Nhu-Thuy Nguyen-Nhat’s research while affiliated with Vietnam Academy of Science and Technology and other places

The seagrass Halophila is one of the genera of Hydrocharitaceae that shows the highest number of species, with around 20 species. Among them, H. ovalis, H. major, H. minor, and H. nipponica are closely related species. It is the first time ITS2 secondary structures and their phylogenetic utility in this genus were reported worldwide. Phylogenetic analysis based on 205 bp of ITS2 showed four clades corresponding to above species. ITS2 secondary structures showed insight into Halophila ovalis from the East coast of Africa. Halophila ovalis from the East coast of Africa showed a distinct variant in Helix 1, 2, and 3 compared to the worldwide populations. Therefore, the ITS2 locus should be used as a DNA barcode for identifying Halophila species.

Halymenia malaysiana, one of the foliose species of Halymenia was first reported from Malaysia and showed some morphological features that distinguish it from sister species, such as Halymenia dilatata, Halymenia maculata, and Halymenia porphyraeformis. In this study, Halymenia samples were collected from nine locations along the coast of Viet Nam (8°–17°N) in the South China Sea and Gulf of Thailand. Morphological observations indicated that almost all samples were Halymenia malaysiana which was supported by two genetic markers, rbcL and COI-5P. However, the presence of Halymenia dilatata in Viet Nam is still unresolved. Based on rbcL, the common haplotype in Viet Nam was R1 as in Malaysia and three new haplotypes were added to H. malaysiana for Southeast Asia. Genetic differences were evident between the Sunda Shelf (Viet Nam and Malaysia) populations and those in Philippine waters. We suggest that a combination of morphology and molecular data would be useful to accurately assess the species diversity and phylogeography of Halymenia in Viet Nam and Southeast Asia region.

Climate change, habitat loss, nutrient pollution, and other anthropogenic impacts cause seagrass degradation globally. Like other seagrass species, Thalassia hemprichii is threatened by these factors. A better knowledge about genetic diversity within and among populations would support and guide prudent conservation strategies. In Viet Nam, T. hemprichii is found mainly in two habitats including hard substratum in the open sea and the soft, muddy sand substratum in lagoons. In this present study, 106 individuals of T. hemprichii collected from eight populations along the southern coast of Viet Nam were used to evaluate the genetic diversity and population structure via 10 loci of microsatellite markers. Based on the pairwise FST, relatively low genetic differentiation was detected among T. hemprichii populations. Statistically significant pairwise population genetic differentiation was found among almost all populations. Cluster, structure, and AMOVA analysis also showed that the eight populations were separated into two groups in agreement with the two different habitat types. Significant positive correlations exist between geographic and genetic distances. The genetic diversity exhibited three of the eight populations suggests that these three populations need protection priority.

Previous studies have documented 836 species of marine macroalgae from Viet Nam. In the present study, marine algal samples were collected along the coast of south central Viet Nam. Morphological observations revealed three new records for the South China Sea, including Zellera tawallina and Grateloupia huangiae (Rhodophyta), and Dictyota grossedentata (Phaeophyceae). The results of morphological observations were supported by phylogenetic analyses. Therefore, the seaweed flora of Viet Nam now comprises 838 marine macroalgal species including 418 Rhodophyta, 149 Phaeophyceae, 183 Chlorophyta, and 88 Cyanobacteria.

The genus Halophila shows the highest species diversity within the seagrass genera. Southeast Asian countries where several boundary lines exist were considered as the origin of seagrasses. We hypothesize that the boundary lines, such as Wallace’s and Lydekker’s Lines, may act as marine geographic barriers to the population structure of Halophila major. Seagrass samples were collected at three islands in Vietnamese waters and analyzed by the molecular maker ITS. These sequences were compared with published ITS sequences from seagrasses collected in the whole region of interest. In this study, we reveal the haplotype and nucleotide diversity, linking population genetics, phylogeography, phylogenetics and estimation of relative divergence times of H. major and other members of the Halophila genus. The morphological characters show variation. The results of the ITS marker analysis reveal smaller groups of H. major from Myanmar, Shoalwater Bay (Australia) and Okinawa (Japan) with high supporting values. The remaining groups including Sri Lanka, Viet Nam, the Philippines, Thailand, Malaysia, Indonesia, Two Peoples Bay (Australia) and Tokushima (Japan) showed low supporting values. The Wallacea region shows the highest haplotype and also nucleotide diversity. Non-significant differences were found among regions, but significant differences were presented among populations. The relative divergence times between some members of section Halophila were estimated 2.15–6.64 Mya.

Along with coral reefs and mangroves, seagrass meadows are being threatened globally Southeast Asia is considered within the area of seagrasses’ cradle of diversity. However, information on the current status of seagrass beds from Southern Viet Nam is limited due to lack of reliable data about seagrass species occurring in the Southeast Asian region. One factor is the difficulty of unambiguous species identification. For example, the leaf morphological characteristics of Halophila ovalis and closely related species are overlapping which leads to misidentifications. In this study, the latest satellite Landsat 8 OLI and SENTINEL-2B image analyses were applied to determine the distribution of seagrass beds in Southern Viet Nam. Detailed morphological and genetic marker analyses were used to determine and update the species composition. The present study together with literature reviews indicate that the total area of seagrass beds from Southern Viet Nam are 10,832.1 ha. 2,562 ha (or 19.1%) of seagrass coverage has been lost. The seagrass beds at Phu Quoc Island are the largest with 7,579 ha. The occurrence of Halophila major is updated for almost all off-shore islands and open-water areas.

In the present paper, Dictyota hauckiana is reported for the first time from Vietnam. It was found growing with other marine macroalgae nearshore, at 1-2 m depth in Ninh Hai waters, Ninh Thuan province. Morphological observations and phylogenetic analysis based on the concatenated rbcL and psbA are presented. There are now 13 species of Dictyota known from Vietnam-ese waters.