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Simple Summary: The present study initially identified four full-length genes (peroxidase1, AmPRX1; peroxidase2 AmPRX2; pectate lyase, AmPL; phytosulfokine, AmPSK) associated with metal tolerance in Avicennia marina, one of the pioneer mangrove species with adaptive features. These genes showed the characteristic features of the respective protein families, indicating their evolutionarily conserved relationship with other plant proteins (e.g., CrPRX in Catharanthus roseus, MtPRX in Medicago truncatula, and NnPSK in Nelumbo nucifera). Real-time quantitative PCR revealed that AmPRX1, AmPRX2, and AmPL were most abundantly expressed in the leaves, while AmPSK displayed similar transcript levels across leaves, stems, and roots. Additionally, increased expression levels of these genes were detected in A. marina leaves under cadmium stress. The study thus suggests the role of these four genes in the cadmium stress response and provides a basis for detailed functional studies. Mangroves colonize the intertidal area of estuaries (e.g., Pichavaram, Payardia, and Mai Po) with remarkable cadmium (Cd) pollution. A study on the mechanism of mangrove plant response to Cd pollution can help to understand the adaptive characteristics of plants under Cd stress. This study explored the roles of peroxidase (PRX), pectate lyase (PL), and phytosulfokine (PSK) genes in cadmium tolerance of mangrove Avicennia marina. Full-length sequences of four genes (i.e., AmPRX1, AmPRX2, AmPL, and AmPSK) associated with metal tolerance were identified with suppression subtractive hybridization and rapid amplification of cDNA ends. These genes showed the characteristic features of the respective protein family, indicating functions similar to other plant proteins. Real-time quantitative PCR analysis demonstrated that cadmium exposure resulted in differences in expression patterns among the tissues. Our findings emphasize the complex regulatory mechanism of these four genes in response to trace metal pollution and reveal their functions in metabolic signaling during the stress response. [ABSTRACT FROM AUTHOR]

Iron‐bound soil organic carbon (Fe‐bound SOC) and microbial residues are vital for SOC preservation, a process crucial for drawdown of anthropogenic carbon. However, the relative importance of Fe‐bound SOC in coastal wetlands, and its interaction with microbial residues remain poorly understood. Here, we used glomalin as a proxy for microbial residues, with the goal of determining microbial residues binding to Fe and Fe‐bound SOC across a 1000 km coastline of China including seven typical mangrove wetlands. As hypothesized, it was found that the glomalin fraction was strongly associated with and likely bound to Fe‐bound SOC complexes, which accounted for 21.4% ± 1.6% of mangrove SOC. Mean level of Fe‐bound glomalin was 1.2 ± 0.1 mg cm−3, accounting for 40.6% ± 1.5% of total glomalin and 35.4% ± 3.2% of Fe‐bound SOC in the top 10 cm mangrove soil layer at regional scale. Due to the growing realization that Fe may be a crucial factor of glomalin cycling, we performed multiple chemical characterization techniques to identify the association between Fe and glomalin. Our results suggested that association to Fe was a main stabilization mechanism of glomalin in soils, and glomalin served as organic ligands in the formation of Fe‐bound SOC. We also found that the bulk glomalin extracts were a glomalin–Fe–OC ternary complex, which constituted an important proportion of mangrove SOC. These findings highlight the important effect of glomalin bound to Fe as a major mechanism of SOC persistence in coastal wetlands. [ABSTRACT FROM AUTHOR]

Mangrove species are undergoing environmental changes from nutrient-poor to enrichment due to the large nutrient input. The potential difference in adaptive strategies between the slow- and fast-growing species may lead to great changes in species interaction and ecosystem stability. This study aims to test whether the slow-growing species Aegiceras corniculatum (L.) Blanco and Kandelia obovata Sheue, Liu & Yong sp. nov. are distinctly different from a fast-growing species Laguncularia racemosa (L.) Gaertn. f. in response to soil nutrient enrichment. With the increase of soil nutrients, L. racemosa shifted from a more conservative to a more acquisitive strategy. The potential causes included the increases in specific leaf area, nutrient resorption efficiency, and photosynthetic capacity as indicated by the increase of leaf δ13C and unchanged leaf succulence, as well as the relocation of photosynthetic products as indicated by the shift toward fast-growing at the cost of constructive and defense compounds. In contrast, A. corniculatum and K. obovata maintained conservative strategies at any soil nutrient levels with only a slight increase in growth. These findings implied that L. racemosa will be more competitive over the slow-growing species in nutrient-rich soils through altering adaptive strategies. [ABSTRACT FROM AUTHOR]

Phosphorous (P) is an essential element that mediates various stresses in plants. In this study, the effects of P on polysaccharides in the root cell walls of two hydroponically cultivated mangrove seedlings ( A. marina and K. obovata ) that differ in Cd accumulation ability were examined in the context of Cd stress. The results showed that A. marina exhibited a higher degree of tolerance to Cd than K. obovata . In both mangrove seedlings, pectin and hemicellulose 1 increased significantly with increasing P levels, the effects of which were greater in A. marina under Cd stress. In addition, cell wall pectin methylesterase (PME) activity was markedly increased in the presence of Cd and P compared with Cd alone. These effects were more pronounced in A. marina than in K. obovata . Taken together, the results of this study provide further insight into the mechanisms of P-mediated alleviation of Cd stress in mangrove seedlings. [ABSTRACT FROM AUTHOR]

The halophytic Avicennia marina (Forsk.) is one of the pioneer mangroves along the south coast of China. It is an appropriate material for understanding molecular mechanisms of heavy metal tolerance in mangrove plants. A forward and a reverse cDNA library was constructed by PCR-based suppressive subtractive hybridization (SSH) to isolate these tolerance-associated genes from A. marina leaves. A total of 99 ESTs obtained from the forward and reverse libraries showed significant differential expressions. Twenty-nine genes selected by SSH were studied by real-time PCR in order to analyze their expression level. Most of these genes' expression increased in leaves under Cd stress, which suggests that these genes contribute to the heavy metal tolerance in A. marina . The diversity of these genes indicated that heavy metal stress resulted in a complex response in mangrove plants. This could prove a useful approach for further exploring the molecular mechanisms behind such heavy metal tolerance. [ABSTRACT FROM AUTHOR]

Phosphorus is an indispensable element for plants, but its role in alleviating the cadmium toxicity of mangrove seedlings is poorly documented. In this study, mangrove seedlings were grown in hydroponics and exposed to various Cd and P treatments. Data suggested that the inhibitory effect of Cd on the rate of radial oxygen loss and root porosity was alleviated by P. A. marina had a higher rate of ROL and POR, indicating that it had a stronger adaptability to anaerobic environment. K. obovata induced a higher Fe concentration in iron plaque under co-application of Cd and P, which may relate to higher biomass. Furthermore, P increased Cd concentration in iron plaque, implying that iron plaque can be an obstacle to prevent Cd entering into the plant, but most Cd was still distributed in its roots. These findings highlight a novel mechanism of Cd detoxification with P addition in mangrove seedlings. [ABSTRACT FROM AUTHOR]

Mangrove ecosystems are vulnerable to environmental threats. In order to elucidate the effect of phosphorus (P) on cadmium (Cd) tolerance and physiological responses in mangroves under Cd stress, a mangrove specie with salt exclusion Kandelia obovata and a specie with salt secretion Avicennia marina were compared in a hydroponic experiment. The results showed that most Cd was accumulated in mangrove roots and that P addition induced Cd immobilisation in them. Cd stress significantly increased malonaldehyde content, whereas P significantly decreased malonaldehyde in mangroves. Phosphorus positively regulated the photosynthetic pigment, proline content and synthesis of non-protein thiols, glutathione and phytochelatins in the leaves under Cd stress conditions. The results suggest different adaptive strategies adopted by two mangroves in a complex environment and A. marina showed a stronger Cd tolerance than K. obovata . The study provides a theoretical basis for P mediated detoxification of Cd in mangrove plants. [ABSTRACT FROM AUTHOR]

This study was carried out to demonstrate the mechanism of phenolic root exudates affecting microbial-mediated cadmium (Cd) speciation transformation thus enhancing the Avicennia marina tolerance to Cd. A rhizo-box experiment was conducted including eight treatments with four Cd levels (0, 1, 2, and 4 mg Cd kg−1) and two phenol levels (0, 15 mg kg−1). The results showed that the addition of phenols increased the pH, reduced the number of iron-reducing bacteria (IRB) and sulfur-oxidizing bacteria (SOB) in the rhizosphere sediments, meanwhile promoted the transformation of Cd to low activity speciation. Furthermore, the sulfate accumulation and synthesis of flavonoid phenols in plants were also enhanced. The results indicated that phenolic root exudates inhibit functional bacteria-mediated Fe and S cycles and promote the immobilization of Cd in the sediments. In conclusion, the mitigation of Cd phytotoxicity induced by phenolic root exudates enhanced the Cd tolerance of A. marina. [Display omitted] • Phenolic root exudates inhibit the mediation of functional bacteria in sediments. • Phenolic root exudates improve Cd transformation to low activity speciation. • Phenolic root exudates promote Cd detoxification in Avicennia marina. • Phenolic root exudates enhance Cd tolerance of Avicennia marina. [ABSTRACT FROM AUTHOR]

The effect of root exudates on the environmental behaviors of phenanthrene in mangrove sediments is poorly understood. In order to evaluate their influence, comprehensive laboratory experiments were performed using batch equilibrium and thin-layer chromatography (TLC) analyses. In the presence of root exudates, sorption of phenanthrene was inhibited, whereas desorption and mobility were promoted, and were elevated as root exudate concentrations increased. Among the three representative low molecular weight organic acids (LMWOAs) (citric, oxalic, and acetic acids), citric acid promoted desorption and mobility of phenanthrene more effectively than the other two. In addition, application of artificial root exudates (AREs) enhanced phenanthrene desorption, and mobility was always lower than that with the same concentration of LMWOAs, suggesting that LMWOAs predominantly affected the fate of phenanthrene in sediments. The results of this study could enhance our understanding of the mobility of persistent organic pollutants in sediment–water system. [ABSTRACT FROM AUTHOR]

Glomalin-related soil protein (GRSP) has received extensive attention due to its ability to immobilize metals in the environment. However, whether it can enter the food chain through digestion is still unclear. Mangroves occupy the transition zone between the sea and land, have important ecological functions. Mangroves suffer from fragmentation due to human activities and urbanization. A variety of waterfowls inhabit near the mangroves and ingest sediment settled on their food inadvertently or for grit; therefore, they are ideal for revealing GRSP's role in metal enrichment. In this study, we investigated the release of metals from mangrove surface sediments and GRSP through a physiologically based extraction test. The investigated metals (As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn) in sediments and those bound to GRSP would be mainly released in the gizzard phase. GRSP appeared to be an efficient carrier of Cu, Zn, Pb, and As from sediments to the waterfowls via direct sediment ingestion. For instance, 3.21% and 3.34% of sediment Cu were released in the gizzard and intestinal phases, respectively, meanwhile GRSP-bound Cu contributed 5.04% and 5.42% to this flux. The continuum of GRSP enrichment - complexation of GRSP and metals - biological accessibility of GRSP-bound metals, influenced by both direct and indirect effects from major nutrients (e.g., C, N, P, and S) and metal contents (e.g., Cu, Cd, Ni), controlled the release of GRSP-bound metals during simulated digestion. Overall, this study provides new insights into the potential risk of GRSP acting as a metal delivery vehicle in the food chain. [Display omitted] • PBET was conducted to study metal bioaccessibility via GRSP. • GRSP is the effective carrier of Cd, Cu, and Mn via direct sediment ingestion. • Metal release in the gizzard of waterfowls was mainly regulated by fertile factors. • Metal release in the intestinal phase was mainly regulated by particle size. • Major nutrient and metal content together control the release of metals from GRSP. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) can be transported from mangrove wetlands to the ocean, as a key link in the global carbon cycle. The continuum between mangrove creeks and the ocean is a prime location for facilitating reactions between DOM and sunlight or microbes. Yet, little is known about the sources, transformation, and fate of DOM in the transport continuum at the critical land-to-sea zone. Here, water samples were collected in the estuary of southeastern China along mangrove creeks. We found that DOC concentrations ranged from 6.65 to 110.48 mg/L along the creek-to-estuary continuum. The high accumulation of DOC can be due to the high residence time of seawater and more sensitivity to environmental disturbance. 1.75 times higher DOC concentrations were showed in the lower area than the upper area of the transporting continuum. DOM materials in mangrove creeks were characterized by mostly labile, highly degradable proteins. Terrestrial index (TI) and marine-derived index (MI) were respectively used to described allochthonous and autochthonous characteristics of DOM. The DOM showed a high TI and low MI in the upper area and low TI and high MI in the lower area of outputting continuum. DOM quality along the mangrove to estuary continuum corresponded with environmental heterogeneity. Biodegradation and photo-biodegradation experiments were conducted on DOM samples over 20-days. A rapid 40–65% decrease was observed in DOC concentration in response to biodegradation. About 45% decrease of DOC concentration in photodegradation, afterward less loss in photo-biodegradation than was examined with biodegradation alone. Chromophoric dissolved organic matter (CDOM) concentrations of the continuum were in fluctuating changes during biodegradation. CDOM was preferentially removed 60–90% then increased significantly in photo-biodegradation. Photo-mediated protein-like substances were demonstrated in strong reactions during biodegradation. Compared to biodegradation alone, DOM quality can be distinctly altered during photo-biodegradation. This study demonstrates that DOM quality was gradually homogenized due to interaction with sunlight and microbes during transport from the mangrove creek to the ocean. [Display omitted] • DOC concentrations range in the threshold regulated by sunlight and microbes. • Photo-altered or highly bioavailable CDOM can be pooled from mangroves to estuary. • DOC, CDOM and FDOM were degraded in similar rules yet distinct levels. • Photo-biodegradation altered DOM much more effectively than biodegradation. • DOM quality was homogenized by sunlight and microbes from mangroves to estuary. [ABSTRACT FROM AUTHOR]

Potential toxic metal pollution in mangroves has attracted extensive attention globally; however, the seasonal variation of potential toxic metals in mangrove wetlands is still poorly understood. Herein, we investigated the variation of content as well as chemical speciation of typical metals (Pb, Cr, Zn and Cu) in the sediments from the Zhangjiang Estuary mangrove wetland, China. The potential risk of metal contamination was also investigated. Compared to the wet season, we found that sediment metal content was higher in the dry season. Mangrove sites show accumulated significant metals than does the mudflat both in wet and dry seasons. Geo-accumulation (Igeo) shows moderate pollution, probably because of the dilution as result of runoff and tidal hydrodynamics in the wet season. Increased concentrations of all metals in the acid-soluble fraction and decreased metal contents in the residue fraction were found in the dry season. Risk assessment indicated that the concentrations of Pb poses a higher environmental risk in the dry season. These results can increase awareness of metal pollution in the dry season and provide information for potential toxic metal management in mangrove wetlands. [ABSTRACT FROM AUTHOR]

Mangroves are inter-tidal ecosystems with important global ecological roles. Today, mangroves around the world are at risk of fragmentation, especially in areas with rapid urbanization. Mangroves experiencing habitat fragmentation may be more intensely affected by human activities and a scenario that might have been ignored by previous studies on trace metal (TM) environmental geochemistry. Here, we investigated the typically fragmented habitats in a subtropical mangrove estuary (the Danshuei Basin in Taiwan Strait) to evaluate how human activities affect the geochemical behaviors of TMs. Ni, Sb, Zn, Cr, Cu, and Cd were the primary contaminants found in the mangrove patches. Metal sequestration from the riverine (Ni, Cr) and in-patch activity (Sb, Zn, Cu, Cd) are primary sources of TM's risk. Using the synthesized pollution risk assessment, we showed that most of the mangrove patches are under moderate pollution risk. A significant relationship between the TMs pollution indicators and the absorption coefficient at 254 nm (a 254), implying that the a 254 could be a potential convenient parameter in the TMs risk assessment, which might be partly explained by the bio-remediation of sulfate-reduction microorganism. This study demonstrates the ecological risks posed by TM pollution on urban mangrove patches and emphasizes the importance of a more comprehensive survey for estuarine mangrove patch environments to achieve Sustainable Development Goals. Image 1 • Ecological risk of trace metals (TMs) in urban mangrove patches were investigated. • Ni, Sb, Zn, Cr, Cu, and Cd were found enriched in the urban mangrove patches. • Riverine and in-patch drivers were both important in explaining TMs' distribution. • Most investigated mangrove patches were under moderate ecological risk of TMs. • Spectral characteristics of dissolved organic matter could help predicting TMs' risk. The spectral feature of cDOM is a potential indicator of trace metal ecological risk in metropolitan mangrove patches. [ABSTRACT FROM AUTHOR]

To better understand the uptake, biotransformation and physiological response to tetrabromobisphenol A (TBBPA) in mangrove plants, a short term 14-day hydroponic assay with two mangrove species, Avicennia marina (A. marina) and Kandelia obovata (K. obovata), was conducted. Results showed that two mangrove species could uptake, translocate and accumulate TBBPA from solution. The hydroxylation and debromination metabolites of TBBPA, including OH-TBBPA, TriBBPA, MonoBBPA, and BPA, were found in both mangroves for the first time. The high-level TBBPA suppressed the growth and increased malondialdehyde (MDA) content of K. obovata , did not pose any negative affect on A. marina. The activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) of K. obovata significantly increased in the 7th day, whereas, SOD and POD activities at high-levels of TBBPA became comparable to the control in the 14th day. Contrastingly, the antioxidant enzymes activities of A. marina were positively stimulated by TBBPA during the 14-day of observation, indicating that A. marina was more tolerant of TBBPA. • K. obovata and A. marina could uptake, translocate and accumulate TBBPA. • Hydroxylation and debromination of TBBPA within two mangrove species • High-level of TBBPA inhibited the growth and development of K. obovata. • SOD, POD, and CAT activities in A. marina were positively induced by TBBPA. • A. marina exhibited higher tolerant to TBBPA stress than K. obovata. [ABSTRACT FROM AUTHOR]

Glomalin-related soil protein (GRSP) is a significant component in the sequestration of heavy metal in soils, but its mechanisms for metal adsorption are poorly known. This study combined spectroscopic data with molecular docking simulations to reveal metal adsorption onto GRSP's surface functional groups at the molecular level. The EXAFS combined with FTIR and XPS analyses indicated that the adsorption of Cd(II), Sr(II), and Ni(II) by GRSP occurred mainly through the coordination of –OH and –COOH groups with the metal. The –COOH and –OH groups bound to the metal as electron donors and the electron density of the oxygen atom decreased, suggesting that electrostatic attraction might be involved in the adsorption process. Two-dimensional correlation spectroscopy revealed that preferential adsorption occurred on GRSP for the metal in sequential order of –COOH groups followed by –OH groups. The presence of the Ni–C shell in the Ni EXAFS spectrum suggested that Ni formed organometallic complexes with the GRSP surface. However, Sr–C and Cd–C were absent in the second shell of the Sr and Cd spectra, which was attributed to the adsorption of Sr and Cd ions with large hydration ion radius by GRSP to form outer-sphere complexes. Through molecular docking simulations, negatively charged residues such as ASP151 and ASP472 in GRSP were found to provide electrostatic attraction and ligand combination for the metal adsorption, which was consistent with the spectroscopic analyses. Overall, these findings provided new insights into the interaction mechanisms between GRSP and metals, which will help deepen our understanding of the ecological functions of GRSP in metal sequestration. [Display omitted] • Spectral data combined with molecular docking reveal adsorption mechanism of GRSP. • Metal is mainly immobilized on the GRSP surface through –OH and –COOH groups. • Preferential adsorption on GRSP for metal in sequential order of –COOH > –OH group. • ASP151 and ASP472 residues in GRSP provide coordination sites for metal adsorption. • Ni with smaller hydrated ionic radii forms organometallic complexes with GRSP. [ABSTRACT FROM AUTHOR]

Aquaculture effluent discharge containing heavy metals affects estuarine mangrove wetlands. Glomalin-related soil protein (GRSP) is recalcitrant organic matter that can be trapped in mangrove wetlands and is critical to metal sequestration. However, studies on the effects of long-term aquaculture effluents on metal pollution in adjacent mangrove wetlands and the ecological role of GRSP are lacking. For the first time, we revealed the effects of discharge histories (0, 8, and 14 years) of shrimp pond effluents on metals (As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn), including the entire process from feed to metals binding with GRSP in mangrove soils. Results showed that mangrove soils receiving the effluents generally had higher or similar metal loadings compared to the control, and long-term effluent discharge increased the potential toxicity of the metals. Aquaculture feed could be a main source of metal input. Redundancy analysis indicated that 14-year effluent discharge increased the pH, bulk density, total nitrogen, and total phosphorus of mangrove soils, reducing the potential of GRSP-bound metals. Scanning electron microscopy and infrared spectroscopy characterisation revealed that effluent disturbances changed the surface morphology and functional group contents of GRSP. This study provides insights into using GRSP as an aquaculture pollution bioindicator. [Display omitted] • Shrimp pond effluents affected the loading and toxicity of metals in wetlands. • The bioavailable metals increased with increasing effluent discharge histories. • Feed is a main metal pollution source in shrimp ponds and aquatic ecosystems. • The 14-year effluent discharge decreased the contents of GRSP and GRSP-bound metals. • GRSP could be a new indicator of aquaculture disturbance based on SEM and FTIR. [ABSTRACT FROM AUTHOR]

• SO 4 2− can help to mitigate As mobility in flooded As-contaminated mangrove sediment. • Sulfate-modified As release variations are always accompanied by Fe/Al release. • Arsenic release was promoted at pH 11, while sulfate decreased arsenic release. • Sulfate alters As form distribution, and strongly adsorbed As increases significantly. Fragile mangrove wetlands are threatened by sea level rise, which results in the introduction of increased sulfate concentrations into mangrove sediment. Mangroves are also susceptible to heavy metal contamination, such as arsenic (As) contamination. However, little is known about whether sulfate affects the bioavailability of As in mangrove sediment. This study focuses on the effects of sulfate on arsenic mobilization and transformation in As-contaminated mangrove sediment, and modeled changes in sulfate concentration, As contamination levels, and pH conditions. The As release from sediment treated with sulfate (30 mM) significantly decreased (P < 0.05). In the moderate arsenic-contaminated sediment, the mean arsenic release was 181.95 ± 33.05 μg kg−1 without sulfate and 157.55 ± 40.98 μg kg−1 with sulfate. In addition, the release of As was affected by both pH and sulfate when pH conditions were changed. Arsenic release was promoted at pH 11 (P < 0.05), but sulfate still mitigating As release at this pH value (P < 0.001). Furthermore, the release of As after sulfate input was always accompanied by the release of iron (Fe) and aluminum (Al). Sulfate altered the distribution of forms of As in the sediment, with a significant increase in the proportion of strongly adsorbed As and silicate-bound states of As. Overall, sulfate intrusion mitigates As release, but it also increases the bioavailability of As in the sediment. This fact makes the toxic element most susceptible to long-term mobilization via biological processes. Therefore, the effect of persistent sulfate intrusion caused by sea level rise on toxic elements in mangrove sediment deserves long-term attention. The significance of this study is that it contributes to understanding the behavior of heavy metals in coastal wetland ecosystems affected by sea level rise. [ABSTRACT FROM AUTHOR]

Deciphering the mobility and transfer of heavy metals in transition buffers is vital to understanding their behavior in mangrove forests. As one of the most redox-sensitive metals, the geochemical fractionation of sediment Chromium in mangrove forests and its health risks to the coastal fishermen folk is not clearly understood. This study investigated the current levels, enrichment, geochemical fractionation, and eco-toxicity on organisms of sediment Chromium from three mangrove forests in southeast China. A health risk assessment for different exposure pathways were also determined with Monte Carlo simulations technique. The results revealed that the concentration of sediment Chromium ranged from 30.75 mg kg−1 to 99.28 mg kg−1. The geochemical fractionations of sediment Chromium were mainly associated with amorphous Fe fraction, crystalline Fe fraction and residual fraction. Notably, 83.12% of samples analyzed in the residual phases of Chromium exceeded the background value of 40.7 mg kg−1. Adverse effect index revealed a considerably negative effect on benthos occurrence in the mangroves. Values of non-carcinogenic risks were below unity at all samples, whereas the cancer risks associated with Cr(VI) exposure via fish consumption at median were close to 1.73 × 10 − 5 . A sensitivity analysis indicated that sediment Cr(VI) concentration and exposure frequency were the most relevant variables in the risk model. As the first attempt to provide information on the human health risks of sediment Chromium in mangrove forests in China, findings from this study can help track potential adverse effects and avoid risks from sediment Chromium. • Chromium were mainly associated with residual phases. • Total carcinogenic risk of Cr(VI) followed the order of CR fish > CR oral > CR derm. • Cr(VI) concentration and exposure frequency were the most influential parameter in calculating CR fish. [ABSTRACT FROM AUTHOR]

Phenols exert a great influence on the dynamic process of Cd in the soil-plant interface. We investigated the influence of phenols on the biogeochemical behavior of cadmium in the rhizosphere of Avicennia marina (Forsk) Vierh. All combinations of four levels of cadmium (0, 1, 2 and 4 mg/kg DW) and two levels of phenol (0 and 15 mg/kg DW) were included in the experimental design. We found that phenols facilitated increasing concentrations of exchangeable cadmium (Ex-Cd), acid volatile sulfide (AVS) and reactive solid-phase Fe (II) in sediments, and iron in plants, but inhibited Cd accumulation in iron plaque and roots. The concentrations of AVS and reactive solid-phase Fe (II) were significantly positively correlated with Cd treatment. As for the biogeochemical behavior of Cd in mangrove sediments, this research revealed that phenols facilitated activation and mobility of Cd. They disturbed the “source-sink” balance of Cd and turned it into a “source”, whilst decreasing Cd absorption in A. marina . Additionally, phenols facilitated iron absorption in the plant and alleviated the Fe limit for mangrove plant growth. [ABSTRACT FROM AUTHOR]

Mangrove wetlands are subjected to pollution due to anthropogenic activities. Mangrove fitness is mainly determined by root exudates and microorganisms activities belowground, but the mechanisms are not yet well known. Rhizospheric interactions among mangrove sediments, microorganisms and root exudates were simulated. In particular, low-molecular-weight organic acids (LMWOA), were examined to explore the metal(loid)s rhizospheric dynamics via batch experiments. Using a combination of comparative sterilised and unsterilised sediments, LMWOA extracts and sediments constituents were examined. Factors such as the solution pH, dissolved organic carbon (DOC), arsenic and iron species and metal(loid)s in the aqueous phase were evaluated. The results show that on an average, the As decreased by 68.3 % and 42.1 % under citric and malic acid treatments, respectively, after sterilisation. In contrast, the As content increased by 29.6 % under oxalic acid treatment. Microorganisms probably facilitate sediment As release in the presence of citric and malic acids but suppress As mobilisation in the presence of oxalic acid. Fe, Mn and Al were significantly (p < 0.05) positively correlated with the trace metal(loid)s (Zn, Pb, Ni, Cu, Cr, Co, Ba, Cd and As). The solution pH was negatively correlated with the solution As. Both DOC and pH reach the peaks at the end of all treatments. The As absorption–desorption dynamics are closely linked to proton consumption, Fe-Mn-Al sedimentation of ageing performance and organic ligand complexation. The study provides an insight into the rhizospheric processes of microbial involvement and gives an enlightening understanding of the metal(loid)s redeployment for plant adaptation in mangrove wetlands. [Display omitted] • Microbes facilitated As extraction of citric and malic acid positively under laboratory conditions. • Under the action of microorganisms, citric acid from root exudates promotes the release of As in mangrove sediments. • Inorganic As species put a potential exposure risk extracted from sediments via LMWOAs. • Desorption-absorption dynamics of As synchronized with sedimentation and ageing processes of Fe, Mn, and Al via STEM-EDAX. [ABSTRACT FROM AUTHOR]

[Display omitted] • We proposed a new framework based on distinct coastal soil textures. • Coastal soil texture explained variation in C:N:P stoichiometry and SOC storages. • SOC stabilization mechanism was revealed by multiple chemical characterizations. • Glomalin-related soil protein contributed to the stability of mangrove SOC. Mangroves are among the most carbon-rich forests in the tropics, but the spatial variation in soil organic carbon (SOC) storage and the mechanisms controlling their stability remain contentious. Here, we proposed a new framework based on distinct coastal soil textures (muddy, sandy, and mud-sand mixed soil) and found that coastal soil texture exhibits regional-scale variations in mangrove soil C:N:P stoichiometry and SOC storage. Using this soil texture classification framework, we identified the important role of mud-sand mixed soil, which functions as a blue carbon hotspot in coastal wetlands. We showed that mangrove SOC storage was underestimated by approximately 37% in mud-sand mixed soil and overestimated by approximately 54% in muddy soil. We further revealed that the chemical composition of SOC and clay mineralogy are responsible for SOC stability by combining physical and chemical characterizations. As a microbial-derived C, glomalin-related soil protein (GRSP) contained higher alkyl C (∼58%) and lower O-alkyl C (∼17%) than mangrove soils, indicating that GRSP contributes to the stable SOC pool by its recalcitrant structure. We further found that the C:N:P stoichiometric signature of GRSP can capture variations in mangrove SOC storage compatible with distinct coastal soil textures. These results highlight the role of GRSP in regulating SOC stabilization mechanisms, potentially attenuating coastal blue carbon-climate feedback. Regional-scale variation in SOC linked to coastal soil textures can provide more robust estimates of the contribution of mangrove SOC to global C dynamics. [ABSTRACT FROM AUTHOR]

The increase levels of tetrabromobisphenol A (TBBPA) in mangrove wetlands is of concern due to its potential toxic impacts on ecosystem. A 93-day greenhouse pot experiment was conducted to investigate the effects of mangrove plants, A. marina and K. obovata , on TBBPA degradation in sediment and to reveal the associated contributing factor(s) for its degradation. Results show that both mangrove species could uptake, translocate, and accumulate TBBPA from mangrove sediments. Compared to the unplanted sediment, urease and dehydrogenase activity as well as total bacterial abundance increased significantly (p < 0.05) in the sediment planted with mangrove plants, especially for K. obovata. In the mangrove-planted sediment, the Anaerolineae genus was the dominant bacteria, which has been reported to enhance TBBPA dissipation, and its abundance increased significantly in the sediment at early stage (0–35 day) of the greenhouse experiment. Compared to A. marina -planted sediment, higher enrichment of Geobater, Pseudomonas, Flavobacterium, Azoarcus , all of which could stimulate TBBPA degradation, was observed for the K. obovata -planted sediment during the 93-day growth period. Our mass balance result has suggested that plant-induced TBBPA degradation in the mangrove sediment is largely due to elevated microbial activities and total bacterial abundance in the rhizosphere, rather than plant uptake. In addition, different TBBPA removal efficiencies were observed in the sediments planted with different mangrove species. This study has demonstrated that K. obovata is a more suitable mangrove species than A. marina when used for remediation of TBBPA-contaminated sediment. Image 1 • Higher TBBPA degradation in mangrove-planted sediments compared to unplanted ones. • Mangrove plants can uptake, translocate, and bioaccumulate TBBPA. • Mangrove plants can enhance microbial activities and abundance in rhizosphere. • Plant uptake contributes less to TBBPA removal than microbial degradation. • K. obovata is more suitable for remediation of TBBPA-contaminated sediments. [ABSTRACT FROM AUTHOR]