Symbiotic relationships between marine invertebrates and microalgae are well-known in cnidarians and mollusks, such as jellyfish, hydroids, and nudibranchs. To explore physiological compatibility between other mollusk species and microalgae, we injected the free-living (FL) strain of the dinoflagellate <i>Effrenium voratum</i> into the body of the cephalopod <i>Octopus minor</i>, nudibranchs <i>Chromodoris orientalis</i> and <i>Dendrodoris fumata</i>, sea snails <i>Nassarius</i> sp., <i>Tectus fenestratus</i>, and <i>Babylonia spirata</i>, and abalone <i>Haliotis discus</i> hannai and monitored the survival of these mollusks and transplanted microalga for 7 d. The transplanted <i>E. voratum</i> (FL) survived for 7 d inside the bodies of all mollusks except <i>H. discus</i> hannai. In additional experiments, the transplanted <i>E. voratum</i> (FL) survived for 25 and 17 d inside the bodies of <i>C. orientalis</i> and <i>O. minor</i>, respectively, until the mollusks died. Therefore, the results obtained in this study suggest that the nudibranch and cephalopod explored herein have the potential for physiological compatibility with microalgae. These findings represent an initial step toward evaluating symbiotic compatibility in novel host-symbiont systems.

Macroalgae host diverse epiphytic eukaryotes that contribute to coastal biodiversity and ecosystem function. Yet how fine-scale host traits shape these communities remains unclear. Using small organelle-enriched metagenomics (SoEM), we profiled epiphytes on eight intertidal macroalgal species from Korea and tested whether host cortical cell layer thickness predicts community structure. Epiphytic composition separated strongly by cortical thickness (thick > 8 cells vs. thin ≤ 5 cells), independent of host phylum. Thick-layered hosts predominantly harbored macrofauna (e.g., Malacostraca, Hexapoda), whereas thin-layered hosts were dominated by microalgae (diatoms). Phylogenetic diversity and the net relatedness index (NRI) indicated phylogenetic clustering in most thick-layered hosts (suggesting habitat filtering), while thin-layered hosts tended toward near-random assembly. Our findings identify cortical cell layer thickness as a key morphological trait structuring epiphytic eukaryotic communities. This trait-based perspective clarifies host–epiphyte interactions and motivates replicated, multi-scale studies integrating morphological and functional traits with genomic data for coastal biodiversity management.

Although Undaria pinnatifida sporophyll is rich in beneficial components such as polysaccharides, proteins, and phytochemicals, its inaccessible structure limits its usability. To overcome this limitation, we ultrasonically extracted <i>U. pinnatifida</i> sporophyll extract (UPE) under optimized conditions (20 kHz, 1,080 W, 30°C) for 8 h and used gradient ethanol precipitation to isolate the UPE fractions (UEF). The yield, composition, and molecular weight of the UEFs changed as fractionation progressed, and the galactose and fucose content increased. Interestingly, the sulfate polysaccharide contents were not related to the fractionation process: UEF3 showed the highest content among the fractions (33.89 ± 0.05%). UEFs had no cytotoxic effects on HaCaT, RAW 264.7, or BEAS-2B cells, and UEF3 prevented ultraviolet B (UVB)-induced cyclooxygenase-2 (COX-2) expression by regulating the ERK kinase pathway. The oral administration of UEF3 prevented epidermal thickening and COX-2 overexpression in UVB-exposed mice. UEF3 also alleviated oxidative stress by reducing reactive oxygen species levels via the activation of the HO-1 / NRF2 signaling pathway. This indicates that polysaccharide precipitation fractionation enhances efficacy by isolating the functional components. In conclusion, fractional processing using ultrasonic-assisted extracts can be an effective approach for developing nutraceutical materials for human health.

Bull kelp (<i>Nereocystis luetkeana</i>) possesses a heteromorphic lifecycle that alternates between two phases: a large, diploid sporophyte and a microscopic, haploid gametophyte. Recent research has suggested that bull kelp may be capable of bypassing the microscopic gametophytic stage via the direct release of structures identified in the literature as embryonic sporophytes. In order to verify the identity of these structures, we isolated and cultured multiple directly released structures over the course of three weeks in the lab and monitored their growth and development. We were able to track 67 directly released structures for more than one time interval, and found no evidence that the development of these structures followed that of embryonic sporophytes. We present the alternative hypothesis that these directly released structures are sporangia that have not yet released their spores. We conclude that even with the direct release of microscopic structures such as sporangia, there is currently no evidence that bull kelp are bypassing their established biphasic life history.

The genus <i>Monoraphidium</i> is a green microalgae distributed globally in freshwater ecosystems, and it is difficult to classify morphologically. Molecular genetic markers have been used for their taxonomy, although they have not been thoroughly evaluated. Here, we investigated three nuclear rRNA molecules (18S, internal transcribed spacer [ITS] + 5.8S, and 28S) and four chloroplast genes (<i>psbB</i>, <i>psbC</i>, <i>rbc</i>L, and <i>tufA</i>) using 16 <i>Monoraphidium</i> strains consisting of 9 species. Upon comparisons of genetic diversity and marker performance evaluation, we found that ITS was the best marker for <i>Monoraphidium</i> to discriminate each species, followed by <i>tufA</i>. The taxonomic discrimination power of the ITS was supported by the neighbor-joining tree. In addition, a secondary structure of ITS2 combined with compensatory base changes showed the distinct differences among individual species of <i>Monoraphidium</i>. These suggest that ITS may be the best marker for species differentiation of the coccoid green algae <i>Monoraphidium</i>.

Southeast Asia is an important marine biodiversity hotspot. Revealing the spatial patterns and environmental drivers related to population genetic structure in this region is a prerequisite for conservation biogeography and genetics. In this study, we applied two chloroplast markers (tufA and rpl2–rpl16) to evaluate population genetic variation and phylogeographic structure of the green macroalga Halimeda macroloba (12 populations, 275 individuals) in Southeast Asia. Both markers showed extremely low genetic variation and haplotype diversity in H. macroloba, with no clear phylogeographic separation between both sides of the Thai-Malay Peninsula (TMP). A postglacial founder effect and predominant asexual reproduction by fragmentation in H. macroloba, together with monsoon-driven ocean currents driving stepping-stone dispersal, may account for the observed remarkable phylogeographic homogeneity around the TMP. However, the tufA and rpl2-rpl16 markers congruently detected a phylogeographic break between the TMP and the eastern South China Sea, despite no obvious observable barrier to gene flow. These results raise the importance to take in situ actions to conserve the indicator species Halimeda in an era of ocean acidification and warming.

Hypoxia often causes the large-scale mortality of benthic organisms and alters the structure and function of pelagic and benthic communities. Protists are a major component of pelagic and benthic communities. Using a metabarcoding analysis, we explored the temporal changes in the structure of protist communities incubated for seven days under normoxic (7.0 mg L<sup>-1</sup>) and hypoxic (1.5 mg L<sup>-1</sup>) conditions. The incubated water was originally collected from Tongyeong Bay, Korea, where hypoxia frequently occurs. Among the phyla, the relative amplicon sequence variant (ASV) abundance of Cercozoa and Ochrophyta increased under hypoxia from day 0 to day 7, whereas that of other phyla declined or remained similar. Moreover, the relative ASV abundances in the phylum Dinoflagellata under both oxygen conditions were highest on days 0, 3, and 7. Among the dinoflagellate orders, the highest dinoflagellate ASV abundance under hypoxia on day 7 belonged to the order Peridiniales, whereas the highest relative read abundance belonged to Prorocentrales. The 35 dinoflagellate species that were detected under the hypoxic condition during incubation were autotrophic (two), phototrophic (autotrophic or mixotrophic) (15), mixotrophic (eight), kleptoplastidic (one), heterotrophic (eight), and parasitic (one), indicating that dinoflagellates with diverse trophic modes are present under hypoxia. Of these detected dinoflagellate species, 14 were present under the hypoxia on day 7. Furthermore, 19 dinoflagellate species were newly determined to be present under hypoxia, 6 of which were present on day 7. These findings highlight the ecological resilience and adaptability of protist communities under the hypoxic condition. The present study provides insights into the potential roles of protists in maintaining ecosystem functions in the oxygen-depleted environments.

The population genetic variability of brackish water species is of fundamental importance for the understanding of their evolutionary history and ecological adaptation to drastic environmental changes. Gracilaria tenuistipitata is a euryhaline and eurythermal species that is used in seaweed salads and agar production in Southeast Asia. However, the genetic variation of this brackish water species remains understudied. Here, we investigated the genetic variability and phylogeography of G. tenuistipitata in Southeast Asia using mitochondrial 5′ region of cytochrome c oxidase subunit I sequences. A total of 16 haplotypes were obtained from 161 specimens including 90 newly sequenced from Vietnam. Haplotype and nucleotide diversity of G. tenuistipitata were similar to those of congeneric marine species. Haplotype network analyses revealed a star-like structure including three haplotypes shared by two to four countries, indicating population connectivity as well as population expansion. At the species level, both mismatch analyses and Bayesian skyline plots revealed a demographic or range expansion in the middle Pleistocene rather than demographic bottlenecks. Migration analysis revealed that G. tenuistipitata populations dispersed southwards and northwards from Vietnam. The combination of genetic variability and phylogeographic analyses revealed that the current populations of G. tenuistipitata have been shaped by a combination of climate, sea-level change, and seasonal monsoon currents.

Species and genera in the dinoflagellate family Kareniaceae have attracted the attention of scientists, aquaculture farmers, and government officials because many species in this family cause harmful algal blooms associated with the mortality of vertebrates and invertebrates. In addition, the genera in Kareniaceae exhibit different morphological, biochemical, and genetic characteristics. To understand bloom dynamics and eco-evolutionary strategies of the genera in Kareniaceae, the biological interactions of kareniacean species and genera with prey and predators should be explored. In the present study, we reviewed the trophic modes, prey taxa and size spectra, feeding mechanisms, growth and ingestion rates, and protistan predators of five genera Gertia, Karenia, Karlodinium, Shimiella, and Takayama in the family. Additionally, we explored the feeding occurrence in Gertia stigmatica, the prey spectrum of Karenia brevis, and the predation of Takayama tasmanica by heterotrophic protists, which have not been fully investigated prior to the present study. Karenia, Karlodinium, Shimiella, and Takayama have different prey taxa and size spectra. Furthermore, within the same genus, different species exhibit different biological interactions with prey and protistan predators, creating different ecological niches. This study provides insights into the eco-evolutionary strategies of kareniacean dinoflagellates.

This study presents a comprehensive analysis of gymnodimine-series toxins produced by Alexandrium ostenfeldii (K- 1354) using liquid chromatography tandem mass spectrometry (LC-MS/MS). Three peaks with identical precursor ions but different retention times were observed, suggesting the presence of isomers identified as gymnodimines B, C, and D. Employing an integrated approach combining LC-MS/MS with electron transfer dissociation, the study revealed the characteristic fragment ions of gymnodimine D. Notably, this study demonstrated the potential for isolating and purifying gymnodimine D from culturable A. ostenfeldii, addressing the current lack of commercially available standards for this isomer. This study contributes remarkably to marine biotoxin monitoring and the understanding of these structurally complex compounds.