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Background: Mangrove ecosystems are considered as hot spots of biogeochemical cycling, yet the diversity, function and coupling mechanism of microbially driven biogeochemical cycling along the sediment depth of mangrove wetlands remain elusive. Here we investigated the vertical profile of methane (CH4), nitrogen (N) and sulphur (S) cycling genes/pathways and their potential coupling mechanisms using metagenome sequencing approaches. Results: Our results showed that the metabolic pathways involved in CH4, N and S cycling were mainly shaped by pH and acid volatile sulphide (AVS) along a sediment depth, and AVS was a critical electron donor impacting mangrove sediment S oxidation and denitrification. Gene families involved in S oxidation and denitrification significantly (P < 0.05) decreased along the sediment depth and could be coupled by S-driven denitrifiers, such as Burkholderiaceae and Sulfurifustis in the surface sediment (0–15 cm). Interestingly, all S-driven denitrifier metagenome-assembled genomes (MAGs) appeared to be incomplete denitrifiers with nitrate/nitrite/nitric oxide reductases (Nar/Nir/Nor) but without nitrous oxide reductase (Nos), suggesting such sulphide-utilizing groups might be an important contributor to N2O production in the surface mangrove sediment. Gene families involved in methanogenesis and S reduction significantly (P < 0.05) increased along the sediment depth. Based on both network and MAG analyses, sulphate-reducing bacteria (SRB) might develop syntrophic relationships with anaerobic CH4 oxidizers (ANMEs) by direct electron transfer or zero-valent sulphur, which would pull forward the co-existence of methanogens and SRB in the middle and deep layer sediments. Conclusions: In addition to offering a perspective on the vertical distribution of microbially driven CH4, N and S cycling genes/pathways, this study emphasizes the important role of S-driven denitrifiers on N2O emissions and various possible coupling mechanisms of ANMEs and SRB along the mangrove sediment depth. The exploration of potential coupling mechanisms provides novel insights into future synthetic microbial community construction and analysis. This study also has important implications for predicting ecosystem functions within the context of environmental and global change. 8kPXxAybXxhPRn3YMgQLJx Video Abstract [ABSTRACT FROM AUTHOR]

Mechanical disturbance from waves and sediment dynamics is a key bottleneck to mangrove seedling establishment. Yet, how species vary in tolerance to sediment dynamics has not been quantified. We identified how tolerance to sediment dynamics differs for three mangrove propagule traits: propagule size, successional stage, and type of embryo development. We selected eight mangrove species growing in south China that vary from small seeds to large elongated propagules, pioneer to climax species, and non‐viviparous to viviparous. In a mesocosm set‐up, we applied bed level treatments to establishing seedlings: erosion, control, or accretion, by removing 2 cm, 0 cm, or adding 1 cm of sediment per week over 3 weeks. We measured seedling survival, shoot, and root lengths, and the critical erosion depth that leads toppling or dislodgement. We identified five relationships between seedling morphology and accretion and erosion thresholds: (1) tall (viviparous) propagules likely had highest accretion thresholds; (2) small pioneer propagules grew relatively fast to increase accretion thresholds; (3) there was a strong correlation between the erosion threshold and root length; and (4) climax species grew longest roots overall, (5) while pioneer species grew longer roots fast in response to sediment erosion. We identify distinct strategies for successful establishment in sediment dynamics that contribute to understanding mangrove zonation and underpin the importance of restoring diverse forests containing not just robust climax species, but also adaptable pioneers. Furthermore, this study reveals maximum shoot and root length as key determinants for seedling stability across species, providing a simple proxy for modeling establishment events. [ABSTRACT FROM AUTHOR]

Use of mangrove ecosystems for coastal flood protection requires reliable predictions of mangrove wave attenuation, especially if this capacity lessens due to storm‐induced forest damage. Quantifying and understanding the variation in drag forces and mechanical properties of mangrove vegetation can improve assessment of mangrove protective capacity. We studied five mangrove species common in the subtropical Pearl River Delta, south China. The studied species range from typically landward‐occurring to more seaward‐occurring pioneer species. We sampled across seven sites in the delta to study the impact of salinity on mechanical properties. We quantified strength and flexibility of branches (branch strength and flexibility related to branch diameter, modulus of rupture and modulus of elasticity), leaf strength (leaf attachment strength related to leaf size, and leaf mass per area) and drag properties (drag force related to surface area and drag coefficient). For all tested species, larger branch diameters resulted in higher mechanical strength. Larger leaf size resulted in larger peak pulling forces and larger branch surface area resulted in stronger drag forces. Notably, species that generally occur lower in the intertidal zone, where exposure to wind and waves is higher, had relatively stronger branches but more easily detachable leaves. This may be regarded as a damage‐avoiding strategy. Across the seven field sites, we found no clear effect of salinity on mangrove mechanical properties. This study provides a mechanistic insight in the storm damage process for individual mangrove trees and a solid base for modeling storm (surge) damage at the forest scale. [ABSTRACT FROM AUTHOR]

Background: The fast-growing introduced mangrove Sonneratia apetala is widely used for mangrove afforestation and reforestation in China. Some studies suggested that this exotic species outperforms native species in terms of carbon sequestration potential. This study tested the hypothesis that multi-species mangrove plantations might have higher carbon sequestration potential than S. apetala monocultures. Results: Our field measurements at Hanjiang River Estuary (Guangdong province, China) showed that the carbon stock (46.0 ± 3.0 Mg/ha) in S. apetala plantations where the native Kandelia obovata formed an understory shrub layer was slightly higher than that in S. apetala monocultures (36.6 ± 1.3 Mg/ha). Moreover, the carbon stock in monospecific K. obovata stands (106.6 ± 1.4 Mg/ha) was much larger than that of S. apetala monocultures. Conclusions: Our results show that K. obovata monocultures may have a higher carbon accumulation rate than S. apetala monocultures. Planting K. obovata seedlings in existing S. apetala plantations may enhance the carbon sink associated with these plantations. [ABSTRACT FROM AUTHOR]

The pulmonate gastropods, Phallomedusa solida (Martens, 1878) and Ophicardelus ornatus (Férussac, 1821), exhibit characteristic distributional patterns at the upper intertidal zones in estuarine mangrove and saltmarsh habitats on the eastern Australian coast. Past studies suggested inundation condition, soil salinity, and percent of vegetation cover were responsible for these patterns. In this study, the role of environmental parameters, food availability, physical stress, and predation pressure in determining the distributional patterns of these gastropods was evaluated along transects spanning saltmarsh, mangrove, and the ecotone habitats. For both species, the maximum population abundance occurred in the upper saltmarsh and the ecotone between mangrove and saltmarsh at 361.0 and 358.0 ind.m −2 , respectively, which was four times that of the lower saltmarsh. Mangroves were evaluated as the optimal habitat for the pulmonates in terms of the environmental parameters moisture content and food availability. However, due to its longer inundation duration within each tidal cycle, use of the mangrove habitat by the pulmonates was impeded because of difficulties in oxygen acquisition under submerged conditions. Laboratory experiments revealed the oxygen intake of the pulmonates dropped abruptly to 4.3–9.0% of aerial rates when submerged. This result indicated that mangroves were not the optimal habitat for the pulmonates. Furthermore, the visiting frequency of predators (yellowfin bream Acanthopagrus australis and toadfishes, Tetraodontidae) was 1.3 times higher in the mangrove compared to those in the ecotone and upper saltmarsh habitats. Underwater video recording also suggested high mortality of these gastropods at 31.7–88.9% in mangrove and 0.80–0.98 times higher than that in saltmarsh, resulting from the predators preying in the mangrove habitat during high tides. Despite the abiotic factors facilitating the distribution of the pulmonates in the mangrove, the higher predation risk restricted the occurrence of P. solida and O. ornatus in the mangrove areas. More than verifying that the distributional pattern of macrobenthos is a complex outcome from environmental factors and interaction with predators, our study also indicated that the influencing strength of the biotic and abiotic factors on the pulmonates distribution might be spatially changeable within a geographically small-scale continuum. [ABSTRACT FROM AUTHOR]

Global recognition that mangroves support coastal ecosystem services has increased; however, studies on trophic dynamics in mangrove ecosystems remain limited. We seasonally analysed the δ 13C and δ 15N of 34 consumers and 5 diets to elucidate the food web dynamics in the Pearl River Estuary (PRE). Fish had a large niche space during the monsoon summer, reflecting increased trophic roles. In contrast, the small niche space of benthos over seasons reflected consistent trophic positions. Consumers mainly utilized plant-derived organic matters in the dry season and particulate organic matters in the wet season. The present study with literature reviews revealed characteristics of the PRE food web with the depleted δ 13C and enriched δ 15N, indicating a high contribution of mangrove-derived organic carbon and sewage input, particularly in the wet season. Overall, this study confirmed the seasonal and spatial trophic dynamics in mangrove forests surrounding megacities for future sustainable mangrove ecosystem management. [Display omitted] • Characterized benthic-pelagic food web dynamics in the Pearl River Estuary (PRE). • Confirmed water discharge as a key factor influencing δ 13C and δ 15N of consumers. • Revealed primary diets (dry, mangrove and halophyte litters; wet, POM) of consumers. • Comparison with previous studies characterized mangrove and sewage input in PRE. [ABSTRACT FROM AUTHOR]

Contrary to the global trend, the area of mangrove in Guangdong Province, southern China, has been increasing over the last two decades. Currently, three exotic mangrove species have been introduced for large-scale afforestation since 1985. A reassessment of the overall status of the mangrove species, habitat change, population of introduced species, was conducted through a comprehensive literature review as well as field investigations covering 96 sites. The success of conservation efforts is also evaluated. Upstream and high intertidal habitats are more vulnerable than downstream and lower intertidal ones, with habitat alteration being the biggest threats. Five mangrove species have narrow distributional extents with small populations, which could incur regional extinction. With the introduced species having naturalized at 42 sites, their role in mangrove management needs to be reconsidered. These findings collectively suggest a need to manage latent species loss and habitat degradation beyond the apparent increase in mangrove area and protection. [ABSTRACT FROM AUTHOR]

In China, large-scale mangrove afforestation has been performed over the last two decades using introduced species. The large areas occupied by the introduced Sonneratia apetala have generated some concerns of biological invasion. The aim of this study was to investigate the feasibility of planting native mangrove species under the canopy of S . apetala plantations and the best species to acclimate under low light conditions in the understory. A 360-day experiment was conducted with four dominant mangrove species in southern China ( S . apetala , and three native species Kandelia obovata , Aegiceras corniculatum , and Bruguiera gymnorhiza ) in four habitats (i.e., bare mudflat, two-year plantation, six-year plantation, and eight-year plantation) with different light irradiance conditions. Results showed that in terms of propagule germination and seedling survival, the low light irradiance condition impeded the early growth of S. apetala , but had less impact on B. gymnorhiza . With decrease in light irradiance, increments in stem height, basal area, and leaf area, and relative growth rate decreased the most for S . apetala and the least for B . gymnorhiza among the four species. In terms of the physiological response parameters of net photosynthesis rate, stomatal conductance, and transpiration rate, B . gymnorhiza exhibited more adaptability than S . apetala under dense canopies. With respect to the response of early growth of seedlings over 12 months, B . gymnorhiza was the most adaptable species to the understory low light condition, while S . apetala was the most adversely affected species. The information from this study should be useful for the introduction of native species into plantations of introduced mangrove species to improve their conservation value. [ABSTRACT FROM AUTHOR]

Small-scale mangroves serve ecological functions similar to large-scale mangroves regarding biological conservation, environmental purification, and supporting biogeochemical processes. The rising aquaculture neighboring mangroves results in their serving as an important sink for massive nutrients and pollutants from aquaculture effluent. We assessed how long-term aquaculture effluent discharge influenced the soil properties of a mangrove-tidal flat continuum using field survey and geostatistics. Common soil physical-chemical properties presented significant spatial variability. Continued aquaculture effluent discharge caused a significant cumulation of soil total organic carbon (SOC) (64.13 g·kg−1), total nitrogen (TN) (2.44 g·kg−1) and total phosphorus (TP) (1.12 g·kg−1) in the mangrove soil, which were as 2–3 times as those on the mudflat. Most of the soil properties changed significantly with increasing distance from the effluent outlet along a tidal channel, and the maximum concentrations of SOC, TN and TP all occurred at 50 m away from the outlet. The results of principal component analysis indicated that aquaculture effluent significantly affected the spatial pattern of soil properties along the mangrove-tidal flat continuum. Continued aquaculture effluent input rendered extensive accumulation of SOC, TN and TP in the mangroves. The spatial heterogeneity of mangrove is the key driver to process the nutrient input spatially differently. • Impacts of continued aquaculture effluent discharge on mangrove soil were assessed. • Soil properties showed spatial variability along the mangrove-tidal flat continuum. • Aquaculture effluent influx caused significant cumulation of nutrients in mangrove. • Heterogeneity of mangrove soil drives different nutrient retention effects. [ABSTRACT FROM AUTHOR]

Early evidence has elucidated that the spread of antibiotic (ARGs) and metal resistance genes (MRGs) are mainly attributed to the selection pressure in human-influenced environments. However, whether and how biotic and abiotic factors mediate the distribution of ARGs and MRGs in mangrove sediments under natural sedimentation is largely unclear. Here, we profiled the abundance and diversity of ARGs and MRGs and their relationships with sedimental microbiomes in 0–100 cm mangrove sediments. Our results identified multidrug-resistance and multimetal-resistance as the most abundant ARG and MRG classes, and their abundances generally decreased with the sediment depth. Instead of abiotic factors such as nutrients and antibiotics, the bacterial diversity was significantly negatively correlated with the abundance and diversity of resistomes. Also, the majority of resistance classes (e.g., multidrug and arsenic) were carried by more diverse bacterial hosts in deep layers with low abundances of resistance genes. Together, our results indicated that bacterial diversity was the most important biotic factor driving the vertical profile of ARGs and MRGs in the mangrove sediment. Given that there is a foreseeable increasing human impact on natural environments, this study emphasizes the important role of biodiversity in driving the abundance and diversity of ARGs and MRGs. [Display omitted] • Multidrug and multimetal were the most abundant resistome classes in mangrove sediments. • Bacterial diversity was the most important driver of the vertically-stratified resistomes. • The negative relationship between bacterial diversity and resistomes was a conserved trait in different mangrove sediments. [ABSTRACT FROM AUTHOR]

Nitrogen (N) is the most critical nutrient that affects the establishment and stability of mangrove ecosystems. Despite the importance of biological nitrogen fixation (BNF) mediated by diazotrophs in mangrove ecosystems, current studies largely overlook the influence of arbuscular mycorrhizal fungi (AMF) on diazotrophs-driven BNF process in the N-limited ecosystems. Here, we conducted a comprehensive study on root-associated AMF and their interspecific interactions with diazotrophs in mangrove ecosystems using high throughput sequencing methods. Our results showed that the richness/diversity of diazotrophs had significant positive correlations with AMF community in mangrove rhizosphere. BNF rate in the rhizosphere of all mangrove species increased with the increasing of positive co-occurrence between AMF and diazotrophs, suggesting that the positive cooperation may promote N fixation efficiency in mangrove ecosystems. We also identified the potential keystone taxa (Zi ≥ 2.5 or Pi ≥ 0.62) and determined their crucial implications for the ecological processes of N fixation in mangrove ecosystems. The random forest analysis further indicated that the AMF keystone taxa were the most important predictors of BNF in mangrove rhizosphere. In addition, path analysis indicated that the α-diversity of AMF and diazotrophs communities, N-related enzymes and sediment nutrition components (e.g., TC, TN, Fe) were the main factors driving BNF process in mangrove ecosystems. This study provides novel insights into the interactions between AMF and diazotrophic communities during BNF process and expands our knowledge of AMF ecological functions of N cycle in mangrove ecosystems. [Display omitted] ● Mangrove species determined the structure of AMF community. ● AMF showed a strong positive co-occurrence network with the diazotrophs. ● The AMF keystone taxa was the most important predictor of BNF in the rhizosphere. ● N-related enzymes were the main factor driven the BNF process in mangrove ecosystem. [ABSTRACT FROM AUTHOR]

Microbially-driven sulfur cycling is a vital biogeochemical process in the sulfur-rich mangrove ecosystem. It is critical to evaluate the potential impact of sulfur transformation in mangrove ecosystems. To reveal the diversity, composition, and structure of sulfur-oxidizing bacteria (SOB) and sulfate-reducing bacteria (SRB) and underlying mechanisms, we analyzed the physicochemical properties and sediment microbial communities from an introduced mangrove species (Sonneratia apetala), a native mangrove species (Kandelia obovata) and the mudflat in Hanjiang River Estuary in Guangdong (23.27°N, 116.52°E), China. The results indicated that SOB was dominated by autotrophic Thiohalophilus and chemoautotrophy Chromatium in S. apetala and K. obovata , respectively, while Desulfatibacillum was the dominant genus of SRB in K. obovata sediments. Also, the redundancy analysis indicated that temperature, redox potential (ORP), and SO 4 2− were the significant factors influencing the sulfur cycling microbial communities with elemental sulfur (ES) as the key factor driver for SOB and total carbon (TC) for SRB in mangrove sediments. Additionally, the morphological transformation of ES, acid volatile sulfide (AVS) and SO 4 2− explained the variation of sulfur cycling microbial communities under sulfur-rich conditions, and we found mangrove species-specific dominant Thiohalobacter , Chromatium and Desulfatibacillum, which could well use ES and SO 4 2−, thus promoting the sulfur cycling in mangrove sediments. Meanwhile, the change of nutrient substances (TN, TC) explained why SOB were more susceptible to environmental changes than SRB. Sulfate reducing bacteria produces sulfide in anoxic sediments at depth that then migrate upward, toward fewer reducing conditions, where it's oxidized by sulfur oxidizing bacteria. This study indicates the high ability of SOB and SRB in ES, SO 4 2− , S 2 − and S2− generation and transformation in sulfur-rich mangrove ecosystems, and provides novel insights into sulfur cycling in other wetland ecosystems from a microbial perspective. • S. apetala and K. obovata sediments had higher TC, ES and AVS. • The elemental sulfur was the key factor driving SOB, total carbon was the driving force for SRB in mangrove sediments. • SOB was more susceptible to environmental changes than SRB. [ABSTRACT FROM AUTHOR]

[Display omitted] • SEM showed how biotic and abiotic factors impact SOC components by path analysis. • Vegetation biomass and soil salinity affect SOC by impacting fine root production. • Vegetation biomass and soil salinity explain 91.2% of production and decomposition. • The production of low-order fine roots explain 90.9% of SOC component ratio change. • Production and decomposition of fine roots change across the sea-land gradient. Mangrove fine roots are an important contributor to soil organic matter (SOC), yet how their production and decomposition affect SOC accumulation across the sea-land gradient is still elusive. This study aimed to investigate the influencing path of biotic and abiotic factors of fine root production and decomposition dynamics on SOC component ratios involving particulate organic carbon (POC)/SOC, dissolved organic carbon (DOC)/SOC, and microbial biomass carbon(MBC)/SOC. Based upon sequential root coring and the litter bag method, the production, and decomposition of mangrove fine roots were investigated. The production and turnover rate of mangrove fine roots increased from 90.9 g·m−2 to 510.9 g·m−2 and 0.44 yr−1 to 1.49 yr−1 across the sea-land gradient respectively. After one year of decomposition, 69.5–80.8% and 58.3–69.2% of low-order and high-order fine roots remained, respectively. Decomposition rates of both low-order and high-order fine roots were significantly slower in the landward zone than in the seaward zones. Redundancy analysis (RDA) illustrated that vegetation biomass and soil salinity were respectively the main biotic and abiotic factors affecting the production and decomposition of mangrove fine roots and they explained 91.2% of mangrove fine root production and decomposition. Stepwise regression analysis indicated that the production of fine roots was mainly influenced by mangrove vegetation biomass while their decomposition was negatively affected by soil salinity. POC and SOC increased significantly from 20.13 t·ha−1 to 58.71 t·ha−1 and 78.42 t·ha−1 to 139.01 t·ha−1 respectively while DOC/SOC and MBC/SOC reduced from 0.33 to 0.24 and from 0.55 to 0.44 across the sea-land gradient. RDA results indicated that low-order fine root production was the major control of the ratio of SOC components among the production and decomposition of low-order and high-order fine roots, explaining 90.9% of SOC component ratio changes. Based upon structural equation modeling, the path coefficient of biotic and abiotic factors POC/SOC was 0.914 (P < 0.001), indicating that mangrove fine root production affecting by vegetation biomass and soil salinity was the main drive of SOC accumulation. Nevertheless, mangrove fine root decomposition played a minor role in SOC component change. The information gained in this study will facilitate policy decisions concerning the restoration of mangrove forests in China. [ABSTRACT FROM AUTHOR]

In order to clarify the distribution, sources, and ecological risk of 16 polycyclic aromatic hydrocarbons (PAHs) in sediments of mangrove wetland located in Shantou, twenty eight surface sediments were collected. The concentration of PAHs ranged from 79.74 to 853.17 ng g−1 dry weight. The concentration of PAHs of sediment in the high tides was higher than that in the low tides and higher in mangrove-planting area than that in non-mangrove growth zone. The spatial distribution of PAHs in the sediment of mangrove wetland was mainly influenced by input sources and tidal currents. The result of the principal component analysis-multiple linear regression model indicated that the major source of PAHs in the studied area was coal burning. However, the adverse effect of PAHs was weak to biota. Undeniably, a lot more systematic explorations are demanded in future to investigate the PAHs contamination in the mangrove wetlands in small–medium cities with the development of coastal zone economy. • The occurrence of PAHs in mangrove sediments from Shantou was studied. • Spatial distribution of PAHs was influenced by input source and tidal current. • PCA-MLR indicated that coal combustion was the principal source of PAHs. • Ecological risk of PAHs was weak. [ABSTRACT FROM AUTHOR]