of natural and unnatural factors, these pollutants exhibit complex behaviors including migration, interception, deposition and transformation, that are closely linked to those of particulate carbon, particularly carbon sequestration processes. MPs and additives hinder the CS function of mangroves by harming the growth of flora and fauna, influencing microbial nitrogen and sulfur cycles, and enhancing the degradation of organic matter in the sediment. The increasing accumulation and widespread occurrence of MPs and additives will greatly influence the carbon cycle. Future work is encouraged on systematic investigation of new alternatives to plastics and additives, and research methods to uncover the impact mechanisms of MPs and additives on BCEs. The developments of management measures and engineering technologies are also required to enhance pollutant control and mangrove CS.

How Does Mangrove Restoration or Reforestation Change Trace Metal Pollution in Mangrove Ecosystems? A Review of Current Knowledge

In recent years, mangrove restoration has gained significant attention due to its carbon storage capacity, role as a pollution sink, and ecological and economic benefits. Moreover, the United Nations Sustainable Development Goals' strategies include mangrove restoration. This review investigates the status of mangrove restoration research and the influence of restoration on trace metal accumulation, speciation, and associated risks in mangrove sediments. Our analysis reveals that research on mangrove restoration is increasing globally, with a notable surge in publications after 2017. However, fewer than 25 articles published between 2007 and 2024 address trace metals in restored mangroves, indicating limited focus from researchers on this topic. Research shows that mangrove restoration can quickly alter sediment properties, such as texture, salinity, and pH. As a result, restored sediments tend to accumulate higher organic carbon content than barren areas. Most studies also suggest that trace metal accumulation is higher in restored sites than in non-restored areas. However, metal speciation varies considerably between sites due to local environmental factors. Furthermore, many studies suggest that restoration may increase the risks posed by trace metals to aquatic biota. The challenges of mangrove restoration research include short study timeframes, low success rates, poorly defined targets, small-scale efforts, conflicts with local communities over resources and benefits, insufficient government funding, and a lack of historical data on community health and pollution status.

Do restoration strategies in mangroves recover microbial diversity? A case study in the Yucatan peninsula

Mangrove forests are fundamental coastal ecosystems for the variety of services they provide, including green-house gas regulation, coastal protection and home to a great biodiversity. Mexico is the fourth country with the largest extension of mangroves of which 60% occurs in the Yucatan Peninsula. Understanding the microbial component of mangrove forests is necessary for their critical roles in biogeochemical cycles, ecosystem health, function and restoration initiatives. Here we study the relation between the microbial community from sediments and the restoration process of mangrove forests, comparing conserved, degraded and restored mangroves along the northern coast of the Yucatan peninsula. Results showed that although each sampling site had a differentiated microbial composition, the taxa belonged predominantly to Proteobacteria (13.2-23.6%), Desulfobacterota (7.6-8.3%) and Chloroflexi (9-15.7%) phyla, and these were similar between rainy and dry seasons. Conserved mangroves showed significantly higher diversity than degraded ones, and restored mangroves recovered their microbial diversity from the degraded state (Dunn test p-value Benjamini-Hochberg adjusted = 0.0034 and 0.0071 respectively). The structure of sediment microbial β-diversity responded significantly to the mangrove conservation status and physicochemical parameters (organic carbon content, redox potential, and salinity). Taxa within Chloroflexota, Desulfobacterota and Thermoplasmatota showed significantly higher abundance in degraded mangrove samples compared to conserved ones. This study can help set a baseline that includes the microbial component in health assessment and restoration strategies of mangrove forests.

Zero-valent iron (ZVI) facilitated in-situ selenium (Se) immobilization and its recovery by magnetic separation: Mechanisms and implications for microbial ecology

Selenium (Se(VI)) is environmentally toxic. One of the most popular reducing agents for Se(VI) remediation is zero-valent iron (ZVI). However, most ZVI studies were carried out in water matrices, and the recovery of reduced Se has not been investigated. A water-sediment system constructed using natural sediment was employed here to study in-situ Se remediation and recovery. A combined effect of ZVI and unacclimated microorganisms from natural sediment was found in Se(VI) removal in the water phase with a removal efficiency of 92.7 ± 1.1% within 7 d when 10 mg L-1 Se(VI) was present. Soluble Se(VI) was removed from the water and precipitated to the sediment phase (74.8 ± 0.1%), which was enhanced by the addition of ZVI (83.3 ± 0.3%). The recovery proportion of the immobilized Se was 34.2 ± 0.1% and 92.5 ± 0.2% through wet and dry magnetic separation with 1 g L-1 ZVI added, respectively. The 16 s rRNA sequencing revealed the variations in the microbial communities in response to ZVI and Se, which the magnetic separation could potentially mitigate in the long term. This study provides a novel technique to achieve in-situ Se remediation and recovery by combining ZVI reduction and magnetic separation.

Biological soil crust elicits microbial community and extracellular polymeric substances restructuring to reduce the soil erosion on tropical island, South China Sea

Soil erosion stands as the preeminent environmental concern globally, attaining heightened significance, particularly within islands where land resources prove notably scarce. Biological soil crusts, referred to as biocrusts, assume a pivotal ecological role in soil conservation. Notably, they augment the horizontal stability of the substrate through the exudation of microbial extracellular polymeric substances (EPS), thereby shielding the soil against shear stress, exemplified in the form of water erosion. While extant research has delved into the anti-erosion mechanisms of biocrusts in arid landscapes, a conspicuous lacuna persists in the exploration of coral island environments. In this study, we collected and assessed 30 samples encompassing dark biocrusts, light biocrusts, and bare soil to scrutinize the potential anti-erosion efficacy of tropical coral island biocrusts within the South China Sea. Employing a cohesive strength meter, we quantified soil shear stress across various stages of biocrust development, revealing a discernible enhancement in soil erosion resistance during the formation of biocrusts. Relative to the exposed bare soil, the soil shear stress exhibited an escalation from 0.33 N m-2 to 0.61 N m-2 and 1.31 N m-2 in the light biocrusts and dark biocrusts, respectively. Mechanistically, we assayed microbial EPS contents, exposing a positive correlation between EPS and soil anti-erodibility, encompassing extracellular protein and polysaccharide. Concurrently, bacterial abundance displayed a significant augmentation commensurate with biocrust formation and development. In pursuit of elucidating the origin of EPS, high-throughput amplicon sequencing was executed to identify microorganisms contributing to biocrust development. Correlation analysis discerned Cyanobacteria, Chloroflexi, Deinococcota, and Patescibacteria as potential microbials fostering EPS production and fortifying erosion resistance. Collectively, our study presents the first evidence that biocrust from tropical coral reef island in the South China Sea promotes resistance to soil erosion, pinpointing key EPS-producing microbials against soil erosion. The findings would provide insights for island environment restoration.

The role of extracellular polymeric substances (EPS) in chemical-degradation of persistent organic pollutants in soil: A review

Persistent organic pollutants (POPs) in soil show high environmental risk due to their high toxicity and low biodegradability. Studies have demonstrated the degradation function of microbial extracellular polymeric substances (EPS) on POPs in various matrices. However, the degradation mechanisms and the factors that influence the process in soil have not been clearly illustrated. In this review, the characteristics of EPS were introduced and the possible mechanisms of EPS on degradation of organic pollutants (e.g., external electron transfer, photodegradation, and enzyme catalysis) were comprehensively discussed. In addition, the environmental conditions (e.g., UV, nutrients, and redox potential) that could influence the production and degradation-related active components of EPS were addressed. Moreover, the current approaches on the application of EPS in biotechnology were summarized. Further, the future perspectives of enhancement on degradation of POPs by regulating EPS were discussed. Overall, this review could provide a new thought on remediation of POPs by widely-existing EPS in soil with low-cost and minimized eco-disturbance.

Bacterial carbonic anhydrase-induced carbonates mitigate soil erosion in biological soil crusts

Soil erosion is a significant environmental issue worldwide, particularly in island regions where land resources are exceedingly scarce. Biological soil crusts play a crucial role in mitigating soil erosion, yet the precise effect and mechanism of biological soil crusts against erosion remain ambiguous. In this study, biological soil crusts at various developmental stages from a tropical coral island in the South China Sea were chosen to investigate the role of carbonic anhydrase in mitigating erosion. A cohesive strength meter, real-time quantitative PCR, and 16S rRNA gene high-throughput sequencing were employed to assess variations in soil antiscouribility as well as bacterial abundance and composition during the formation and development of biological soil crusts. Scanning electron microscopy was utilized to detect carbonates induced by bacterial carbonic anhydrase and elucidate their role in the solidification of sand particles. The findings indicate that the formation and development of biological soil crusts significantly enhance anti-scouribility. Comparison to those of bare coral sand, the shear stress increased from 0.35 to 1.11 N/m2 in the dark biocrusts. Moreover, significantly elevated carbonic anhydrase activity was observed in biological soil crusts, demonstrating a positive correlation with antiscouribility. In addition, there was a significant increase in bacterial abundance within the biological soil crusts. The enrichment of Cyanobacteriales and Chloroflexales potentially contributed to the increased carbonic anhydrase activity and antiscouribility. Furthermore, three cyanobacterial strains with carbonic anhydrase activity were isolated from biological soil crusts and subsequently confirmed to enhance sand solidification through microbial carbonate precipitation. This study presents initial evidence for the role of microbial carbonic anhydrase in enhancing the antiscouribility of biological soil crusts during their formation and development. These findings offer novel insights into the functional and mechanistic dimensions underlying the mitigation of soil erosion facilitated by biological soil crusts, which are valuable for implementing sustainable biorestoration and environmental management technologies to prevent soil erosion.

Biocrust reduces the soil erodibility of coral calcareous sand by regulating microbial community and extracellular polymeric substances on tropical coral island, South China Sea

Tropical coral islands assume a pivotal role in the conservation of oceanic ecosystem biodiversity. However, their distinctive environmental attributes and limited vegetation render them highly susceptible to soil erosion. The biological soil crust (biocrust), owing to its significant ecological role in soil stabilization and erosion prevention, is deemed an effective means of mitigating soil erosion on coral island. However, existing research on the mechanisms through which biocrusts resist soil erosion has predominantly concentrated on arid and semi-arid regions. Consequently, this study will specifically delve into elucidating the erosion-resistant mechanisms of biocrusts in tropical coral island environments, South China Sea. Specifically, we collected 16 samples of biocrusts and bare soil from Meiji Island. High-throughput amplicon sequencing was executed to analyze the microbial community, including bacteria, fungi, and archaea. Additionally, quantitative PCR was utilized to assess the abundance of the bacterial 16S rRNA, fungal ITS, archaeal 16S rRNA, and cyanobacterial 16S rRNA genes within these samples. Physicochemical measurements and assessments of extracellular polymeric substances (EPSs) were conducted to characterize the soil properties. The study reported a significantly decreased soil erodibility factor after biocrust formation. Compared to bare soil, soil erodibility factor decreased from 0.280 to 0.190 t h MJ-1 mm-1 in the biocrusts. Mechanistically, we measured the microbial EPS contents and revealed a negative correlation between EPS and soil erodibility factor. Consistent with increased EPS, the abundance of bacteria, fungi, archaea, and cyanobacteria were also detected significantly increased with biocrust formation. Correlation analysis detected Cyanobacteria, Chloroflexi, Deinococcota, and Crenarchaeota as potential microbials promoting EPSs and reducing soil erosion. Together, our study presents the evidence that biocrust from tropical coral island in the South China Sea promotes resistance to soil erosion, pinpointing key EPSs-producing microbials against soil erosion. The findings would provide insights for island soil restoration.

Radial Oxygen Loss from the Roots of Mangrove Seedlings Enhances the Removal of Polycyclic Aromatic Hydrocarbons

The presence of polycyclic aromatic hydrocarbons (PAHs) in soil poses a significant global environmental concern, particularly in coastal wetlands. Mangrove ecosystems exhibit enormous potential in environmental purification; however, the underlying mechanisms involved in the degradation of pollutants (e.g., PAHs) remain ambiguous. In the present investigation, a soil pot experiment was conducted with the addition of pyrene to evaluate the effect of radial oxygen loss (ROL) from roots on PAH degradation using three mangrove seedlings (Rhizophora stylosa, Aegiceras corniculatum, and Avicennia marina). The results showed that mangrove plantation can significantly promote the efficiency of pyrene removal. As for the three mangrove species studied, the greatest removal rate (90.75%) was observed in the soils associated with A. marina, followed by A. corniculatum (83.83%) and R. stylosa (77.15%). The higher PAH removal efficiency of A. marina can be partially attributed to its distinctive root anatomical structure, characterized by a thin exodermis and high porosity, which facilitates ROL from the roots. The results from qPCR further demonstrate that ROL is beneficial for promoting the abundance of PAH-ring hydroxylating dioxygenase gene, leading to a higher removal efficiency. Additionally, Rhizobiales, Defferrisomatales, and Ardenticatenales may also play important roles in the process of pyrene degradation. In summary, this study provides evidence for elucidating the mechanism of PAH removal from the perspective of ROL, thereby contributing valuable insights for species selection during mangrove restoration and remediation.