Nha Trang Bay Research Articles

A long-term dataset of <i>in-situ</i> temperature data reveals wind-induced up- and downwelling in the coastal waters of Nha Trang, South Central Vietnam

In order to implement successful coastal management and protect corals, it is imperative to understand the Nha Trang Bay’s coastal processes and take adequate measures to protect corals and reef structure. This paper aimed to analyze whether sudden variations in physical parameters, such as temperature, could be potentially harmful to coastal coral reefs, in addition to anthropogenic factors such as pollution and intensive fishing. In this paper, the first long-term observation (2008–2019) of temperatures, not only from SST data, but also in situ in coral reefs (10 and 18 m depth) at Nha Trang Bay, South Central Vietnam, was investigated. The data showed that wind-induced upwelling during summer mainly govern the coastal region. In contrast, wind-induced downwelling was found during winter, visible in all three investigated water layers (SST, 10 and 18 m). In winter, the vertical mixing is strong and there is virtually no time-lag between the layers. In summer a scattering layer was formed, the phenomenon where a layer of water with different properties (such as temperature or salinity) is formed, blocking the sinking of water. In summer, correlations with air temperature were not significant, nor were correlations with night cooling, thus having implications for the distribution of nutrients and the health of the coral reefs. However, this was only the situation near the coast. Wavelet analysis shows that the short-term variability is significantly more substantial, caused by the shallow depth of the thermocline, which is much stronger affected by tidal and weather events than in winter. As a result of the combination of large yearly temperature variations (21oC to 31oC) plus increased sediment deposition in the rainy seasons, reefs close to the shore are generally not well-developed. This paper strongly advocates for science-based monitoring of coral reef conditions and underscores the need for law enforcement within the Marine Protected Area of Nha Trang Bay.

A new endemic insular species of the genus Colubroelaps (Squamata, Serpentes, Colubroidea) from Khanh Hoa Province, Vietnam

A new species of the poorly known genus Colubroelaps Orlov, Kharin, Ananjeva, Nguyen & Nguyen, 2009, is described based on a single female specimen collected from Hon Tre Island, Khanh Hoa Province, Vietnam. The new species, Colubroelaps adlerisp. nov., can be distinguished from its sister species Colubroelaps nguyenvansangi Orlov, Kharin, Ananjeva, Nguyen & Nguyen, 2009 by the following combination of morphological characters: small body size (TL 402 mm); tail relatively short (ratio TaL/TL 0.10); dorsal scales in 14–14–14 rows; supralabials six, third and fourth entering orbit; infralabials seven; loreal present; ventral scales 234; subcaudals 30, all divided; cloacal plate divided; dorsal coloration pale brown, with narrow and interrupted dark stripe along spine; body flanks dark-gray lacking bluish iridescence; ventrally uniform off-white; head black with rostral, nasals, prefrontals, preoculars, loreal, and the two anterior supralabials, as well as anterior parts of supraoculars and frontal dirty yellowish-brown with dark-brown spots. This discovery represents the second known species of the genus Colubroelaps and the first record of the genus in the coastal areas of southern-central Vietnam. The new species is likely micro-endemic to the small island of Hon Tre within Nha Trang Bay in Khanh Hoa Province, Vietnam, at an elevation of 30 m a.s.l., and was recorded only from the heavily disturbed secondary forest area of the island. The new species is under threat due to the intensifying development of tourist infrastructure on Hon Tre Island. We suggest that the new species be considered Endangered (EN) following the IUCN’s Red List categories.

Anticancer Activity In Vitro of Sulfated Polysaccharides from the Brown Alga Spatoglossum vietnamense.

Sulfated polysaccharides SpvF1, SpvF2, SpvF3, and SpvF4 from the brown alga S. vietnamense collected north of Hon Do (Nha Trang Bay, Vietnam) were isolated and studied. The structure of the obtained polysaccharide was studied using chemical methods and NMR spectroscopy. Fucoidans were low-sulfated (SpvF1, SpvF2) and medium-sulfated (SpvF3, SpvF4) heterogeneous polysaccharides. The molecular weight of the polysaccharides obtained was in the range of 16 to 44 kDa. All investigated fucoidans until 400 µg/mL were not cytotoxic for human colon carcinoma cells DLD-1, HCT-116, and HT-29 in vitro. Fucoidans SpvF1 and SpvF2 have inhibited the colony formation and growth of investigated cells from 20 to 30%. Fucoidans SpvF3 and SpvF4 have the strongest inhibitory effect for investigated cancer cells: from 40 to 50%.

Metazoan parasite fauna of grey mullet (Mugilidae) from the coastal waters of Vietnam

A total of 201 fishes of the family Mugilidae caught off central Vietnam (Nha Trang Bay) were examined for parasites. Eight species of Mugilidae were identified based on cytb and 16S rRNA sequences. Protozoa were not revealed. The metazoan parasites belong to six higher taxa. Myxozoa Grassé, 1970 is represented by six genera, three of which were first found in grey mullets off Vietnam. The specimens of Monogenea Van Beneden, 1858 belong to two genera: Ligophorus Euzet et Suriano, 1977 and Metamicrocotyla Yamaguti, 1953. Twelve species of the former were recorded for the first time both in the coastal waters of Vietnam and in the East Sea. Trematoda Rudolphi, 1808 is represented by the members of the four families, of which only three species, Haplosplanchnus pachysoma (Eysenhardt, 1892), Pseudohaploporus planilizum Atopkin, Besprozvannykh, Ha, Nguyen, Nguyen & Chalenko, 2019 and Spiritestis arabii Nagaty, 1948, were identified. The latter was first recorded in the East Sea. Acanthocephala Rudolphi, 1808 specimens belong to the species Neoechinorhynchus dimorphospinus Amin & Sey, 1996. Crustacea Brünnich, 1772 are represented by five families, of which only the copepod Lernanthropsis mugilii (Shishido, 1898) and the isopod Nerocila orbignyi (Guérin, 1832) were identified. Hirudinea Savigny, 1822 were not determined to genus and species level. A total of 25 species of parasites were found in the Mugilidae fishes examined in Nha Trang Bay. In general, 96 species of parasites are currently documented in these fishes in the East Sea, 55 of which occur off Vietnam. As a result of this study, carried out off central Vietnam, 16 parasite species were recorded in grey mullet for the first time in Vietnamese coastal waters and 15 for the first time in the East Sea. Thus, extending the geographical range of the studies may result in new records of parasites in these fishes.

Antioxidant and cytoprotective activities of marine fungi isolated from brown seaweeds in Nha Trang Bay, Khanh Hoa Province, Central Vietnam

This study aims to evaluate the antioxidant and cytoprotective activities of 32 fungal strains isolated from brown seaweeds collected from Nha Trang Bay, Khanh Hoa Province, Central Vietnam. These fungal extracts were screened for antioxidant activity by 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2’-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) free radical scavenging assay. In contrast, cytoprotective activity on rat cardiomyocytes H9c2 cell line was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT) assay. The results indicated the capacity for free DPPH and ABTS radical scavenging of fungal crude extracts with 31.25% (n = 10) and 81.25% (n = 26), respectively. Among the fungal strains with high antioxidant activity, the three fungal extracts 2104NT-1.3, 2104NT-3.3, and 2104NT-7.7 increased the viability of rotenone-exposed cardiomyocyte cells by 9.9%, 15.2%, and 13.6%, respectively. Three fungal strains with significant antioxidant and cytoprotective activities were identified as Penicillium chermesinum 2104NT-1.3, Aspergillus sp. 2104NT-3.3, and Penicillium sp. 2104NT-7.7 based on sequence analysis of internal transcribed spacer (ITS) region. This study provided the potential fungal strains isolated from Nha Trang Bay for further investigation of antioxidant and cytoprotective compounds.

Cryptic invasions of Pseudopolydora (Annelida: Spionidae), with description of a new species from Queensland, Australia.

During surveys worldwide, we collected adult and larval specimens of Pseudopolydora Czerniavsky, 1881 similar to P. achaeta Radashevsky & Hsieh, 2000 and P. rosebelae Radashevsky & Migotto, 2009 far from their type localities in Taiwan and Brazil, respectively. Analysis of sequence data of five gene fragments: mitochondrial COI and 16S rDNA, nuclear 18S rDNA and 28S rDNA, and Histone 3 (3114 bp in total) of individuals from Nha Trang Bay (Vietnam), São Paulo (Brazil), Florida (USA), and the Caribbean Sea (Martinique) confirmed their conspecificity with P. achaeta from Taiwan. Based on the morphology, we also report this species for the first time for the East Sea (South Korea) and provide new records for the north-western part of the Sea of Japan (Russia). The analysis also showed the conspecificity of worms from Vietnam with P. rosebelae from Brazil. Based on the morphology, we also report P. rosebelae for the first time for the Caribbean Sea (Belize), South China Sea (Thailand), and Hawaii. Developed pelagic larvae of P. achaeta and P. rosebelae from Nha Trang Bay are described and illustrated. A worm from the Great Barrier Reef (Queensland, Australia) earlier identified as P. cf. rosebelae, is referred to the new species Pseudopolydora nivea sp. nov. Following our earlier hypothesis about the origin of Pseudopolydora in the Indo-West Pacific Ocean, we explain the occurrence of these worms, including Pseudopolydora floridensis Delgado-Blas, 2008, on the Atlantic coasts of North and South America by unintentional transportation in ballast water, followed by successful invasions. We assume that the penetration of warm waters of the East China Sea through the Korea Strait and the warming of the waters of the Sea of Japan have led to the extension of the range of tropical-subtropical P. achaeta to the north-western part of the Sea of Japan.

Bioactivity responses to changes in mucus-associated bacterial composition between healthy and bleached Porites lobata corals

This study aims to investigate how bioactivities of the coral surface mucus layer (SML) respond to changes in mucus-associated bacterial communities between bleached and healthy Porites lobata corals in Nha Trang Bay, Vietnam. The findings suggested that significant shifts in the mucus-associated bacterial communities were related to changes in coral health states from bleached to healthy P. lobata colonies (p < 0.05), while bacterial compositions were not significantly different across seasons and locations (p > 0.05). Of which 8 genera, Shewanella, Fusibacter, Halodesulfovibrio, Marinifilum, Endozoicomonas, Litoribacillus, Algicola, and Vibrio were present only in the SML of bleached coral while absent in the SML of the healthy one. As compared with the bleached SML, the healthy SML demonstrated stronger antibacterial activity against a coral bleaching pathogen, V. coralliilyticus, higher antitumor activity against HCT116 cell accompanied with increased induction of cleaved PARP and accelerated cell nucleic apoptosis and cycle arrest at S and G2/M phases exhibiting several typical characteristics, cell shrinkage, lost cell contact, and apoptotic body formation. Moreover, putative compounds detected at 280 nm in the healthy SML were obviously higher than those in the bleached one, probably they could be bioactive molecules responsible for competitively exclusion of pathogens, Algicola and Vibrio, from the healthy SML.

Phylogenetic analysis of Ellipsomyxa species (Myxosporea) and description of Ellipsomyxa gordeyi n. sp. from the gall bladder of mullets (Mugiliformes: Mugilidae) in Nha Trang Bay of the East Sea, Vietnam

The coastal waters of Vietnam are home to a wide diversity of fishes, but the parasite diversity of these potential hosts is much less well characterized. To begin addressing this knowledge gap, we carried out surveys of myxozoan parasites in fishes collected from Nha Trang Bay in Vietnam's East Sea in 2018–2019. Mugilid fishes were collected in March–April 2018, January–February 2019, and November–December 2019, and examined for myxozoans. Myxospores consistent with those of the genus Ellipsomyxa were found in the gall bladder of four mullet species, and we thoroughly characterized those from Planiliza melinoptera. Myxospores were elliptoid and devoid of striation, with a distinct sinuous suture line. Polar capsules were pyriform and oriented toward the poles of the spore. Morphological features were compared to nominal species and this species from Vietnam was distinct. Phylogenetic analysis based on partial small subunit rDNA sequence revealed that broadly, Ellipsomyxa species split into three phylogenetic lineages, and although in some branches there are groupings by host family, habitat or locality, there are no clear phylogenetic patterns. The new species we encountered in P. melinoptera had a close sister relationship with Ellipsomyxa adlardi, with both species part of a larger subclade within the Ellipsomyxa lineage. Despite this phylogenetic similarity, these species were morphologically distinct, and partial large subunit DNA sequences were only 93% similar to each other. A combination of the morphological characteristics and molecular data suggest that this is an undescribed species and we propose the name Ellipsomyxa gordeyi n. sp.

Sea Surface Temperature Variability in NHA Trang Bay, Viet Nam: Patterns and Mechanisms

Sea Surface Temperature Variability in NHA Trang Bay, Viet Nam: Patterns and Mechanisms

Fauna of Anemonefishes (Pomacentridae, Amphiprioninae) and Their Host Sea Anemones (Cnidaria, Actiniaria) of the Hon Rom Coral Reefs (Southern Central Vietnam, Nha Trang Bay) and its Changes for the Period from 2003 to 2018

Fauna of Anemonefishes (Pomacentridae, Amphiprioninae) and Their Host Sea Anemones (Cnidaria, Actiniaria) of the Hon Rom Coral Reefs (Southern Central Vietnam, Nha Trang Bay) and its Changes for the Period from 2003 to 2018

Seasonal bleaching and partial mortality of Pocillopora verrucosa corals of the coast of central Vietnam

IntroductionCoral reefs are one of the most productive yet vulnerable ecosystems on Earth. An important step in understanding the functioning of coral communities is studying the environmental impact on the state of reefs. The present study aimed to assess the impacts of the water temperature, precipitation, wave action, and population density on the seasonal bleaching, partial and complete mortality of shallow-water branching corals of Pocillopora verrucosa species complex. Materials and methodsThe present study was conducted at the Dam Bay research station (Nha Trang Bay) from 26 April 2020 to 2 April 2021. The environmental data was collected from the Dam Bay weather station and using a temperature logger planted on the nursery, coral bleaching and mortality were assessed visually using photos taken underwater. ResultsThroughout the study, the percentage of bleached coral colonies varied from 1 to 41.5% with two peaks, in autumn (October) and spring (April). Rates of the development of partial mortality varied from 0 to 10%, with a pronounced peak in December. At the end of the experiment, the percentage of coral colonies suffering from partial mortality reached 47%. Only 4,5% of experimental colonies died to the end of the study. Analysis revealed that water temperature and partial mortality are the drivers of bleaching, while wave action, temperature and bleaching are the factors with a significant impact on partial mortality.DiscussionThus, both bleaching and patrial mortality are interlinked and the effect of partial mortality on bleaching is stronger than the opposite. Both phenomena have pronounced seasonal variation, but their maximum manifestation does not coincide in time allowing corals to avoid their synergetic effect. Moreover, it was found that the impact of individual stress on mortality prevailed over the impact of seasonal bleaching and partial mortality events. The main cause of coral death was the development of algal fouling at the sites of colony lesions, which gradually spread over the entire surface of the colony.

Assessing impacts of climate change-driven sea level rise on hydrodynamics and sediment dynamics along Ba Lang beaches in Nha Trang Bay, Khanh Hoa, Vietnam

The Ba Lang sand beaches, located north of the Nha Trang Bay in Central Vietnam, are famous tourist attractions. However, they are experiencing shoreline and coastal erosion retreat, which is attributed to natural causes (such as tropical depressions, storms, and monsoons) as well as human impacts (such as hydropower generation, sand dredging, and coastal works). According to the forecast of the Vietnam Ministry of Natural Resources and Environment, global climate change will cause the sea level to rise by 74 cm along the coast from the Dai Lanh Cape to the Ke Ga Cape (including the Ba Lang beaches) by the end of this century in the representative concentration pathway (RCP) 8.5 scenario. Sea level rise (SLR) due to global climate change is expected to aggravate the coastal erosion and shoreline retreat problems. In this study, coupled numerical models with the spectral wave module (MIKE 21 SW), hydrodynamic module (MIKE 21 HD), and sand transport module (MIKE 21 ST) in the MIKE 21 package were used to simulate waves, current fields, and sediment dynamics along the Ba Lang beaches considering the impact of SLR. These models were calibrated with the field data measured in December 2016. The results showed that SLR caused the wave height to increase and reduced the current speed and total sediment load in monsoon conditions. The increase in wave height was even intensified under the joint impact of SLR and extreme events.

The third, brackish water-dwelling, species of the genus Balssipotamon (Decapoda: Potamidae) from Tre Island of Nhatrang Bay, Vietnam

A third species of the genus Balssipotamon Ðặng et Hồ, 2008 (Brachyura: Decapoda: Potamidae), B. tre sp. nov., is described from Tre Island located in Nhatrang Bay, Vietnam. The new species clearly differs from the congeners in morphology, as well as ecologically, inhabiting a coastal brackish pond, cohabited by the typical brackish-water crabs Varuna litterata (Fabricius, 1798) (Varunidae) and Somanniathelphusa cf. sinensis (H. Milne-Edwards, 1853) (Gecarcinucidae).

Degradation of Coral Reefs under Complex Impact of Natural and Anthropogenic Factors with Nha Trang Bay (Vietnam) as an Example

Degradation of Coral Reefs under Complex Impact of Natural and Anthropogenic Factors with Nha Trang Bay (Vietnam) as an Example

Comparative analysis of eukaryotic microbial communities associated with Acropora formosa, sediment, and seawater in a coral reef ecosystem of Whale island, Nha Trang bay, Vietnam

In this study, eukaryotic microbial communities associated with coral Acropora formosa and its natural surroundings, sediment and seawater, in a coral reef ecosystem of Whale Island, Nha Trang Bay, Vietnam were investigated. First, genetic material was taken from Acropora formosa’s surface mucus layer (SML) as well as the sediment underneath and seawater above the colonies from four different sampling locations in a coral reef ecosystem. Subsequently, the data were sequenced using 18S rRNA gene amplicon sequencing method. Sequences (reads) were then analyzed in Rstudio version 4.2.0. Bioinfomatic tools such as DADA2 pipeline clustered the sequences into amplicon sequence variants (ASVs), to which the taxonomy was assigned using SILVA 132 database. The majority of the sequences was categorized at the kingdom and phylum levels, but fewer sequences were identified at genus and species level. The visualization of the results revealed changes in abundance and composition of the eukaryotic communities in all samples. The results demonstrated that phylum Dinoflagellata had the highest relative abundance in coral samples. Meanwhile, Ochrophyta was the most prevalent phylum in seawater samples. Notably, after filtering out the sequences with abundance less than 2%, only genus Symbiodinium appeared significantly in coral samples. The composition of samples from coral sampling sites was more consistent. The same was true for samples of seawater, whereas the composition of sediment samples varied more. Alpha and beta diversity indices confirmed that there were significant differences (p &lt; 0.05) in abundance and composition of eukaryotic communities among three different habitats. These findings come as the first effort to explore the diversity of eukaryotic communities in different habitats and could be valuable for further study in functional profiling or metabolic functions of microbial communities in the coral ecosystem.

Estimating the Chlorophyll-a in the Nha Trang Bay using Landsat-8 OLI data

The pigment chlorophyll-a (Chl-a) is used to evaluate aquatic ecological health. Using remote sensing techniques to estimate this pigment and spatially mapping its distribution becomes essential for measuring and assessing water quality in coastal areas. Nha Trang Bay is famous not only for its scenery but also for its biodiversity values, especially the existence of coral reefs. In this study, Landsat-8 OLI was taken on June 3, 2015, and field measurements of Chl-a at 13 survey sites from June 6-8, 2015, were used to build a local algorithm to monitor the spatio-temporal distribution of Chl-a content in Nha Trang Bay. The ACOLITE processor was employed for the atmospheric modification of Landsat-8 OLI images to obtain atmospherically corrected surface reflectance products. Four types of simple regression models, including linear, exponential, logarithmic, and power models, were used to describe the relationship between the in-situ measurement of Chl-a and remote sensing reflectance of Landsat-8 OLI data. The results of correlation analysis have shown a statistically significant relationship between field measurement of Chl-a concentration and the remote sensing reflectance ratio of Landsat-8 band 3 versus band 2 using an exponential model with a coefficient of determination of 0.88 (p < 0.001) and root mean square error (RMSE) of 0.40 mg/m3. This empirical relationship was applied to map the spatial distribution of Chl-a concentration from 27 cloudless Landsat-8 OLI images from 2013 to 2021. The spatio-temporal distribution of Chl-a indicated that the Chl-a concentration in the Bay has a low value (less than 1 mg/m3). However, this concentration becomes high in the coastal areas (greater than 1 mg/m3) and the Cai and Tac rivers (greater than 2 mg/m3). It is also noted that the content of Chl-a in the rainy season is fairly higher than in the dry season, by an average of 0.68 mg/m3 and 0.53 mg/m3, respectively. This research highlights that Landsat-8 OLI data can be an effective and valuable tool in monitoring Chl-a for coastal areas.

Structure and cytotoxic activity of a sulfated polysaccharide from green seaweed Caulerpa racemose collected on Nha Trang bay, Viet Nam

AbstractIn this work, a new sulfated polysaccharide was isolated from green seaweed Caulerpa racemosa harvested at Nha Trang sea. The structure of obtained polysaccharide was determined and its cytotoxic activity was evaluated. The results showed that structure of the polysaccharide has a main chain of (1→4)‐β‐D‐galactopyranose residues and branches are (1→3)‐α‐L‐glucopyranose‐2,4‐disulfate residues linked to the main chain at C‐6 position. The sulfated polysaccharide showed cytotoxic activity against three cancer cell lines Hela (cervical cancer) HT29 (human colorectal adenocarcinoma), and 8505c (human undifferentiated thyroid carcinomas) with IC50 values of 57.52±2.08, 51.16±1.33 and 45.68±1.78 μg/mL, respectively

THE POPULATION STRUCTURE OF THE SYMBIOTIC CRAB, &lt;i&gt;TRAPEZIA&lt;/i&gt; &lt;i&gt;SEPTATA&lt;/i&gt; (DECAPODA, TRAPEZIIDAE) AT THE EARLY STAGES OF CORAL COLONY DEVELOPMENT

Symbiotic invertebrates play crucial roles in coral reef ecosystems due to their extremely high abundance and biomass, and by maintaining coral fitness and protecting them from predation. Among these invertebrates, symbiotic crabs of the family Trapeziidae are especially important for branching corals of the family Pocilloporidae. In this research, we studied the spatial arrangement and parameters of the populations of the symbiotic crab, Trapezia septata at the early stage of Pocillopora verrucosa colony formation. The study was conducted based on symbionts sampled from small coral colonies exposed for three months at a coral nursery in the Nha Trang Bay, Vietnam. As much as 59.5% of the study colonies are shown to be inhabited by the T. septata crab. The percentage of inhabited colonies grows proportionately to their volume. The male to female ratio is 1.4, being skewed to males. The number of males and females is increased with the volume of inhabited colonies, as is the average size of individuals. Individual colonies are commonly inhabited by a single crab or heterosexual pairs. Sexual dimorphism was found, females being larger than males. A positive relationship between the sizes of males and females in pairs was revealed, as in 73% pairs females were larger than males. The size of the clutch correlated positively to the size of the female. We hypothesize that the predominance of males in the population, as well as the individuals located among the hosts one by one, is the result of intersexual competition

Density and Bleaching of Corals and Their Relationship to the Coral Symbiotic Community

Corals provide an important habitat for diverse fauna. The habitat is especially rich in branching species. Each colony harbors several species and dozens of individuals, forming an integrated symbiotic community. Factors affecting the diversity and abundance of coral symbionts are poorly understood. The present study tested experimentally the relationship between the population density of coral Pocillopora verrucosa and the diversity and abundance of symbionts in planted coral fragments exposed for three months. Colony size and bleaching were also considered as two additional factors. We demonstrated that, even at the early stage of colony formation, the diversity of obligate symbionts on planted corals in Nha Trang Bay was higher than the diversity of any other studied local faunas of the world’s ocean. Species richness and the abundance of symbionts were positively related to colony size and density, while no impact of bleaching was found. Species richness and the abundance of obligate symbionts increased with host population density, while in facultative symbionts, these indices decreased. We hypothesized that the negative effect of host population density on species richness and the abundance of facultative symbionts is caused by the competitive pressure of obligates, which increases with their abundance. The reasons for the different responses of obligate and facultative symbionts to host density were considered.

Diversity and composition of bacterial communities associated with healthy and bleached coral Fungia sp. in Nha Trang bay, Vietnam

Coral bleaching is probably caused by the loss of endosymbiotic algae from the host tissue or disturbance of the microbial community composition of corals. In particular, bacteria inhabiting the surface mucus layer of corals are supposed to mediate coral health, but their role in coral bleaching has not been fully clarified. In the present study, we collected mucus samples from bleached and healthy Fungia sp. colonies in Nha Trang bay to investigate biodiversity and bacterial community composition using 16S rRNA gene amplicon next-generation sequencing. The results indicated rich biodiversity and significant changes in bacterial communities between bleached and healthy corals. Two phyla, Proteobacteria and Bacteroidetes, making up approximately 80% of the total bacterial abundance, were predominant in both bleached and healthy samples. Three phyla, Actinobacteria, Planctomycetes, and Cyanobacteria identified as minor taxa, were low in abundance in both samples. However, there were significant differences in bacterial communities at the genus level. Three bacterial genera, Erythobacteria, Synechcococcus CC9902, and Candidatus Actinomarina, involved in coral health protection, were mostly determined in the healthy coral samples. Whereas, five genera, Algicola, Fusibacter, Halodesulfovibrio, Marinifilum, and especially the genus Vibrio, were mainly detected in the bleached corals with a notable increase in relative abundance. Moreover, analysis of alpha and beta diversity (NMDS) also confirmed that there were significant changes in bacterial composition between the bleached and healthy corals (p-value &lt;0.05). These findings suggest that the disturbance of the bacterial community composition living on coral is one of the factors causing coral bleaching, beside environmental factors like pH, temperature, and dissolved oxygen.