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.

This study assessed variations in demersal fish assemblages with respect to the study site and water depth. Seasonal samplings from May 2018 to March 2019 were conducted along the northern (Ayajin, Goseong) and southern (Hupo, Uljin) sites of the East Sea off the Korean coast, using commercial gill nets. Samples were collected at depths of ~50, ~80, ~150 m across the study sites, with concurrent monitoring of water column structures. A total of 73 species and 6250 specimens were collected. Distinctive fish species compositions were observed according to the study site and depth. Although Glyptocephalus stelleri was the most abundant fish species in both Ayajin and Hupo, Gadus macrocephalus, Icelus cataphractus, and Alcichthys elongatus were most predominant in Ayajin, whereas Cleisthenes pinetorum, Hippoglossoides dubius, and Gymnocanthus herzensteini were more prevalent in Hupo. In terms of depth layer, in Ayajin, G. stelleri dominated in both intermediate and deeper layers, with Hemilepidotus gilberti, A. elongatus, Enophrys diceraus common in shallower depths. Conversely, in Hupo, G. stelleri, C. pinetorum, and A. nadeshnyi dominated across all depth layers, whereas Dasycottus setiger and G. herzensteini dominated in deeper and shallower depths, respectively. Significant influences of the study site and water depth on fish assemblage structures were observed due to variations in water temperature at the seasonal thermocline boundary.

The influences of oceanographic changes on diet composition and trophic level for pollock (Gadus chalcogrammus) inhabiting the East Sea off the Korean coast were examined based on stomach content and stable isotope analyses during 2016 and 2017. The diets of pollock consisted mainly of benthic crustaceans (particularly carid shrimps and euphausiids) and cephalopods, with a predominance of teleosts in the diets of larger individuals in deeper habitats. In 2016, amphipods, carid shrimps and cephalopods featured strongly in pollock diets, and the contribution of amphipods decreased in the diets of larger individuals and deeper depths. In 2017, euphausiids dominated at shallower depths, whereas the contributions of carid shrimps and teleosts increased in deeper habitats. Body-size-related differences in carbon stable isotope (δ13C) values were present in both 2016 and 2017, but size-related differences in nitrogen stable isotope (δ15N) values were only observed in 2017. The increased contribution of euphausiids during 2017 resulted in a distinct decrease in the trophic level of pollock compared to co-occurring higher trophic level predators, which can be linked to changes in habitat water temperature. Combined stomach contents and isotopic analyses provide a more comprehensive understanding of how fish diets and trophic levels fluctuate with changes in the type and abundance of prey resources in response to environmental changes.

UNASSIGNED: The Laophontidae is a very large and diverse family containing more than 320 species and 74 genera in the Harpacticoida. According to records published until recently, 25 species of 12 genera of Laophontidae were reported to appear in Korean waters. The most common laophontid species in Korean waters is Paralaophontecongenera (Sars, 1908). During research on the meiobenthic community in the subtidal zone near the Korean coast in the East Sea, we found an undescribed genus of the Laophontidae family. The character traits of this undescribed specimen of the Laophontidae family do not match any existing genus.
UNASSIGNED: Herein, a new genus of the interstitial marine benthic copepods family Laophontidae T. Scott, 1905 is described from the subtidal zone near Hupo Harbour on the east coast of Korea. This genus was named Strictlaophonte gen. nov. and has been classified into the family Laophontidae. This was based on the presence of seven segmented female antennules, reduced antennary exopod, first endopodal segments having no inner setae from the second leg to the fourth leg and P5 with a distinct exopod that is not fused at the basis. The distinguishing features of Strictlaophonte gen. nov. are P5 exopod having only four setae, the P1 exopod having two segments and the antenna exopod having four setae. In particular, this new genus has unique characteristics in that the caudal rami are very tightly attached to each other.

It is crucial to establish appropriate background concentrations to discern heavy metal pollution in the marine environment. In this study, we analyzed heavy metals in deep-sea sediment cores to determine regional background concentrations at the East-Sea Byeong Ocean dumping site. The vertical profiles of heavy metals were categorized into three groups based on their contamination characteristics, and regional background levels for 12 metals were determined using pre-1900 averages. The enrichment factor, contamination factor, and pollution load index, calculated using regional background concentrations, indicated significant contamination by Cr, Co, Cu, Zn, Cd, Hg, and Pb during the ocean dumping period. These results differ from those obtained using global average concentrations. This underscores the importance of considering regional characteristics to minimize the risk of misinterpreting anthropogenic impacts. The approach based on local information is considered useful when sediment quality guidelines are absent or inapplicable.

The spatiotemporal distribution of MASTs (MArine STramenopiles), mostly affiliated with heterotrophic protists, and their interactions with Synechococcales were investigated in an anthropogenically polluted bay of the East Sea using 18S rRNA and 16S rRNA gene sequences. The bay was characterized by strong stratification between the surface and bottom layers and cold and nutrient-rich water intrusion in summer, whereas the bay water was well mixed in winter. MAST-3, MAST-6, MAST-7, and MAST-9 were the major MAST clades, whereas the dominance of MAST-9 declined from >80 % in summer to <10 % in winter and the diversity of MAST communities increased in winter. Co-occurrence network analysis via the sparse partial least squares revealed that MAST-3 had a Synechococcales-specific interaction during the study periods but prey-specific interactions with other MAST clades were not detected. Temperature and salinity markedly influenced the relative abundance of major MAST clades. The relative abundance of MAST-3 increased at temperatures above 20 °C and salinities above 33 ‰, however, the abundance of MAST-9 decreased under the same conditions. Analysis of the metabolic functions of cyanobacteria using the FAPROTAX (Functional Annotation of Prokaryotic Taxa) indicated that the response of photosynthetic cyanobacteria to NH4+ and PO43- was significant in summer but these functions were not tightly coupled with the abundance of Synechococcales. Similarly, strong associations of MAST-3 with high temperature/salinity and Synechococcales were indicative of coupled cascading during bottom-up processes. However, other major MAST clades were likely decoupled with Synechococcales and environmental conditions in which cyanobacteria can thrive. Therefore, our results demonstrated that MAST communities can couple or decouple with environmental variables and potential prey depending on MAST clades. Collectively, our findings provide novel insights into the role of MAST communities in microbial food webs in eutrophic coastal waters.

Studies of the land-sea-air interactions of aerosol are scarce considering their significant role in global environmental changes. Here, we investigated potential sources of sub-micron aerosols over the East Sea (Sea of Japan), which is strongly influenced by continental and marine aerosols. A high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was used to measure the size-resolved chemical compositions of sub-micron aerosols during the period March 10-15, 2020. Concentrations of all AMS species, black carbon (BC), PM10 (particulate matter <10 μm) and PM2.5 (particulate matter <2.5 μm) were higher when cruising in industrialized coastal areas compared to the offshore region. A positive matrix factorization (PMF) model identified five distinct sources, i.e., hydrocarbon-like organic aerosol, semi-volatile and low-volatile oxygenated aerosols, methanesulfonic acid (MSA), and dimethyl sulfide (DMS; C2H6S) oxidation, which accounted for 5.98 %, 21.6 %, 28.3 %, 34.5 %, and 9.64 % of the total organic mass, respectively. The spatiotemporal variation of MSA, as well as the MSA to sulfate ratio (MSA:SO42-) over the East Sea, was determined for the first time. The mass concentrations of MSA displayed a similar time series distribution pattern to those of DMS. The time series distributions of the MSA:SO42- ratio displayed distinct differences, with higher ratios downwind of the ocean (0.216 ± 0.083 μg·m-3) than land (0.089 ± 0.030 μg·m-3). The growth of ultrafine particles (10-35 nm) was observed during two of the elevated MSA:SO42- ratio events, suggesting a potential role of MSA in new particle formation.

In March and April 2018, we carried out a survey of myxosporean parasites in coastal fishes collected from Nha Trang Bay in Vietnam\'s East Sea. Of the 159 fish specimens, 18 fish species were represented, belonging to 10 families. In 8 host species, 7 myxosporean species were found, representing the genera Kudoa and Unicapsula. Two of these species were new to science: Kudoa igori n. sp. from the gallbladder of Longarm mullet Osteomugil cunnesius (Valenciennes, 1836) (prevalence 10%) and Kudoa borimiri n. sp. from skeletal muscles of Longarm mullet and Longfinned mullet Osteomugil perusii (Valenciennes, 1836) (prevalence 30% and 60%, respectively). Vegetative stages were not found. Spores of K. igori n. sp. were small, orbicular to quadrate in apical view, and with four equal valves. In lateral view, spores were shallowly ovoid with a slightly protruding anterior pole. The 4 polar capsules were slightly unequal sizes and were oriented toward the apex of the spore. Dimensions were as follows: spore length 4.56 ± 0.22 (4.18-4.56), thickness 4.42 ± 0.39 (3.55-5.13), width 5.74 ± 0.46 (4.66-6.50), length of biggest polar capsule 1.38 ± 0.14 (1.1-1.65), middle 1.28 ± 0.12 (1.03-1.53), smallest 1.13 ± 0.11 (0.94-1.30), width of biggest polar capsule 1.03 ± 0.14 (0.83-1.4), middle 0.92 ± 0.12 (0.76-1.08), smallest 0.74 ± 0.12 (0.55-0.94). Spores of K. borimiri n. sp. had four equal valves with slightly rounded ends and were quadrate in apical view. In lateral view, spores were broadly deltoid. Four equal-sized polar capsules opened at the apex of the spore. In O. cunnesius, spore dimensions were as follows: length 3.51 ± 0.15 (3.34-3.92), thickness 3.65 ± 0.38 (2.79-4.21), width 4.97 ± 0.37 (4.13-5.97), length of the polar capsules 1.23 ± 0.18 (0.99-1.57), width 0.88 ± 0.07 (0.70-1.00). Overlapping dimensions for K. borimiri n. sp. spores were also found in O. perusii. Other known myxozoan species we encountered were Kudoa thyrsites and Kudoa whippsi, which had not previously been reported from the East Sea and Vietnam. We also encountered Kudoa monodactyli, which had not previously been reported from Nha Trang Bay. In addition, we report 2 additional species, a Kudoa sp. and a Unicapsula sp., that were not attributable to previously described myxozoans and need further investigation to completely characterize.

Mercury (Hg) contamination in open coastal areas has attracted public concern regarding safe fish consumption and management of the coastal environment, especially in areas of accidental Hg spills on inland coasts. This study investigated the temporal and spatial distribution of Hg in sediments of Youngil Bay and the Hupo Basin, East Sea, Korea; it also discussed the sources and transport of anthropogenic Hg. Hg hot spots were found in the northern Hupo Basin (elevated by 2-3×) and the river mouth area in Youngil Bay (elevated by approximately 70×). The river mouth contamination resulted from the destruction of a dam impounding landfill waste, while the basin contamination was attributed to atmospheric deposition and Hg enrichment associated with increased organic carbon concentrations driven by high biological production in the coastal upwelling area. Spilled Hg was transported to open coastal areas up to 36.6°N.

A Gram-stain-negative, aerobic, yellow-pigmented and non-motile rod-shaped bacterium, designated as GrpM-11T, was isolated from coastal seawater collected from the East Sea, Republic of Korea. Strain GrpM-11T could grow at 10-40 °C (optimum, 35 °C), at pH 5.5-9.5 (optimum, pH 7.0) and in the presence of 0-8 % (w/v) NaCl (optimum, 3-4 %). Cells hydrolysed aesculin, gelatin and casein, but could not reduce nitrate to nitrite. The 16S rRNA gene sequence analysis showed that this strain formed a distinct phylogenic lineage with Parasphingopyxis algicola ATAX6-5T (96.2 % sequence identity) and Parasphingopyxis lamellibrachiae DSM 26725T (96.2 % identity) and belonged to the genus Parasphingopyxis. The predominant isoprenoid quinone was ubiquinone-10. The polar lipid profile of strain GrpM-11T consisted of diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylcholine, sphingoglycolipid and three unknown glycolipids. Cellular fatty acid analysis indicated that summed feature 8 (C18 : 1 ω7c and/or C18 : 1 ω6c; 42.8 %), C16 : 0 (19.0 %), C18 : 1 ω7c 11-methyl (13.3 %) and C18 : 1 ω7c (8.0 %) were the major fatty acids. The DNA G+C content of strain GrpM-11T was 63.7 mol%. Through whole genome sequence comparisons, the digital DNA-DNA hybridization and average nucleotide identity values between strain GrpM-11T and two species of the genus Parasphingopyxis were revealed to be in the ranges of 19.0-22.0 % and 76.3-79.7 %, respectively. Based on the results of polyphasic analysis, strain GrpM-11T represents a novel species of the genus Parasphingopyxis, for which the name Parasphingopyxis marina sp. nov. is proposed. The type strain is GrpM-11T (KCCM 43343T=JCM 34665T).