Metals in mangrove ecosystems and associated biota: A global perspective

Metal contamination in mangrove crab (Ucides cordatus) and human risk assessment on the Brazilian Amazon coast

The mangrove crab, Ucides cordatus, plays a relevant role in nutrient cycling in mangroves and is a significant source of food security and income by sustaining the artisanal fishing economy in South America. However, increasing metal contamination threatens the coastal systems, mangrove crabs, and the human population depending on this resource. This study investigates the concentrations of five metals (Al, Co, Fe, Ni, Zn) in the muscle, gills, hepatopancreas, and carapace via metal transfer from the sediment to these crab tissues in "low" and "potentially impacted" areas in the states of Pará and Maranhão on the Brazilian Amazon coast. Our results indicate that Fe (2403.35 mg kg-1 ww) and Zn (65.94 ± 4.48 mg kg-1 ww) levels exceed the most conservative limits established for Fe (EU and FAO/WHO = 45.00 mg kg-1 ww) and Zn (EU and ANVISA = 50.00 mg kg-1 ww), particularly in the gills and hepatopancreas. Bioaccumulation of Zn was observed in all tissues, with this concentration being up to 7.76-fold higher in crab meat. Risk assessment analyses suggest that estimated daily intake values for muscle consumption alone pose no health risk. In contrast, PIS and SIS methods for consuming white and brown meat suggest potential risks. These findings reveal the urgent need for comprehensive assessments of metal contamination in different fish and shellfish species and for improved risk assessment protocols, which are crucial to supporting public health policies and mitigating the impacts of this kind of contamination in mangrove areas.

An assessment of metal concentrations in leaves, roots, and associated sediments of mangrove plant (Avicennia marina) in the Myeik area, Myanmar

The Myeik region in Myanmar hosts extensive mangrove forests, however, increasing human activities like mining and agriculture have caused heavy metal pollution. This study investigates the role of Avicennia marina in metal dynamics and contamination status in Myeik through metal concentration analysis in leaves, roots, and sediments. Bioconcentration factors (BCF) and translocation factors (TF) were calculated, and ecological indices were used to assess contamination levels and ecological risks. Sediments had higher metal concentrations than plant parts, with nonessential elements (As, Cd, Co, Cr, Pb) averaging 83.2 ± 21.4, 3.16 ± 0.58, 25.6 ± 4.3, 55.1 ± 14.2, and 87 ± 18 mg/kg, and essential elements (Cu, Fe, Mn, Ni, Zn) averaging 21.0 ± 8.0, 18,283 ± 1614, 719 ± 312, 81 ± 30, and 108 ± 32 mg/kg. A. marina showed high BCF for Cd and Cu, and essential elements (Ni, Zn, Mn) exhibited higher TF than nonessential elements (As, Pb, Co). Ecological indices indicated significant contamination and risks due to As, Cd, and Pb, mainly sourced from mining and industrial activities. The contamination levels are ranked as Inlaymyine > Kyweku > Lighthouse. A. marina's metal uptake and translocation were influenced by local environmental factors and physiological responses. Mangrove sediments retained more metals than mudflats, showing habitat-specific differences. These findings highlight the potential of A. marina as a bioindicator of heavy metal pollution and its significant contribution to natural bioremediation processes in the Myeik.

Water and sediment characteristics in the Avicennia marina environment of the Arabian Gulf: A review

Mangroves are halophytic woody plants inhabiting the sea-land confluence of tropical and subtropical regions, influenced by marine and terrestrial factors. Among various others, Avicennia marina is the only mangrove species that thrives the extreme climatic conditions of the Arbian Gulf (the Gulf). Highly varying temperatures, hyper salinity, freshwater scarcity and anthropogenic pressures have resulted in a unique mangrove ecosystem in the Gulf. However, the water and sediment characteristics of the Gulf mangroves evolved due to natural and anthropogenic factors are not well documented yet. This review evaluates the existing literature on the water and sediment characteristics of the Gulf mangroves, and compares them with the international thresholds and regional standards reported in literature. The study reveals that the hydrological factors, such as precipitation, river runoff and groundwater, along with varying temperature and elevated salinity, significantly impacted the growth and distribution of mangroves in the Gulf. Trace elements show elevated concentrations at a few mangrove regions of the Gulf influenced by coastal developments. Furthermore, industrial expansion has resulted in the accumulation of petroleum hydrocarbons and toxic elements, raising concerns about their potential for bioaccumulation and ecological risk. On the other hand, the restoration of mangrove ecosystems in the Gulf poses significant challenges, including identification of potential sites and the reestablishment of suitable environmental conditions that disrupted by natural and anthropogenic pressures. Therefore, this review emphasizes the need for long-term monitoring and comprehensive data on hydrological and chemical parameters impacting Gulf mangrove ecosystems for the implementation of effective conservation strategies.

A systematic review on distribution, sources and impacts of heavy metals in mangrove ecosystems

Mangrove ecosystems play a vital role in providing habitat for numerous plant and animal species, protecting coastlines from erosion and storm damage, and supporting coastal communities with natural resources. Several studies have been conducted on heavy metals due to advancements in technology, which have improved the accuracy in detecting, identifying, and quantifying the metals. However, our understanding on the distribution, sources, and impacts of heavy metals on mangrove ecosystem health and productivity is currently scattered and unorganized. This review aims to bridge knowledge gaps, inform conservation strategies, and promote interdisciplinary collaboration by synthesizing ecological, socio-economic, and policy-related insights. We reviewed 139 publications from 2003 to 2024 across 20 countries, sourced from four databases (Scopus, Web of Science, PubMed, and EBSCO) and one search engine (Google Scholar). The results revealed a rapid increase in publications on this topic globally, particularly during the period between 2010 and 2023. China (29%) and India (22%) emerged as the leading contributors to research on mangrove heavy metal contamination. The most frequently studied heavy metals were Cadmium followed by Lead, Copper, Zinc and Chromium. The major sources of heavy metals contamination in mangrove ecosystems were industrial effluents (36 %) and rapid land-use conversion (31 %) and sediments (51 %) were the most commonly investigated component of mangrove ecosystem. Heavy metals have negative effects on human health (44 %) and reduce mangroves growth and development (25 %). Future studies should prioritize investigating the interactions among heavy metals contamination, and mangrove productivity and health, as well as the resilience of associated species over time.

Evaluation and mitigation of potentially toxic elements contamination in mangrove ecosystem: Insights into phytoremediation and microbial perspective

Mangroves, essential coastal ecosystems, are threatened by human-induced Potentially-toxic-elements (PTEs) pollution. This study analyzed PTEs distribution, phytoremediation potential, and rhizosphere microbial communities in Taiwan's Xinfeng mangrove forest. Significant variations in physicochemical and PTEs concentrations were observed across adjacent water bodies, with moderate contamination in the river, estuary, and overlying water of mangroves sediment. The partition-coefficient showed the mobility of Bi, Pb, Co, and Sr at the water-sediment interface. The geochemical-indices revealed high Bi and Pb contamination and moderate Zn, Sr, Cu, and Cd contamination in sediment. The overall pollution indices indicated the significant contamination, while moderate ecological risk was found for Cd (40 ≤ Eri < 80). Mangroves Kandelia obovata and Avicennia marina exhibited promising PTEs phytoremediation potential (Bi, Cd, Mn, Sr, and Co). Metagenomics indicated a diverse microbial community with N-fixation, P-solubilization, IAA synthesis, and PTEs-resistance genes. These findings underscore the need for targeted conservation to protect these critical habitats.

Seasonal variations in microplastics in a coastal wetland in southwest India as well as their risks to Sillago sihama and Gerres filamentosus

Microplastics are minute plastic particles ranging from 1 µm to 5 mm in size. Mangroves are crucial ecosystems with roles in carbon sequestration, shoreline protection, and habitat for diverse species. Despite their significance, the extent of microplastic pollution in mangroves, especially in India, remains inadequately understood. To address this gap, we conducted a seasonal sampling in the Kota mangrove ecosystem at different water column depths. Our analysis revealed average microplastic abundances of 0.93 (monsoon), 3.71 (post-monsoon), and 2.92 MPs/L (pre-monsoon). The average microplastic abundances were 19.88 and 15.86 microplastics/individual for Gerres filamentosus and Sillago sihama, respectively. Fibrous microplastics smaller than 1 mm were dominant. Transparent microplastics dominated the water column (28.57% in monsoon, 77.45% in post-monsoon, and 49.24% in pre-monsoon), and they were also prevalent in S. sihama (49.55%) and G. filamentosus (41.51%). This points towards greater bioavailability and suggests that transparent microplastics are often mistaken for prey. Anthropogenic influence is a major factor that governs microplastic distribution than season in Kota mangroves. Fourier transform infrared spectroscopy revealed that polypropylene was the dominant polymer in both water column as well as in S. sihama and G. filamentosus. We identified aquaculture, tourism, and local activities as probable sources of microplastic pollution. The monitoring data is crucial as it provides insights into microplastics pollution in two economically important fish species that are largely consumed by the local population. Exposure to microplastics from the consumption of these fish may cause serious health issues for human beings.

Halotolerant Endophytic Bacteria Priestia flexa 7BS3110 with Hg2+ Tolerance Isolated from Avicennia germinans in a Caribbean Mangrove from Colombia

The mangrove ecosystems of the Department of Atlántico (Colombian Caribbean) are seriously threatened by problems of hypersalinization and contamination, especially by heavy metals from the Magdalena River. The mangrove plants have developed various mechanisms to adapt to these stressful conditions, as well as the associated microbial populations that favor their growth. In the present work, the tolerance and detoxification capacity to heavy metals, especially to mercury, of a halotolerant endophytic bacterium isolated from the species Avicennia germinans located in the Balboa Swamp in the Department of Atlántico was characterized. Diverse microorganisms were isolated from superficially sterilized A. germinans leaves. Tolerance to NaCl was evaluated for each of the obtained isolates, and the most resistant was selected to assess its tolerance to Pb2+, Cu2+, Hg2+, Cr3+, Co2+, Ni2+, Zn2+, and Cd2+, many of which have been detected in high concentrations in the area of study. According to the ANI and AAI percentages, the most halotolerant strain was identified as Priestia flexa, named P. flexa 7BS3110, which was able to tolerate up to 12.5% (w/v) NaCl and presented a minimum inhibitory concentrations (MICs) of 0.25 mM for Hg, 10 mM for Pb, and 15 mM for Cr3+. The annotation of the P. flexa 7BS3110 genome revealed the presence of protein sequences associated with exopolysaccharide (EPS) production, thiol biosynthesis, specific proteins for chrome efflux, non-specific proteins for lead efflux, and processes associated with sulfur and iron homeostasis. Scanning electron microscopy (SEM) analysis showed morphological cellular changes and the transmission electron microscopy (TEM) showed an electrodense extracellular layer when exposed to 0.25 mM Hg2+. Due to the high tolerance of P. flexa 7BS3110 to Hg2+ and NaCl, its ability to grow when exposed to both stressors was tested, and it was able to thrive in the presence of 5% (w/v) NaCl and 0.25 mM of Hg2+. In addition, it was able to remove 98% of Hg2+ from the medium when exposed to a concentration of 14 mg/L of this metalloid. P. flexa 7BS3110 has the potential to bioremediate Hg2+ halophilic contaminated ecosystems.

Distribution, ecological risk, and sediment-influencing mechanisms of heavy metals in surface sediments along the intertidal gradient in typical mangroves in Hainan, China

The relative importance of each sediment physicochemical property to sediment heavy-metal (HM) contents has not yet been quantitatively evaluated. Differences in the HM contents of mangrove surface sediments among the high, middle, and low intertidal zones, and their quantitative relationships to sediment physicochemical properties, were investigated in Dongzhaigang and Qinglan Harbor reserves, Hainan, China. In both reserves, the Cu and Ni concentrations increased significantly from the low to high intertidal zones; the patterns of change in the Mn and Pb contents were opposite in the two reserves. The Cr concentration was significantly lower and the Pb concentration was significantly higher in the dry season than in the wet season. Ecological risks of HM were higher in Dongzhaigang than in Qinglan Harbor. Regression and redundancy (hierarchical partitioning) analyses showed that the sediment total sulfur, nitrogen and potassium contents and pH were key factors affecting the HM contents of mangrove surface sediments.

Evaluation and health risk assessment of arsenic and potentially toxic elements pollution in groundwater of Majha Belt, Punjab, India

Concentrations of potentially toxic elements (PTEs) like arsenic, uranium, iron, and nitrate in the groundwater of the Majha Belt (including Tarn Taran, Amritsar, Gurdaspur, and Pathankot districts) in Punjab, India were measured to evaluate the health risks associated with its consumption and daily use. The average concentrations of these elements in some locations exceeded the WHO-recommended values. Arsenic and iron toxicity levels were found to be higher in the Amritsar district, while uranium toxicity was more prevalent in Tarn Taran. The Trace Element Evaluation Index suggests that Amritsar is one of the districts most affected by toxic elements. According to the US Environmental Protection Agency's (USEPA) guidelines, the HQ values of U, Fe, and nitrate were less than one, indicating that there is no non-carcinogenic health risk for adults and children. However, the hazard quotient (HQ) value for arsenic was greater than one, indicating a higher possibility of health risk due to arsenic in the study area. The total hazard index values of 44.10% of samples were greater than four for arsenic, indicating that people in the Majha Belt are at a very high health risk due to the usage of water for drinking and domestic purposes. The cancer risk assessment values for arsenic in children (5.69E + 0) and adults (4.07E + 0) were higher than the accepted limit of USEPA (10-4 to 10-6) in the Majha Belt. The average radiological cancer risk values of U for children and adults were 8.68E-07 and 9.45E-06, respectively, which are well below the permissible limit of 1.67 × 10-4 suggested by the Atomic Energy Regulatory Board of DAE, India. The results of this study confirm that the residents of the Majha Belt who use contaminated groundwater are at a serious risk of exposure to arsenic in the Amritsar district and uranium in Tarn Taran district.

Trophodynamics and potential health risk assessment of heavy metals in the mangrove food web in Yanpu Bay, China

Mangroves are the cradle of coastal water biodiversity and are susceptible to heavy metal pollution. However, the trophic transfer mechanism of heavy metals in the mangrove food web and the resulting human health risks are not fully understood. Heavy metal concentration (Cr, Ni, Cu, Zn, As, Cd, Pb, V, Co) and stable isotope ratios of carbon and nitrogen (δ13C and δ15N) were evaluated in sediments and particulate organic matter, litter, and aquatic organisms (plankton, arthropods, mollusks, omnivorous fish, and carnivorous fish) from the Yanpu Bay mangroves. The results revealed that heavy metals exhibited different trophic transfer patterns. As and Hg were efficiently biomagnified, with trophic magnification factors of 1.17 and 1.42, respectively; while Cr, Ni, Cu, Cd, Pb, V, and Co were efficiently biodiluted. Zn exhibited a trophic magnification factor > 1 and was not significantly correlated with δ15N (p > 0.05), suggesting no biomagnification or biodilution. The heavy metals in the important fishery species (omnivorous fish and carnivorous fish) were below the permissible limits, except for Zn in Ophichthus apicalis. The assessment of probabilistic health risks revealed that fish consumption in adults and children posed an acceptable risk (total target hazard quotient <1).

Persistent organic pollutants and trace metals in selected marine organisms from the Akanda National Park, Gabon (Central Africa)

Akanda National Park (ANP) is composed of mangrove ecosystems bordering Libreville, Gabon's capital. The contamination of aquatic resources from the ANP by persistent organic pollutants (POPs) and trace metals (TMs) was never evaluated. To provide a basis for their monitoring in the ANP, five species (two fish, two mollusks, and one crustacean) were analyzed from three sampling sites in 2017. Contamination levels for POPs and TMs were below maximum acceptable limits for seafood, including Cd and Pb. No DDT was found in any sample. Inter-specific differences were more obvious than the differences among sites, although the results may be biased by an unbalanced sampling design. The oyster Crassostrea gasar was the most contaminated species, making this species a good candidate to assess environmental contamination in the area. The studied species also contained essential elements, such as Fe, Zn and Mn at interesting levels in a nutritional point of view.

Microplastics in mangroves with special reference to Asia: Occurrence, distribution, bioaccumulation and remediation options

Microplastics (MPs) are a new and lesser-known pollutant that has intrigued the interest of scientists all over the world in recent decades. MP (<5mm in size) can enter marine environments such as mangrove forests in a variety of ways, interfering with the health of the environment and organisms. Mangroves are now getting increasingly exposed to microplastic contamination due to their proximity to human activities and their position as critical transitional zones between land and sea. The present study reviews the status of MPs contamination specifically in mangrove ecosystems situated in Asia. Different sources and characteristics of MPs, subsequent deposition of MPs in mangrove water and sediments, bioaccumulation in different organisms are discussed in this context. MP concentrations in sediments and organisms were higher in mangrove forests exposed to fishing, coastal tourism, urban, and industrial wastewater than in pristine areas. The distribution of MPs varies from organism to organism in mangrove ecosystems, and is significantly influenced by their morphometric characteristics, feeding habits, dwelling environment etc. Mangrove plants can accumulate microplastics in their roots, stem and leaves through absorption, adsorption and entrapment helping in reducing abundance of microplastic in the surrounding environment. Several bacterial and fungal species are reported from these mangrove ecosystems, which are capable of degrading MPs. The bioremediation potential of mangrove plants offers an innovative and sustainable approach to mitigate microplastic pollution. Diverse mechanisms of MP biodegradation by mangrove dwelling organisms are discussed in this context. Biotechnological applications can be utilized to explore the genetic potential of the floral and faunal species found in the Asian mangroves. Detailed studies are required to monitor, control, and evaluate MP pollution in sediments and various organisms in mangrove ecosystems in Asia as well as in other parts of the world.

Prediction of heavy metals in polluted mangrove soils in Brazil with the highest reported levels of mercury using near-infrared spectroscopy

Infrared reflectance spectroscopy has demonstrated potential as a tool for monitoring and preventing contamination in different environments. The objective of this study was to evaluate the usage of near-infrared spectroscopy for predicting heavy-metal contamination in mangrove soils from the Botafogo River estuary located in Pernambuco State, Northeastern Brazil. These soils exhibit the highest mercury (Hg) levels ever reported for Brazilian mangrove soils. Sixty-one samples (obtained at depths ranging from 0 to 5 cm) were collected and measured using near-infrared (1000-2500 nm) reflectance spectroscopy. Preprocessing methods were applied, and partial least squares regression was used to build prediction models for attributes such as clay content, soil organic matter (SOM), pH, Eh, and concentrations of Cr, Cu, Hg, Ni, Pb, and Zn. The models were evaluated using root mean squared error (RMSE), the adjusted coefficient of determination (R2adj), bias, the ratio of performance to interquartile distance (RPIQ), and Lin's concordance correlation coefficient (CCC). The best outcomes were noted for concentrations of Cr, Cu, Hg, Ni, and Pb (RPIQ > 2.5 and R2adj > 0.80); second-best outcomes were found for Zn and SOM (RPIQ > 1.5 and R2adj > 0.70). Clay content, pH and Eh exhibited the poorest outcomes (RPIQ < 1.5). The importance of spectral preprocessing is highlighted, notably with Savitzky-Golay derivatives and Multiplicative Scatter Corrections, which boosted performance for most of the variables. Near-infrared spectroscopy can be efficiently used to predict Cr, Cu, Hg, Ni, Pb and SOM and represents a technique complementary to traditional analyses.

Contamination in mangrove ecosystems: A synthesis of literature reviews across multiple contaminant categories

Mangrove forests are exposed to diverse ocean-sourced and land-based contaminants, yet mangrove contamination research lags. We synthesize existing data and identify major gaps in research on five classes of mangrove contaminants: trace metals, persistent organic pollutants, polycyclic aromatic hydrocarbons, microplastics, and pharmaceuticals and personal care products. Research is concentrated in Asia, neglected in Africa and the Americas; higher concentrations are correlated with waste water treatment plants, industry, and urbanized landscapes. Trace metals and polycyclic aromatic hydrocarbons, frequently at concentrations below regulatory thresholds, may bioconcentrate in fauna, whereas persistent organic pollutants were at levels potentially harmful to biota through short- or long-term exposure. Microplastics were at variable levels, yet lack regulatory and ecotoxicological thresholds. Pharmaceuticals and personal care products received minimal research despite biological activity at small concentrations. Given potential synergistic effects, multi-contaminant research, increased monitoring of multiple contaminant classes, and increased public outreach and involvement are needed.

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.

Challenges towards the Sustainability and Enhancement of the Indian Sundarban Mangrove's Blue Carbon Stock

The Sundarban is the world's largest contiguous mangrove forest and stores around 26.62 Tg of blue carbon. The present study reviewed the factors causing a decline in its blue carbon content and poses a challenge in enhancing the carbon stock of this region. This review emphasized that recurrent tropical cyclones, soil erosion, freshwater scarcity, reduced sediment load into the delta, nutrient deficiency, salt-stress-induced changes in species composition, mangrove clearing, and anthropogenic pollution are the fundamental drivers which can potentially reduce the total blue carbon stock of this region. The southern end of the Ganges-Brahmaputra-Meghna Delta that shelters this forest has stopped its natural progradation due to inadequate sediment flow from the upper reaches. Growing population pressure from the north of the Sundarban Biosphere Reserve and severe erosion in the southern end accentuated by regional sea-level rise has left minimal options to enhance the blue carbon stock by extending the forest premises. This study collated the scholarly observations of the past decades from this region, indicating a carbon sequestration potential deterioration. By collecting the existing knowledge base, this review indicated the aspects that require immediate attention to stop this ecosystem's draining of the valuable carbon sequestered and, at the same time, enhance the carbon stock, if possible. This review provided some key recommendations that can help sustain the blue carbon stock of the Indian Sundarban. This review stressed that characterizing the spatial variability of blue carbon with more sampling points, catering to the damaged trees after tropical cyclones, estuarine rejuvenation in the upper reaches, maintaining species diversity through afforestation programs, arresting coastal erosion through increasing sediment flow, and combating marine pollution have become urgent needs of the hour. The observations synthesized in this study can be helpful for academics, policy managers, and decision makers willing to uphold the sustainability of the blue carbon stock of this crucial ecosystem.

Genome-wide functional characterization of Canavalia rosea cysteine-rich trans-membrane module (CrCYSTM) genes to reveal their potential protective roles under extreme abiotic stress

Cysteine-rich transmembrane module (CYSTM) proteins constitute small molecular protein families and have been identified across eukaryotes, including yeast, humans, and several plant species. Plant CYSTMs play vital roles in growth regulation, development, phytohormone signal transduction, pathogen defense, environmental stress response, and even heavy metal binding and detoxification. Canavalia rosea (Sw.) DC is a perennial halophyte with great semi-arid and saline-alkali tolerance. In this study, the CrCYSTM family including 10 members were identified in the C. rosea genome, with the purpose of clarifying the possible roles of CrCYSTMs in C. rosea plants development and stress resistance. The phylogenetic relationships, exon-intron structure, domain structure, chromosomal localization, and putative cis-acting elements in promoter regions were predicted and analyzed. Transcriptome analysis combined with quantitative reverse transcription PCR showed that different CrCYSTM members exhibited varied expression patterns in different tissues and under different abiotic stress challenges. In addition, several CrCYSTMs were cloned and functionally characterized for their roles in abiotic stress tolerance with yeast expression system. Overall, these findings provide a foundation for functionally characterizing plant CYSTMs to unravel their possible roles in the adaptation of C. rosea to tropical coral reefs. Our results also lay the foundation for further research on the roles of plant CYSTM genes in abiotic stress signaling, especially for heavy metal detoxification.

Distribution and risk assessment of microplastics in typical ecosystems in the South China Sea

Microplastic pollution in the marine environment has attracted worldwide attention. The South China Sea is considered a hotspot for microplastic pollution due to the developed industries and high population density around the South China Sea. The accumulation of microplastics in ecosystems can adversely affect the health of the environment and organisms. This paper reviews the recent microplastic studies conducted in the South China Sea, which novelty summarizes the abundance, types, and potential hazards of microplastics in coral reef ecosystems, mangrove ecosystems, seagrass bed ecosystems, and macroalgal ecosystems. A summary of the microplastic pollution status of four ecosystems and a risk assessment provides a more comprehensive understanding of the impact of microplastic pollution on marine ecosystems in the South China Sea. Microplastic abundances of up to 45,200 items/m³ were reported in coral reef surface waters, 5738.3 items/kg in mangrove sediments, and 927.3 items/kg in seagrass bed sediments. There are few studies of microplastics in the South China Sea macroalgae ecosystems. However, studies from other areas indicate that macroalgae can accumulate microplastics and are more likely to enter the food chain or be consumed by humans. Finally, this paper compared the current risk levels of microplastics in the coral reef, mangrove, and seagrass bed ecosystems based on available studies. Pollution load index (PLI) ranges from 3 to 31 in mangrove ecosystems, 5.7 to 11.9 in seagrass bed ecosystems, and 6.1 to 10.2 in coral reef ecosystems, respectively. The PLI index varies considerably between mangroves depending on the intensity of anthropogenic activity around the mangrove. Further studies on seagrass beds and macroalgal ecosystems are required to extend our understanding of microplastic pollution in marine environments. Recent microplastic detection in fish muscle tissue in mangroves requires more research to further the biological impact of microplastic ingestion and the potential food safety risks.

Characterization of microplastic pollution in the Pasur river of the Sundarbans ecosystem (Bangladesh) with emphasis on water, sediments, and fish

An emerging concern of today's world, due to their universal dispersion worldwide, is the environment's microplastic pollution. The Sundarbans, the world's largest mangrove, have unique and dynamic environmental settings with numerous pollution risk exposures, including microplastics (MPs). Thus, the present study has focused on the MP pollution in water, sediment, and fish samples of the Sundarbans of Bangladesh for the first time. Water and sediment samples were collected (n = 30/each) from sampling locations along the Pasur river (Bangladesh). Furthermore, nine species of fish samples were collected from a local fish market situated at the Mongla port. Results show that 100 % of the analyzed samples have evidence of MPs. On average, 2.66 × 103 plastic particles/L and 1.57 × 105 particles/kg were found in water and sediment samples, respectively. Furthermore, results show a higher number of MPs in the animals' gastrointestinal tract (GIT) (10.41 particles/g), concerning the average concentration recorded in the muscles (4.68 particles/g). O. pama and H. nehereus were the species that showed the highest MPs accumulation in the GIT. In the muscles, the highest MP levels were observed in T. ilisha and L. calcarifer. Most of the particles were smaller than 1 mm; black-colored particles dominated the pool. FT-IR analysis revealed the presence of seven polymer types where polyamide was abundant in water and sediment samples. SEM analysis showed morphological structures and adsorbed particles on the surface of plastic samples, and the spatial distribution of MPs indicates that the location with high human intervention has elevated levels of MPs. Therefore, our study demonstrates that Sundarbans mangrove forests are highly contaminated with MPs and that its fisheries can be a potential source of human exposure to these pollutants.

Health risk-benefit assessment of the commercial red mangrove crab: Implications for a cultural delicacy

Mangrove forests, provide vital food resources and are an endangered ecosystem worldwide due to pollution and habitat destruction. A risk-benefit assessment (RBA) was performed on the red mangrove crab (Ucides occidentalis) from the threatened Guayas mangroves in Ecuador. It was aimed to assess the combined potential adverse and beneficial health impact associated with crab consumption and define a recommended safe intake (SI) to improve the diet of the Ecuadoran population while ensuring safe food intake. Target hazard quotients (THQs), benefit quotients (Qs), and benefit-risk quotients (BRQs) were calculated based on the concentrations of the analyzed contaminants (121 pesticide residues, 11 metal(loid)s, antimicrobial drugs from 3 classes) and nutrients (fatty acids, amino acids, and essential nutrients). Except for inorganic arsenic (iAs), the THQ was below 100 for all investigated contaminants, suggesting that the average crab consumer is exposed to levels that do not impose negative non-carcinogenic or carcinogenic health effects in the long and/or short term. Concentrations of iAs (average AsIII: 25.64 and AsV: 6.28 μg/kg fw) were of the highest concern because of the potential to cause negative health effects on long-term consumption. Despite the thriving aquaculture in the Guayas estuary, concentrations of residues of the antimicrobial drugs oxytetracycline (OTC), florfenicol, and nitrofurans still were low. Based on the fact that different risk reference values exist, related to different safety levels, four SI values (0.002, 0.04, 4, and 18 crabs/day) were obtained. The strictest intake values indicate a concern for current consumption habits. In conclusion, the red mangrove crab contains various important nutrients and can be part of a balanced diet for the Ecuadorian population when consumed in limited portions. The present study emphasizes the importance of safeguarding the quality of the environment as a prerequisite for procuring nutritious and safe food.

Spatial distribution of total mercury and methylmercury in the sediment of a tropical coastal environment subjected to heavy urban inputs

Jurujuba Cove is located in Guanabara Bay (adjacent to highly populated city of Rio de Janeiro, Brazil), which receives diffuse sources of contaminants along with two main freshwater inputs (the Cachoeira and Icaraí rivers), and hosts mussel farms. The main goal of this work was to evaluate the total mercury (THg) and methylmercury (MeHg) concentrations distributions in the sediments of the cove and their associations with physical and chemical parameters, thereby assessing their geochemical behavior. Twenty samples of surface sediments were collected and characterized for grain size, pH, redox potential, organic carbon, total phosphorus, THg and MeHg. Spatial distribution maps were produced for each parameter and a principal components analysis was carried out, to assess THg and MeHg behavior and their relationships with other parameters. The principal components analysis showed that grain size functions as the main diluting agent. The highest THg concentrations were observed in the mussel-farm area (656.1 ng g^-1), and were related to fine grain size and elevated organic carbon values. High MeHg concentrations also occurred in the center of the cove, probably favored by high organic carbon content (low-energy environment). Total phosphorus concentrations indicate that Cachoeira River is a possible source of sewage, but little mercury seems to come from it. The results showed that although total mercury concentrations are elevated, with exception of a few locations, small methylmercury convertion rates were recorded in the sediments.

Biogeochemical behaviour of cadmium in sediments and potential biological impact on mangroves under anthropogenic influence: A baseline survey from a protected nature reserve

Cadmium is a toxic element and its effects are well understood for human health, but its biogeochemical behaviour is still poorly studied and understood in natural ecosystems. This work addresses knowledge gaps concerning its presence, biogeochemical behaviour and impacts in mangrove ecosystems. Through geochemical data and multivariate analysis (i.e., factor and cluster analysis) of data from mangroves of Isla del Carmen, one of the largest extents in Mexico we explored the biogeochemical behaviour of Cd, a potentially toxic element, to identify its anthropogenic sources and interactions with sediments. Pollution indices, including enrichment factor (EF), geo-accumulation index (Igeo), sediment quality guidelines (SQG) and toxicological studies were used to assess the biological impacts of Cd and infer the natural levels tolerated by mangrove trees that form the basis of this natural ecosystem. Our results highlighted that Cd accumulation is driven by interactions between organic matter (OM), sulphur and fine particles; whereas enrichment factor showed values of 6.9 (EF) and 3.5 (EF) associated with point sources and ranged between 2 and 2.9 (EF) in relation to non-point sources. Finally, our geochemical approach revealed that Cd enrichment originates from urban activities and from the poor management of urban residuals.

Response of bacterial diversity and community structure to metals in mangrove sediments from South China

Human activities have given rise to metal contamination in the constituents of mangrove ecosystems, posing a critical threat to sediment microorganisms; hence, it is of great importance to comprehend the effects of metals on the microbial communities in mangrove sediments. This study was the first to explore the response of the bacterial diversity and community structure to nine metals (As, Co, Cr, Cu, Mn, Ni, Pb, V and Zn) and organic matter fractions (including total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS)) in mangrove wetlands from Zhanjiang, China, using 16S rRNA high-throughput sequencing technology and Spearman correlation analysis. The results showed that these nine metals were scattered differently in different mangrove sediments, and the metals and organic matter fractions jointly affected the bacterial communities in the sediments. Several metals displayed significant positive correlations with the abundances of the phylum Bacteroidetes and the genera Actibacter and Sphingobacterium but significant negative correlations with the abundances of two genera Holophaga and Caldithrix. Furthermore, the abundances of the phylum Actinobacteria and many bacterial genera showed significant positive or negative responses to the levels of the three organic matter fractions. Interestingly, the levels of a number of bacterial genera that exhibited increased abundance with high levels of metals and TS might be reduced with high TOC and TN, and vice versa: the levels of genera that exhibited decreased abundance with high levels of metals and TS might be increased with high TOC and TN. Overall, many bacterial groups showed different response patterns to each metal or organic matter fraction, and these metals together with organic matter fractions influenced the bacterial diversity and community structure in mangrove sediments.

Polycyclic aromatic hydrocarbons (PAHs) in mangrove ecosystems: A review

Polycyclic aromatic hydrocarbons (PAHs) are organic pollutants of increasing concern in the different fields of the environment and human health. There are 16 of them that are recognized as priority pollutants by the US environmental protection agency due to their mutagenic and carcinogenic potentials. Due to their hydrophobicity and stability, they are persistent in the environment and can be transported over long distances. Their toxicological effects on multiple species, including humans, as well as their bioaccumulation in the food web became major topics in organic pollutants research this last decade. In the environment, multiple studies have been conducted on their accumulation in the soil and their degradation processes resulting in numerous review papers. However, the dynamics of PAHs in mangrove ecosystems is not yet completely understood. In this review paper, an exhaustive presentation of what is known about PAHs and their transfer, accumulation, and degradation in mangrove ecosystems is offered. This article brings to light the knowledge already acquired on the subject and the perspective research necessary to fully comprehend PAHs dynamics in mangrove ecosystems.

Accumulation of nutrients and potentially toxic elements in plants and fishes in restored mangrove ecosystems in South China

Mangroves are highly dynamic ecosystems that offer important services such as maintaining biodiversity, filtering pollutants, and providing habitats for fishes. We investigated the uptake and accumulation of nutrients and potentially toxic elements in mangrove plants and fish to better understand the role of mangrove restoration in maintaining mangrove biota quality. In mangrove plants, the average bioconcentration factors of nutrients and potentially toxic elements were in the order P > Pb > Mn > Mg > Se > Zn > Hg > Cu > Cd > As > Co > Cr > Ni > Fe > V > Sb, where only P (all plant species) and Pb (Sonneratia apetala Buchanan-Hamilton) had a BCF > 1.0 in mangrove plants. In general, Sonneratia spp. had better performances than Kandelia candel (Linn.) Druce, Aegiceras corniculatum (Linn.) Blanco and Acanthus ilicifolius L. Sp. in terms of nutrient uptake and toxic metal(loid)s accumulation, and the best uptake capacity was found in S. apetala. Fast growth and easy adaptation make S. apetala suitable for a restored mangrove ecosystem, but continual management is needed to prevent its suppression of mangrove species diversity. The concentration of As, Cd, Hg, Cu, Cr and Pb in the mangrove sediment were 30-220% higher than the Chinese National Standard of Marine Sediment Quality Class I limits, suggesting that the sediments were unsuitable for aquaculture and nature reserves. Although a higher toxic metal(loid)s concentration in the sediment was found, the target hazard quotient (THQ) of this toxic metal(loid)s in 5 mangrove habitat fishes was <1.0, except THQ of Pb in Boleophthalmus pectinirostris Linnaeus was 1.17, and THQ of Cr in Bostrychus sinensis Lacépède was 1.12. The low THQ (less than 1.0) of mangrove habitat fishes suggested that the restored mangrove system could alleviate the bioaccumulation of toxic metal(loid)s in mangrove fish.

Multiple heavy metals affect root response, iron plaque formation, and metal bioaccumulation of Kandelia obovata

Multiple heavy metal pollution in mangrove wetlands is serious. Kandelia obovata seedlings were cultured in pots in which lead (Pb), zinc (Zn) and copper (Cu) were added separately and in combinations. The results showed that heavy metal stress improved the rate of root oxygen leakage, enhanced root activity, and reduced root porosity. The root under single heavy metal stress was impacted by the addition of other heavy metals, demonstrating antagonistic or synergistic effects. Iron plaque (IP) formation was improved under single Zn or Cu stress, and inhibited in binary stress of Pb + Cu. The adsorptions of IP on heavy metals in combined stress (Pb, 62–116 μg g⁻¹; Zn, 194–207 μg g⁻¹; Cu, 35–52 μg g⁻¹) were higher than that in single stress (Pb, 18 μg g⁻¹; Zn, 163 μg g⁻¹; Cu, 22 μg g⁻¹). K. obovata accumulated higher levels of heavy metals in root (Pb, 7–200 μg g⁻¹; Cu, 4–78 μg g⁻¹), compared with IP (Pb, 18–116 μg g⁻¹; Cu, 22–52 μg g⁻¹), stem (Pb, 3–7 μg g⁻¹; Cu, 9–17 μg g⁻¹), and leaf (Pb, 2–4 μg g⁻¹; Cu, 4–7 μg g⁻¹). Correlation analysis showed that single and binary stresses affected K. obovata, with more significant effect of trinary stress. Regression path analysis showed that multiple heavy metal stress firstly affected root, then indirectly contributed to IP formation, as well as heavy metal in IP and root; at last, heavy metal in IP directly contributed to heavy metal bioaccumulations in root.

Bioaccumulation and Heavy Metal Contamination in Fish Species of the Dhaleswari River of Bangladesh and Related Human Health Implications

The release of a large quantity of heavy metals into the Dhaleswari River from the tannery, dyeing, and other industrial setups and their subsequent transfer to food chains through fish consumption have been an alarming issue in Bangladesh. To study the pollution level, a total of seven fish species, namely Heteropneustes fossillis, Channa punctata, Nandus nandus, Chanda nama, Anabas testudineus, Mystus gulio, and Colisa fasciata, were collected in winter from the Dhaleswari River and the total concentrations of Cr, Pb, Ni, and Zn in head and body tissues were analyzed separately. The concentrations of Cr, Pb, and Zn were found 300, 20, and 10 times higher, respectively, than the guideline value of the Food and Agriculture Organization (FAO)/World Health Organization (WHO), indicating possible health risks to humans. In most cases, bioaccumulation factors (BAFs) exceeded the highest limit, expressing that most of the species, especially C. nama, A. testudineus, and C. fasciata, were in the highly bioaccumulative state. The health risks associated with fish consumption were determined in terms of estimated daily intake (EDI), non-carcinogenic risks (THQ), and carcinogenic risk (TR) factors. The THQs for Cr and Pb crossed the maximum value of 1 in all the fish species except Pb in Mystus gulio, which might cause different non-carcinogenic diseases upon consumption of these fishes. In all the fish species, the carcinogenic risk factor for Cr exceeded the standard value (10^-4), indicating chronic cancer risk to humans. Although the estimated daily intake (EDI) values did not cross the permissible limit, continuous consumption of contaminated fish from the target area may cause serious health complications. This study revealed that consumption of these fishes exposed people to a higher risk of non-carcinogenic and carcinogenic consequences in terms of human health.

Degraded mangroves as sources of trace elements to aquatic environments

Mangrove forests have been reported as sinks for metals because of the immobilization of these elements in their soils. However, climate change may alter the functioning of these ecosystems. We aimed to assess the geochemical dynamics of Mn, Cu, and Zn in the soils of a mangrove forest dead by an extreme weather event in southeastern Brazil. Soil samples were collected from dead and live mangroves adjacent to each other. The physicochemical parameters (total organic carbon, redox potential, and pH), total metal content, particle size, and metal partitioning were determined. Distinct changes in the soil geochemical environment (establishment of suboxic conditions) and a considerable loss of fine particles was caused by the death of the mangroves. Our results also showed a loss of up to 93 % of metals from soil. This study highlights the paradoxical role of mangroves as potential metal sources in the face of climate change.

Spatial distribution and ecological risk assessment of potentially toxic metals in the Sundarbans mangrove soils of Bangladesh

At present, there are growing concerns over the increasing release of trace metals in the Sundarbans mangrove areas in Bangladesh due to nearby shipbreaking and metallurgical industries, untreated waste discharge, navigation activities, and other natural processes that deposit trace metals into soils. The current study investigated the spatial distribution, contamination level, and ecotoxicity of eight trace metals (Fe, Mn, Cu, Zn, Pb, Cd, Cr, Ni) in Sundarbans soils. Results revealed that all the trace metals except Cr were present in higher concentrations compared to Earth’s shale and/or upper continental crust. Principal component analysis and Pearson correlation showed strong positive correlations (p < 0.05) between Fe, Mn, Cu, and Zn; Ni with Mn and Cr. There were significant associations (p < 0.05) of % clay and total organic carbon (TOC) with Pb-Ni-Cr and negative correlations of pH with all the trace metals. The hierarchical cluster analysis grouped Pb, Ni, and Cd into one distinct cluster, suggesting they are derived from the same sources, possibly from anthropogenic activities. Geo accumulation index (I-geo), enrichment factor (EF), contamination factor (CF), and spatial distribution showed moderately polluted soils with Ni, Pb, and Cd (EF = 3–7.4, CF = 1–2.8, I-geo = 0–0.9) and low pollution by Zn, Cu, Fe, and Mn (EF < 3, CF < 1, I-geo < 0). The ecological risk index (RI) revealed that S-4 (RI = 114.02) and S-5 (RI = 100.04) belonged to moderate risk, and other areas posed a low risk (RI < 95). The individual contribution of Cd (25.9–73.7%), Pb (9.2–29.1%), and Ni (9.6–26.4%) to RI emphasized these metals were the foremost concern in the Sundarbans mangroves due to their long persistence time and high toxicity, even if they were present in low concentrations.

Seasonal effect on biomarker responses in sentinel species: innovation in mangrove conservation status assessment

We evaluated the environmental quality in mangrove areas of the Western Atlantic with different levels and history of contamination, considering biomarkers for the crab Ucides cordatus. For this purpose, specimens were collected in two climatic seasons (rainy and dry seasons) and assays of genotoxicity (MN, micronucleus), cytotoxicity (NRRT, neutral red retention time) and biochemical (MT, metallothionein; and LPO, lipoperoxidation) were conducted. In the most impacted mangroves, there was an increase in the mean of micronucleus (frequency of MN/1000), which was associated with a shorter retention time (minutes of NRRT). In contrast, the most pristine areas showed MN < 3 and NRRT < 100 min, with no seasonal effect, indicating a lower effect of degenerative processes by xenobiotics. The rainy season was more harmful, especially regarding cytogenotoxicity. The use of bioindicator species for environmental monitoring should be guided by an analysis of biomarkers considering the season, because during the period of highest rainfall, biomarkers values can change.

Impacts of environmental pollution on mangrove phenology: Combining remotely sensed data and generalized additive models

Mangrove ecosystems worldwide have been affected by anthropogenic activities that modify natural conditions and supply trace elements that affect mangrove health and development. In order to gain a better understanding of these ecosystems, and assess the influence of physicochemical (granulometry, pH, salinity and ORP) and geochemical variables (concentrations of V, Cr, Co, Ni, Cu, Zn, Pb, Rb, Sr and Zr) on mangrove phenology, we combined field and satellite derived remotely sensed data. Phenology metrics in combination with Generalized Additive Models showed that start of the season was strongly influenced by Pb and Cu pollution as well as salinity and pH, with a large percentage of deviance explained (92.10%) by the model. Start of season exhibited non-linear delays as a response to pollution. Other phenology parameters such as the length of season, timing of the peak of season, and growth peak also indicated responses to both trace elements and physicochemical and geochemical variables, with percentages of deviance explained by the models ranging between 33.90% and 97.70%. While the peak of season showed delays as a response to increased pH and decreased salinity, growth peak exhibited a non-linear decrease as a response to increased Sr concentrations. These results suggest that trace element pollution is likely to lead to altered phenological patterns in mangroves.

Phytoextraction and Antioxidant Defense of Mangrove Seedling (Kandelia obovata) to Inorganic Arsenate Exposure

Increasing arsenic (As) pollution is posing potential endangerment to mangrove wetland ecosystems. Mangrove phytoextraction, translocation, and responses to As exposure must be urgently addressed. In this study, the growth and physiological response of Kandelia obovata seedlings were examined after addition of 25−200 mg kg−1 As under sediment culture conditions. Results showed that the seedling morphological variations were not significant below 100 mg kg−1 compared to the control group, indicating superoxide dismutase, peroxidase, and catalase synergetic interaction to resist the As exposure. High As concentrations (150–200 mg kg−1) inhibited the seedling growth accompanied by a significant increase in malondialdehyde content and decrease in activities of antioxidant enzymes. Toxicity symptoms and mortality appeared in 200 mg kg−1 As, presumably because the plant reached the limit of As tolerance. Besides, As accumulated mainly in roots, accounting for 87.04–97.85% of the total As, and the bioaccumulation factor (BCF) was >100%. However, the BCF and translocation factor (TF) in stems and leaves were below unity, illustrating a weak capacity of transferring As to aerial parts of the seedlings. Overall, K. obovata is a potential remediated species in polluted coastal wetlands due to high phytoextraction capacity and high tolerance to As exposure.

Heavy metal pollution in coastal wetlands: A systematic review of studies globally over the past three decades

Coastal wetlands are ecosystems lying between land and ocean and are subject to inputs of heavy metals (HMs) from terrestrial, oceanic and atmospheric sources. Although the study on HM pollution in coastal wetlands has been rapidly developing over the past three decades, systematic reviews are still unavailable. Here, by analyzing 3343 articles published between 1990 and 2019, we provided the first holistic systematic review of studies on HM pollution in coastal wetlands globally. The results showed a trend of rapid increases in publications in this field globally, especially over the past ten years. Trends varied greatly among coastal countries, and global trends were primarily driven by the US before 2000, and in China after 2010. We also found that mercury (Hg), cadmium (Cd), and copper (Cu) were the most widely studied HM elements globally, but patterns differed geographically, with Hg being most widely examined in the Americas, Cd in China and India, and lead (Pb) in the western Europe and Australia, respectively. Among different types of coastal wetlands, salt marshes, mangrove forests, and estuaries were the most widely studied, in contrast to seagrass beds and tidal flats. As for ecosystem components, soils/sediments and plants were most extensively investigated, while algae, microbes, and animals were much less examined. Our analysis further revealed rapid emergence of topics on anthropogenic sources, interactions with other anthropogenic environmental changes (climate change in particular), and control and remediation methodology in the literature in the recent ten years. Moving forward, we highlight that future studies are needed to i) better understand the impacts of HM pollution in less studied coastal wetland systems and species, ii) deepen current understanding of the biogeochemical behaviors of HMs under anthropogenic activities, iii) examine interactions with other anthropogenic environmental changes, iv) conceive ecological remediation (i.e., "ecoremediation" as compared to traditional physiochemical remediation and bioremediation) strategies, and v) develop advanced analysis instruments and methods. The perspectives we brought forward can help stimulate many new advances in this field.

Roles of Canavalia rosea metallothioneins in metal tolerance and extreme environmental adaptation to tropical coral reefs

Canavalia rosea (Sw.) DC is a perennial twining herb distributed in the semi-arid and saline-alkali areas of coastal regions and has evolved halotolerance. In this study, we present the first comprehensive survey of the metallothionein (MT) gene family in C. rosea. MT proteins belong to a family of low-molecular-weight polypeptides with a high content of cysteine residues, which have an affinity to bind with heavy metal ions. MTs also play important roles in stress responses as reactive oxygen species (ROS) scavengers. A total of six CrMTs were identified in the C. rosea genome and classified into four subgroups by phylogenetic analysis. An analysis of the cis-acting elements revealed that a series of hormone-, stress-, and development-related cis-acting elements were present in the promoter regions of CrMTs. The expression of CrMTs also showed habitat- and environmental stress-regulated patterns in C. rosea. CrMT overexpression in yeast enhanced tolerance to heavy metals and ROS, as well as high osmotic and alkalinity stress, which is consistent with their predicted roles as metal-chelating proteins and ROS scavengers. Our results indicate that the CrMT genes might contribute to the detoxification of plants to metals and provide marked tolerance against abiotic stress. The expression patterns of CrMTs in C. rosea also indicate that CrMTs play important roles in this species' response to extreme environments on tropical islands and reefs, probably by improving the thermotolerance of C. rosea plants.

Dynamics of low-molecular-weight organic acids for the extraction and sequestration of arsenic species and heavy metals using mangrove sediments

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.

Chemometric study on the biochemical marker of the manglicolous fungi to illustrate its potentiality as a bio indicator for heavy metal pollution in Indian Sundarbans

The study represents in vitro chemometric approach for assessing the heavy metal pollution in Indian Sundarbans. Physio-chemical and elemental characterisation of the sediment samples of Indian Sundarbans had shown high enrichments of toxic metal ions. It was characterised by elevated enrichment factors (2.16-10.12), geo-accumulation indices (0.03 -1.21), contamination factors (0.7-3.43) and pollution load indices (1.0-1.25) which showed progressive sediment quality deterioration and ecotoxicological risk due to metal ions contamination. The physio-chemical parameters of the sediments were replicated and computational chemometric modeling was utilized to assess fungal metabolic growth. All the fungi isolates had shown maximum metabolic activity in high temperature, alkaline pH, and high salinity. Further, the fungal metabolic activity was assessed in different gradient of heavy metal concentration. The significant deterioration of biochemical marker with increasing concentration of heavy metal indicates the status of the microbial health due to toxic metal pollution in the mangrove habitat.

Understanding potentially toxic metal (PTM) induced biotic response in two riparian mangrove species Sonneratia caseolaris and Avicennia officinalis along river Hooghly, India: Implications for sustainable sediment quality management

Elevated human-induced activities have prompted significant uncontrolled release of potentially toxic metals (PTM) to the undisturbed ecosystem throughout the globe. Riparian mangrove vegetations act as a natural purifier of wastewaters and assist in maintaining a healthy ecosystem. We have investigated the elevated PTM-induced stress and biotic response of two riparian mangrove species e.g. Sonneratia caseolaris and Avicennia officinalis by river Hooghly. The increased PTM concentrations were observed throughout the river bank; with the maximum pollution load at Chemaguri (S9). Except Co, Cr and Pb, higher enrichment factor (1.97-8.89) and contamination factor (0.64-2.88) values were observed for Cd, Cu, Fe, Zn. Mn, and Ni. Geo-accumulation index (-2.2 - 0.92) values indicates natural geogenic accumulation of Cu in the riparian mangrove sediment. Thus, sediment quality indices suggest except Cu, enrichment of all studied PTMs was sourced from anthropogenic activities. The sediment of the region when compared with consensus-based sediment quality guidelines shows considerable ecotoxicological risks and threat towards human health considering Ni accumulation. The highest potential ecological risk index value was observed in Chemaguri (S9). The biotic response of riparian mangroves was characterized by reduced photosyhthetic pigments (Chlorophyll a and Chlorophyll b) and increased activity of antioxidative stress enzymes (POD, CAT and SOD). Significant statistical relationship between antioxidative enzyme activity, photosynthetic pigments and bioaccumulated PTMs reflects active functioning of detoxification mechanism in the riparian mangrove species.

Potential and mechanism of glomalin-related soil protein on metal sequestration in mangrove wetlands affected by aquaculture effluents

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.

Effects of heavy metals and organic matter fractions on the fungal communities in mangrove sediments from Techeng Isle, South China

Heavy metal pollution has become a serious environmental problem in mangrove ecosystems and has attracted more attention. Most of previous studies have mainly focused on the effects of heavy metals on bacterial communities in mangrove sediments. This study was the first to investigate the effects of heavy metals (e.g., As, Co, Cr, Cu, Mn, Ni, Pb, V and Zn) and organic matter fractions (including total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS)) on the fungal communities in mangrove sediments from Techeng Isle, South China. The results of this study indicated that the average contents of Mn, Pb and V of 8.30-161.80 μg/g presented relatively higher pollution levels, while the concentrations of Zn, Cr, Cu and Ni of 0.80-21.93 μg/g were lower than those recorded in other mangrove ecosystems. Furthermore, the sediment fungal community structures responded differently to the nine heavy metals and three organic matter fractions. Heavy metals Cr, Pb and V displayed significant positive correlations with Eutypella (P < 0.05), whereas significant negative correlations with Cystobasidium, Lulworthia, Cladosporium, Lulwoana and Cephalotheca (P < 0.05). In addition, the effects of heavy metals and TS on many fungal genera were opposite to those of TOC and TN. Fungal genera that decreased with high TOC and TN contents may be increased with high heavy metal contents and TS, and vice versa, and the genera that increased with high TOC and TN contents may be decreased with high heavy metals and TS. Our results suggested that many heavy metals, such as Cr, Pb and V, were sensitive to several fungal genera in mangrove sediments, and heavy metals together with organic matter fractions may participate and shape the fungal communities in mangrove sediments.

Molecular impacts of dietary exposure to nanoplastics combined with arsenic in Canadian oysters (Crassostrea virginica) and bioaccumulation comparison with Caribbean oysters (Isognomon alatus)

Despite the urge need to address the possible impact of plastic debris, up to now, little is known about the translocation of nanoplastics through the trophic web. Plus, due to their surface reactivity, nanoplastics could sorb and thus increase metals bioavailability to aquatic filter-feeding organisms (e.g., bivalves). In this study, we investigated the dietary exposure route on the oyster Crassostrea virginica through microalgae themselves exposed to three nanoplastic dispersions (PSL, PSC and NPG) at reportedly environmental concentrations combined or not with arsenic. Interactive effects of nanoplastics on arsenic bioaccumulation were studied, along with the expression of key genes in gills and visceral mass. The investigated gene functions were endocytosis (cltc), oxidative stress (gapdh, sod3, cat), mitochondrial metabolism (12S), cell cycle regulation (gadd45, p53), apoptosis (bax, bcl-2), detoxification (cyp1a, mdr, mt), and energy storage (vit). Results showcased that nanoplastic treatments combined with arsenic triggered synergetic effects on gene expressions. Relative mRNA level of 12S significantly increased at 10 and 100 μg L^-1 for NPG combined with arsenic and for PSC combined with arsenic. Relative mRNA level of bax increased for PSL combined with arsenic and for PSC combined with arsenic at 10 and 100 μg L^-1 respectively. We also observed that relative arsenic bioaccumulation was significantly higher in Crassostrea virginica gills compared to Isognomon alatus'. These results are the first comparative molecular effects of nanoplastics alone and combined with arsenic investigated in farmed C. virginica oysters. Together with I. alatus results we thus shed light on species different sensitivity.

Seasonal Variation and Ecological Risk Assessment of Heavy Metal in an Estuarine Mangrove Wetland

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.

Genome-Wide Identification, Structure Characterization, Expression Pattern Profiling, and Substrate Specificity of the Metal Tolerance Protein Family in Canavalia rosea (Sw.) DC

Plant metal tolerance proteins (MTPs) play key roles in heavy metal absorption and homeostasis in plants. By using genome-wide and phylogenetic approaches, the origin and diversification of MTPs from Canavalia rosea (Sw.) DC. was explored. Canavalia rosea (bay bean) is an extremophile halophyte with strong adaptability to seawater and drought and thereby shows specific metal tolerance with a potential phytoremediation ability. However, MTP genes in leguminous plants remain poorly understood. In our study, a total of 12 MTP genes were identified in C. rosea. Multiple sequence alignments showed that all CrMTP proteins possessed the conserved transmembrane domains (TM1 to TM6) and could be classified into three subfamilies: Zn-CDFs (five members), Fe/Zn-CDFs (five members), and Mn-CDFs (two members). Promoter cis-acting element analyses revealed that a distinct number and composition of heavy metal regulated elements and other stress-responsive elements existed in different promoter regions of CrMTPs. Analysis of transcriptome data revealed organ-specific expression of CrMTP genes and the involvement of this family in heavy metal stress responses and adaptation of C. rosea to extreme coral reef environments. Furthermore, the metal-specific activity of several functionally unknown CrMTPs was investigated in yeast. These results will contribute to uncovering the potential functions and molecular mechanisms of heavy metal absorption, translocation, and accumulation in C. rosea plants.

Biological responses of mangrove oysters (Crassostrea rhizophorae) and mercury contamination in an urban tropical estuary

This study aimed to assess the biological responses of oysters from an urban estuary in Northeast Brazil, through the evaluation of biochemical and physiological biomarkers, and integrate these responses with the investigation of mercury seasonal contamination. Oysters and sediment were collected from three sites in the estuary of the Ceará River during dry and rainy seasons. Biomarkers (AchE, CaE, GST, CAT, and Condition Index) were analyzed in different tissues. Hg bioaccumulation was higher in animals sampled in the rainy season, with increases varying from 5% to 136%, compared to the dry season. The changes in biomarkers highlight already elevated stresses for the organisms at the inner portion of the estuary, near the confluence with the Maranguapinho River, mainly during the rainy season, corroborating other studies that showed ecotoxicological effects with water and sediment samples. Finally, no correlation between Hg in sediment/oyster and biomarker results was observed.

Molecular Impacts of Dietary Exposure to Nanoplastics Combined or Not with Arsenic in the Caribbean Mangrove Oysters (Isognomon alatus)

Nanoplastics (NPs) are anthropogenic contaminants that raise concern, as they cross biological barriers. Metals' adsorption on NPs' surface also carries ecotoxicological risks to aquatic organisms. This study focuses on the impacts of three distinct NPs on the Caribbean oyster Isognomon alatus through dietary exposure. As such, marine microalgae Tisochrysis lutea were exposed to environmentally weathered mixed NPs from Guadeloupe (NPG), crushed pristine polystyrene nanoparticles (PSC), and carboxylated polystyrene nanoparticles of latex (PSL). Oysters were fed with NP-T. lutea at 10 and 100 µg L-1, concentrations considered environmentally relevant, combined or not with 1 mg L-1 pentoxide arsenic (As) in water. We investigated key gene expression in I. alatus' gills and visceral mass. NP treatments revealed significant induction of cat and sod1 in gills and gapdh and sod1 in visceral mass. As treatment significantly induced sod1 expression in gills, but once combined with any of the NPs at both concentrations, basal mRNA levels were observed. Similarly, PSL treatment at 100 µg L-1 that significantly induced cat expression in gills or sod1 in visceral mass showed repressed mRNA levels when combined with As (reduction of 2222% and 34%, respectively, compared to the control). This study suggested a protective effect of the interaction between NPs and As, possibly by decreasing both contaminants' surface reactivity.

Trophic transfer of emerging metallic contaminants in a neotropical mangrove ecosystem food web

Emerging metallic contaminants (EMCs) are of concern due their presence in aquatic ecosystems and the lack of environmental regulations in several countries. This study verifies the presence of EMCs in two neotropical mangrove estuarine ecosystems (Espírito Santo Brazil) by evaluating abiotic and biotic matrices across six trophic levels (plankton, oyster, shrimp, mangrove trees, crabs and fish) and hence interrogates the trophic transfer of these elements and their possible input sources. Using the oyster Crassostrea rhizophorae as a biomonitor, ten EMCs (Bi, Ce, La, Nb, Sn, Ta, Ti, W, Y and Zr) were determined. Bi input was from iron export and pelletizing industries; Ce, La and Y inputs were mainly associated with solid waste from steel production, while Zr, Nb and Ti were related to atmospheric particulate matter emissions. EMCs were detected at various trophic levels, showing biomagnification for most of them in the Santa Cruz estuary but biodilution in Vitória Bay. These contrasting results between the estuaries could be attributed to different pollution degrees, needing further research to be fully understood. This is the first report demonstrating EMCs trophic pathways in situ, constituting an essential baseline for future research and safety regulations involving EMCs in the environment.

Trace metals in estuarine organisms from a port region in southern Brazil: consumption risk to the local population

Metal contamination is a threat to estuarine environments. They can accumulate in the food chain and cause toxic effects on aquatic organisms and human health. This study evaluated the concentrations of metals (Cd, Cr, Cu, Fe, Ni, Pb, and Zn) in aquatic organisms of Antonina Bay (southern Brazil) to analyze whether the metal concentrations were in accordance with Brazilian food legislation and estimate the human risk of local seafood consumption. All analyzed organisms (Centropomus parallelus, Mugil curema, Genidens genidens, Crassostrea brasiliana, and Xiphopenaeus kroyeri) showed traces of metals in their tissues with different metal concentrations among species. Metal concentrations were generally higher in oyster C. brasiliana, and biomagnification was not observed. Cr and Zn concentrations were above the limits established by legislation for all species in at least one sample. The concentrations of the other metals were within permitted levels. However, concentrations of Cd, Cr, Fe, and Zn posed a human consumption risk. In general, the C. brasiliana oyster presented the highest risk for human consumption, probably due to its filtering habit. Thus, the results indicated that metal concentrations in the tissues of the Antonina Bay seafood can pose a risk to human health, and this chronic exposure to metals also can cause toxic effects on local aquatic biota.

Microplastics pollution in mangrove ecosystems: A critical review of current knowledge and future directions

Over the last decade, microplastics (MPs, plastic particles <5 mm) as emerging contaminants have received a great deal of international attention, not only because of their continuous accumulation in both marine and terrestrial environment, but also due to their serious threats posed to the environment. Voluminous studies regarding sources, distribution characterization, and fate of MPs in the different environmental compartments (e.g., marine, freshwater, wastewater, and soil) have been reported since 2004, whereas MPs pollution in unique marine ecosystems (e.g., coastal mangrove habitat) receives little scientific attention. Mangrove ecosystem, an important buffer between the land and the sea, has been identified as a potential sink of MPs caused by both marine and land-based activities. Moreover, the source and distribution characteristics of MPs in this ecosystem are significantly different from other coastal habitats, mainly owing to its unique features of high productivity and biomass. With the impetus to provide a more integrated view of MPs pollution in mangrove habitats, a literature review was conducted based on the existing studies related to this topic. This is the first review to present the current state of MPs pollution in mangrove ecosystems, specially including (i) the possible sources of MPs in mangrove areas and their pathways entering into this habitat; (ii) MPs pollution in the different mangrove compartments (including surface seawater, sediments, and biotas); and (iii) factors influencing MPs distribution in mangrove areas. Toward that end, the research gaps are proposed to guide for future research priorities.

Past and present anthropic environmental stress reflect high susceptibility of natural freshwater ecosystems in Romania

The concentrations of twelve heavy metals and trace elements (Cr, Mn, Co, Ni, Cu, As, Cd, Pb, Hg, Zn, Fe, and Al) in bed sediment and river freshwater that received sewage discharge, industrial wastewater inputs and mining residue were discussed. Spatial distribution, intra-annual trends and diffuse flux in 2019 in the middle and lower reaches of Olt River Basin (ORB) were investigated using inductively coupled mass spectrometry (ICP-MS) and atomic absorption spectroscopy (AAS). We applied correlation and principal component analysis (PCA) to quantify metal distribution relationship within environmental factors (pH, air temperature) and organic matter existing in the ORB. Moreover, the ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁷Pb isotope ratios analysis was employed to conclude the possible origin of the contamination. PCA analysis categorized metal presence in the four-component model, which explains 91% (May), 92% (July) and 93% (September) of the variance and indicates the potential origins of pollutants. The HCA and correlation analysis emphasized the relationship between trace elements, heavy metals in water and sediments and physicochemical characteristics of water. It was observed a high discrepancy in metal distribution between riverbed sediments and water body. In September, correlation indices highlighted sparse positive relationship with trace elements in water and mainly negative correlation values with trace elements from sediments. The origin of pollutants in sediments and water appear to be both natural and human-related activities. In all seasons increased the total exchangeable concentration of Ni, Cu and Zn in the sediments downstream sewage treatment plants and upstream of dams. The consideration of environmental factors and physicochemical characteristics of water is required to develop strategies for pollution management, assessment and mitigation in the actual condition of climate change. This study evaluated the heavy metals pollution in the Olt River Basin over three periods in 2019 under human-induced changes.