Tropical Coral Island Research Articles

Chromolaena odorata affects soil nitrogen transformations and competition in tropical coral islands by altering soil ammonia oxidizing microbes

Invasive plants can change the community structure of soil ammonia-oxidizing microbes, affect the process of soil nitrogen (N) transformation, and gain a competitive advantage. However, the current researches on competition mechanism of Chromolaena odorata have not involved soil nitrogen transformation. In this study, we compared the microbially mediated soil transformations of invasive C. odorata and natives (Pisonia grandis and Scaevola taccada) of tropical coral islands. We assessed how differences in plant biomass and tissue N contents, soil nutrients, N transformation rates, microbial biomass and activity, and diversity and abundance of ammonia oxidizing microbes associated with these species impact their competitiveness. The results showed that C. odorata outcompeted both native species by allocating more proportionally biomass to aboveground parts in response to interspecific competition (12.92 % and 22.72 % more than P. grandis and S. taccada, respectively). Additionally, when C. odorata was planted with native plants, the available N and net mineralization rates in C. odorata rhizosphere soil were higher than in native plants rhizosphere soils. Higher abundance of ammonia-oxidizing bacteria in C. odorata rhizosphere soil confirmed this, being positively correlated with soil N mineralization rates and available N. Our findings help to understand the soil N acquisition and competition strategies of C. odorata, and contribute to improving evaluations and predictions of invasive plant dynamics and their ecological effects in tropical coral islands.

Plant invasion alters soil phosphorus cycling on tropical coral islands: Insights from Wollastonia biflora and Chromolaena odorata invasions

Plants may modify soil biogeochemical properties that facilitate their invasion of new sites which may be mediated by soil microorganisms. We compared microbially-mediated soil phosphorus (P) cycling among sites with different degrees of invasion by Wollastonia biflora and Chromolaena odorata on two coral islands. Wollastonia biflora and C. odorata outcompeted all native species and invested more P in leaves (45–136% more than the native species). We found a decrease in the proportion of HCl–P and organic P, and a corresponding increase in the proportion of labile inorganic P (Pi) of the soil total P concentrations following plant invasion. Leaf P concentrations in invasive species were positively correlated with the proportion of labile Pi, suggesting that HCl–P and organic P were the major P sources for invasive plants, and that plant invasion increased soil P-cycling rates as a result of root carboxylate release (evidenced by greater leaf manganese concentrations) and enhanced soil alkaline phosphatase activity. The latter was confirmed by an increased soil phoD gene abundance after plant invasion. Soil P availability was elevated after plant invasion which was associated with increases in the release of root carboxylates and microbial activities that are involved in mobilization of HCl–P and mineralization of organic P, respectively, especially in the wet season. This, in turn, strongly increased leaf P concentrations of invasive species to sustain rapid plant growth, and presumably depriving competitors of P. These novel insights into soil P cycling through the enrichment of a specific soil microbial community and root carboxylate release in invaded ecosystems enhance our understanding of plant invasion mechanisms on coral islands.

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

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

Linking Transpiration to Reef Nitrogen Supply on a Tropical Coral Island

AbstractCoral reef islands are biodiversity hotspots with high conservation value, but we have a poor understanding of how island vegetation, through transpiration, influences groundwater nutrient supply to adjacent reef systems. Here we combine stable isotope tracing, geophysical surveys, and satellite analysis to unravel the links between transpiration and the discharge of groundwater nitrate to the waters surrounding a coral reef island (Lady Elliot Island in the Great Barrier Reef). Over a 2‐year study period (2020–2021) there was a net loss of freshwater from the island, that is, evapotranspiration (ET) exceeded rainfall, largely caused by high rates of transpiration (73% of ET). Transpiration was higher in forested areas, and groundwater salinity appeared to be higher there. By tracing the nitrogen stable isotope signature of groundwater nitrate into reef organism tissue we were able to map groundwater nitrogen discharge spatially and temporally. Groundwater nitrogen discharge was focussed on one side of the island and did not vary seasonally as expected, despite reduced rainfall and seabird guano inputs over the austral winter/spring. Based on our results, we propose that transpiration by island vegetation slows groundwater flow and concentrates nitrate in the groundwater being released to the surrounding reef system. High concentrations of nitrate in groundwater (up to 27 mmol L−) were observed on Lady Elliot Island and these seem to have increased since 2014, but further work is required to understand if this is a normal scenario for tropical coral islands or the result of the islands' revegetation program.

Giant African snail invasion homogenizes seasonal soil biodiversity in tropical coral islands

Giant African snail invasion homogenizes seasonal soil biodiversity in tropical coral islands

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

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

Using water‐landing, fixed‐wing UAVs and computer vision to assess seabird nutrient subsidy effects on sharks and rays

AbstractBird colonies on islands sustain elevated productivity and biomass on adjacent reefs, through nutrient subsidies. However, the implications of this localized enhancement on higher and often more mobile trophic levels (such as sharks and rays) are unclear, as spatial trends in mobile fauna are often poorly captured by traditional underwater visual surveys. Here, we explore whether the presence of seabird colonies is associated with enhanced abundances of sharks and rays on adjacent coral reefs. We used a novel long‐range water‐landing fixed‐wing unoccupied aerial vehicle (UAV) to survey the distribution and density of sharks, rays and any additional megafauna, on and around tropical coral islands (n = 14) in the Chagos Archipelago Marine Protected Area. We developed a computer‐vision algorithm to distinguish greenery (trees and shrubs), sand and sea glitter from visible ocean to yield accurate marine megafauna density estimation. We detected elevated seabird densities over rat‐free islands, with the commonest species, sooty tern, reaching densities of 932 ± 199 per km−2 while none were observed over former coconut plantation islands. Elasmobranch density around rat‐free islands with seabird colonies was 6.7 times higher than around islands without seabird colonies (1.3 ± 0.63 vs. 0.2 ± SE 0.1 per km2). Our results are evidence that shark and ray distribution is sensitive to natural and localized nutrient subsidies. Correcting for non‐sampled regions of images increased estimated elasmobranch density by 14%, and our openly accessible computer vision algorithm makes this correction easy to implement to generate shark and ray and other wildlife densities from any aerial imagery. The water‐landing fixed‐wing long‐range UAV technology used in this study may provide cost effective monitoring opportunities in remote ocean locations.

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

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

Genome Assembly of Cordia subcordata, a Coastal Protection Species in Tropical Coral Islands.

Cordia subcordata trees or shrubs, belonging to the Boraginaceae family, have strong resistance and have adapted to their habitat on a tropical coral island in China, but the lack of genome information regarding its genetic background is unclear. In this study, the genome was assembled using both short/long whole genome sequencing reads and Hi-C reads. The assembled genome was 475.3 Mb, with 468.7 Mb (99.22%) of the sequences assembled into 16 chromosomes. Repeat sequences accounted for 54.41% of the assembled genome. A total of 26,615 genes were predicted, and 25,730 genes were functionally annotated using different annotation databases. Based on its genome and the other 17 species, phylogenetic analysis using 336 single-copy genes obtained from ortholog analysis showed that C. subcordata was a sister to Coffea eugenioides, and the divergence time was estimated to be 77 MYA between the two species. Gene family evolution analysis indicated that the significantly expanded gene families were functionally related to chemical defenses against diseases. These results can provide a reference to a deeper understanding of the genetic background of C. subcordata and can be helpful in exploring its adaptation mechanism on tropical coral islands in the future.

High phosphorus availability and low light intensity reduce the competitive ability of the invasive plant Chromolaena odorata in tropical coral islands

Chromolaena odorata is one of the most common invasive plants, as the phosphorus input from guano in the coral islands continuously decreasing, causing substantial harm to the native vegetation in recent years. In the current study, we investigated the effects of soil phosphorous content, light intensity and competition on several physiological traits (plant height, leaf area, maximum net photosynthetic rate, and relative growth rate) of C. odorata and the native species Pisonia grandis and Scaevola taccada based on a greenhouse experiment with two light intensities and three levels of soil available phosphorus (P) content. We also evaluated the effects of light intensity and soil phosphorus content (and their interaction) on the relative yield and aggressivity coefficient of the invasive species C. odorata. The results showed that low light intensity significantly inhibited the growth of the three species. However, the high P treatment significantly inhibited the growth of C. odorata and P. grandis and significantly increased the growth of S. taccada under full-light conditions. The effect of soil P content on the interspecific competition among C. odorata, P. grandis, and S. taccada was mediated by light intensity and species combination. The high P treatment significantly reduced the competitive advantage of C. odorata over P. grandis. The results demonstrate that shaded habitats with a high soil P content could restrict invasion by C. odorata. This suggests that the invasiveness of C. odorata in tropical coral islands can be reduced by protecting native vegetation and thus increasing shade and soil P content.

Spatial variation of soil seed banks along a gradient of anthropogenic disturbances in tropical forests on coral islands

Poor regeneration of natural vegetation is a major factor contributing to the degradation of tropical coral islands. Soil seed banks (SSB) are important for maintaining the resilience of plant communities. However, the community characteristics and spatial distribution of SSBs and the controlling factors along human disturbance on coral islands are unclear. To fill this gap, we measured the community structure and spatial distributions of forest SSBs on three coral islands in the South China Sea, with varying degrees of human disturbance. The results showed that strong human disturbance increased the diversity, richness, and density of SSBs, as well as increased the richness of invasive species. With increased human disturbance, the heterogeneity pattern of SSBs spatial distribution changed from difference between forest east and west to forest center and edge. The similarity between the SSBs and above-ground vegetation also increased, and the distribution of invasive species extended from the edge to the central area of the forests, demonstrating that human disturbance limited the outward dispersal of seeds of resident species but increased the inward dispersal of seeds of invasive species. Interaction between soil properties, plant characteristics, and human disturbance explained 23–45% of the spatial variation of forest SSBs on the coral islands. However, human disturbance reduced the correlations of plant communities and spatial distribution of SSBs with soil factors (i.e., available phosphorus and total nitrogen) and increased the correlations of the community characteristics of SSB with landscape heterogeneity index, road distance, and shrub and litter cover. Resident seed dispersal on tropical coral islands might be enhanced by reducing building height, constructing buildings in down-wind locations, and preserving corridors that support animal movement among forest fragments.

Functional Characterization of Heat Shock Factor (CrHsf) Families Provide Comprehensive Insight into the Adaptive Mechanisms of Canavalia rosea (Sw.) DC. to Tropical Coral Islands.

Heat shock transcription factors (Hsfs) are key regulators in plant heat stress response, and therefore, they play vital roles in signal transduction pathways in response to environmental stresses, as well as in plant growth and development. Canavalia rosea (Sw.) DC. is an extremophile halophyte with good adaptability to high temperature and salt-drought tolerance, and it can be used as a pioneer species for ecological reconstruction on tropical coral islands. To date, very little is known regarding the functions of Hsfs in the adaptation mechanisms of plant species with specialized habitats, especially in tropical leguminous halophytes. In this study, a genome-wide analysis was performed to identify all the Hsfs in C. rosea based on whole-genome sequencing information. The chromosomal location, protein domain or motif organization, and phylogenetic relationships of 28 CrHsfs were analyzed. Promoter analyses indicated that the expression levels of different CrHsfs were precisely regulated. The expression patterns also revealed clear transcriptional changes among different C. rosea tissues, indicating that the regulation of CrHsf expression varied among organs in a developmental or tissue-specific manner. Furthermore, the expression levels of most CrHsfs in response to environmental conditions or abiotic stresses also implied a possible positive regulatory role of this gene family under abiotic stresses, and suggested roles in adaptation to specialized habitats such as tropical coral islands. In addition, some CrHsfAs were cloned and their possible roles in abiotic stress tolerance were functionally characterized using a yeast expression system. The CrHsfAs significantly enhanced yeast survival under thermal and oxidative stress challenges. Our results contribute to a better understanding of the plant Hsf gene family and provide a basis for further study of CrHsf functions in environmental thermotolerance. Our results also provide valuable information on the evolutionary relationships among CrHsf genes and the functional characteristics of the gene family. These findings are beneficial for further research on the natural ecological adaptability of C. rosea to tropical environments.

Solar-driven atmospheric water harvesting with a super-hygroscopic composite modified activated carbon fiber for tropical island ecological farm

Atmospheric water harvesting is a promising strategy to address the water scarcity in islands. In this work, an activated carbon fiber (ACF) templated hybrid water adsorbent ACF-cobalt(II)-ethanolamine (ACF-Co-EA) was fabricated and used to build an ecological farm (Eco-farm) for potential application on tropical coral islands. ACF-Co-EA took the advantage of both the pore structure of ACF and the superabsorbent property of the Co-EA complex, thus, exhibiting superior water harvest capacity over ACF or Co-EA. The equilibrium water adsorption of ACF-Co-EA was 763 ​mg ​g−1 ​at 25 ​°C and 70% RH (typical environment of tropical coral islands). Under 1 ​kW ​m−2 simulated solar irradiation, the surface temperature of ACF-Co-EA increased rapidly to 50 ​°C within 12 ​min and stabilized at 54 ​°C in 30 ​min because of the superior light absorbance of ACF, which made more than 90% of captured water released. ACF-Co-EA-based Eco-farm could harvest 6.9 ​g ​g−1·day−1 of water to ensure plant growth in the tropical coral islands' environment without any additional energy or water supply. The study provided novel ideas to alleviate the problems of freshwater scarcity and food shortage in the tropical coral islands.

Changes in vegetation types affect soil microbial communities in tropical islands of southern China

Soil microbial communities are the key drivers of nutrient cycling in ecosystems. However, the functional response of soil microbial community composition to contrasting vegetation types in tropical coral islands is still unclear. Tropical coral islands provide a unique, extreme habitat characterized by higher soil pH and P, but lower N and soil water contents. To determine the responses of soil microbial communities to changes in vegetation types, soil microbial biomass and community composition were investigated by determination of phospholipid fatty acids (PLFAs) under three vegetation types (including tree, shrub, and herb-vine) on Dong Island and Yongxing Island of southern China. Redundancy analysis (RDA) has been used to determine the driving factors (soil properties) for shaping soil microbial community composition. The results showed that the total biomass of PLFAs, as well as the specific microbial taxa [such as bacteria, Gram-positive bacteria (G+), Gram-negative bacteria (G-), fungi, arbuscular mycorrhizal fungi (AMF), and actinomycetes] increased in the soils from herb-vine via shrub to tree. Furthermore, along the above vegetation types gradient, the ratios of Gram-positive to Gram-negative bacteria (G+:G-), total saturated to total monounsaturated fatty acids (sat:mono), and fungi to bacteria (F:B) ratio decreased, indicating a shift in soil microbial community towards lower stress and copiotrophic dominance. Our findings indicate that soil microbial groups have a sensitive response to shifting plant communities in tropical coral islands, and soil water content, the ratios of soil organic matter and N content to P content, and soil pH might be the critical drivers of microbial community composition and structure in the study region.

Comprehensive Analysis of the Hsp20 Gene Family in Canavalia rosea Indicates Its Roles in the Response to Multiple Abiotic Stresses and Adaptation to Tropical Coral Islands

Heat shock protein 20 (Hsp20) is a major family of heat shock proteins that mainly function as molecular chaperones and are markedly accumulated in cells when organisms are subjected to environmental stress, particularly heat. Canavalia rosea is an extremophile halophyte with good adaptability to environmental high temperature and is widely distributed in coastal areas or islands in tropical and subtropical regions. In this study, we identified a total of 41 CrHsp20 genes in the C. rosea genome. The gene structures, phylogenetic relationships, chromosome locations, and conserved motifs of each CrHsp20 or encoding protein were analyzed. The promoters of CrHsp20s contained a series of predicted cis-acting elements, which indicates that the expression of different CrHsp20 members is regulated precisely. The expression patterns of the CrHsp20 family were analyzed by RNA sequencing both at the tissue-specific level and under different abiotic stresses, and were further validated by quantitative reverse transcription PCR. The integrated expression profiles of the CrHsp20s indicated that most CrHsp20 genes were greatly upregulated (up to dozens to thousands of times) after 2 h of heat stress. However, some of the heat-upregulated CrHsp20 genes showed completely different expression patterns in response to salt, alkaline, or high osmotic stresses, which indicates their potential specific function in mediating the response of C. rosea to abiotic stresses. In addition, some of CrHsp20s were cloned and functionally characterized for their roles in abiotic stress tolerance in yeast. Taken together, these findings provide a foundation for functionally characterizing Hsp20s to unravel their possible roles in the adaptation of this species to tropical coral reefs. Our results also contribute to the understanding of the complexity of the response of CrHsp20 genes to other abiotic stresses and may help in future studies evaluating the functional characteristics of CrHsp20s for crop genetic improvement.

Characteristics habitat and molecular identity of Calophyllum inophyllum L. (Calophyllaceae) in Spratly Islands, Vietnam

Calophyllum inophyllum L. (Calophyllaceae) has excellent medicinal value and is grown in many coastal and island zones of Vietnam and other Asian countries. The species distribution has been threatened in recent decades due to the destruction of its native habitat and over-exploitation. This study aimed to determine habitat and conduct a molecular phylogenetic analysis of C. inophyllum in the Spratly Islands. Climate, soil physics and soil chemistry data were all analyzed in this study. Phylogenetic analysis using maximum likelihood (ML) and Bayesian inference (BI) revealed that all Vietnam samples are closely related to C. inophyllum (MLBS = 100%, BPP = 100%). Trees grow in climates with annual precipitation ranging from 750 to 5000 mm. These species differ from others in the mangrove forest in that they could still tolerate indefinite waterlogging and do not flourish beneath the jungle canopy. The vegetation in the region is identified as tropical rain forest and tropical seasonal forest on medium sandy, alkaline, low in humus, nitrogen, phosphorus and potassium. The findings demonstrated many ecological variables impacting the growth of C. inophyllum and confirmed the ITS-rDNA region as a molecular marker for species identification, thereby, providing a valuable scientific basis for breeding and conserving this species and other species of tropical coral islands worldwide.

Genome-Wide Identification, Primary Functional Characterization of the NHX Gene Family in Canavalia rosea, and Their Possible Roles for Adaptation to Tropical Coral Reefs.

Canavalia rosea, distributed in the coastal areas of tropical and subtropical regions, is an extremophile halophyte with good adaptability to high salinity/alkaline and drought tolerance. Plant sodium/hydrogen (Na+/H+) exchanger (NHX) genes encode membrane transporters involved in sodium ion (Na+), potassium ion (K+), and lithium ion (Li+) transport and pH homeostasis, thereby playing key roles in salinity tolerance. However, the NHX family has not been reported in this leguminous halophyte. In the present study, a genome-wide comprehensive analysis was conducted and finally eight CrNHXs were identified in C. rosea genome. Based on the bioinformatics analysis about the chromosomal location, protein domain, motif organization, and phylogenetic relationships of CrNHXs and their coding proteins, as well as the comparison with plant NHXs from other species, the CrNHXs were grouped into three major subfamilies (Vac-, Endo-, and PM-NHX). Promoter analyses of cis-regulatory elements indicated that the expression of different CrNHXs was affected by a series of stress challenges. Six CrNHXs showed high expression levels in five tested tissues of C. rosea in different levels, while CrNHX1 and CrNHX3 were expressed at extremely low levels, indicating that CrNHXs might be involved in regulating the development of C. rosea plant. The expression analysis based on RNA-seq showed that the transcripts of most CrNHXs were obviously decreased in mature leaves of C. rosea plant growing on tropical coral reefs, which suggested their involvement in this species’ adaptation to reefs and specialized islands habitats. Furthermore, in the single-factor stress treatments mimicking the extreme environments of tropical coral reefs, the RNA-seq data also implied CrNHXs holding possible gene-specific regulatory roles in the environmental adaptation. The qRT-PCR based expression profiling exhibited that CrNHXs responded to different stresses to varying degrees, which further confirmed the specificity of CrNHXs’ in responding to abiotic stresses. Moreover, the yeast functional complementation test proved that some CrNHXs could partially restore the salt tolerance of the salt-sensitive yeast mutant AXT3. This study provides comprehensive bio-information and primary functional identification of NHXs in C. rosea, which could help improve the salt/alkaline tolerance of genetically modified plants for further studies. This research also contributes to our understanding of the possible molecular mechanism whereby NHXs maintain the ion balance in the natural ecological adaptability of C. rosea to tropical coral islands and reefs.

Morphological, biochemical and physiological responses of a tropical coastal plant Guettarda speciosa to salt stress

Selection and utilization of plants with high salt tolerance can be efficient to rapidly construct or restore the vegetation of the tropical coral islands. Here, we investigated the response of Guettarda speciosa , a typical tropical coastal plant, to salt stress. Different morphological, physiological and biochemical changes in G. speciosa plants after being subjected to NaCl stress based on the salt content of the reference seawater were studied. The results showed that G. speciosa can be tolerant to 25 g L −1 NaCl which corresponds to 2.5% salt content of seawater. A series of adaptive mechanisms were triggered to cope with salt-stress, including the increase in the thickness of leaf epidermis and palisade tissue to limit water loss, changes in photosynthetic pigments, and antioxidant defense mechanisms. Osmoregulatory substances and antioxidant enzymes worked together to cope with membrane lipid peroxidation caused by the accumulation of reactive oxygen species under salt stress and maintain the stability of osmotic potentials. Our findings indicated that G. speciosa has a strong salt tolerance but probably could not tolerate prolonged seawater immersion, which limit its coastal distribution. Therefore, G. speciosa can be considered for vegetation construction and ecological restoration in tropical coral islands such as those in South China Sea.

Soil water dynamics and water balance on a tropical coral island

AbstractUnderstanding soil water dynamics and the water balance of tropical coral islands is important for the utilization and management of their limited freshwater resources, which is only from rainfall. However, there is a significant knowledge gap in the influence of soil water on the water cycle of coral islands. Soil water dynamics and the water balance of Zhaoshu Island, Xisha Archipelago were thus investigated using soil moisture measurements and the Hydrus‐1D model from October 2018 to September 2019. Over the study period, vegetation transpiration, soil evaporation, groundwater recharge and storage in the vadose zone were approximately 196, 330, 365 and 20 mm, occupying 22%, 36%, 40% and 2% of annual rainfall total (911 mm), respectively. For the wet season (from May to October) these values became 75, 202, 455 and 40 mm, occupying 10%, 26% and 59% and 5% of the seasonal rainfall total (772 mm), respectively. During the dry season (from November to April), a dry soil layer between 40 and 120 cm depth of the soil profile was identified that prevented water exchange between the upper soil layers and the groundwater resulting in the development of deep roots so that vegetation could extract groundwater to supplement their water requirements. Vegetation not only consumes all dry season rainfall (140 mm) but extracts water deeply from groundwater (90 mm) as well as from the vadose layer (20 mm). As such, the vegetation appears to be groundwater‐dependent ecosystems. The research results aid us to better understand the process of water dynamics on coral islands and to protect coral island ecosystems.

Which traits are necessary to quickly select suitable plant species for ecological restoration?

Abstract Traditionally, restoration ecologists and land managers have used the trial‐and‐error method to select candidate restoration species. This method, however, is time consuming (usually more than 3 years) and has a relatively low success rate. Recently, Wang et al. (2020) developed a trait‐based species selection framework which can quickly (within 1 month) and successfully select many appropriate species for ecological restoration. They used 28 traits that are associated with tolerance to harsh environmental conditions to select candidate restoration species for a tropical coral island in Hainan Province, China. However, it is likely that some of the 28 traits used in this study may not be very important for species selection, providing the potential use of fewer overall traits. This is important since in many situations land managers will have limited data and resources on species traits. In this study, we used Wang et al. (2020)’s trait data to test which traits are necessary to achieve a similar success rate when screening species for restoration applications. We performed principal component analysis (PCA) to compute each trait's relative contribution. Then, we used the backward stepwise approach where the trait that had the least contribution among all remaining traits was removed one at a time, and the screening model was then run again using the smaller set of traits. Species which are proven very appropriate for ecological restoration in Wang et al. (2020) were the standard to quantify how many and which traits should be used to acquire similar screening results. We also classified all 28 traits into four types of functional traits to test if a small set of traits can mimic Wang et al. (2020)’s selection results. Our results indicate that it is hard to simultaneously reduce trait numbers and maintain the right screening results; especially for tree species. Likewise, vine species and herbaceous species still required most of the original traits used by Wang et al. (2020). Our results also indicate that multiple trait types are required, rather than one single group of functional traits. Our results reject the possibility of using fewer and more targeted traits for species screening. Although investigations in other ecosystems are needed to test the generality of the conclusion, our results suggest that multiple functional traits are still required to be measured to select appropriate plant species for the restoration of tropical coral islands.