Search

Rising seawater temperatures are causing geographic shifts in the composition and abundance of the marine plankton that make up the microbial food web. Over the past few years, the water temperature around the Korea Strait has been increasing rapidly. Twelve cruise surveys were conducted in the Jeju Strait between 2017 and 2019 to monitor the presence of the tropical dinoflagellate genera Ornithocercus and Triposolenia, which are brought in by the Jeju Warm Current. Additionally, passive tracer experiments were performed using the Regional Ocean Modeling System to understand how the warm currents impact the Jeju Strait. The results of these simulations were then compared against the distribution ranges and abundance of Ornithocercus and Triposolenia. The study found that there were significant seasonal variations in abundance and spatial distribution of the tropical dinoflagellates during the sampling period, particularly during September and November when they were highly abundant and widely distributed. These patterns were closely related to the intensity and path of the Jeju Warm Current, particularly from southeastern offshore (EKB02; Eastern Kuroshio Branch02) rather than from southwestern offshore (EKB01; Eastern Kuroshio Branch01) of Jeju Island. The study also found no tropical dinoflagellates at any of the stations near the south coast of Korea during the sampling period. These findings suggest that the Ornithocercus and Triposolenia tropical dinoflagellate genera may serve as useful biological indicators to monitor the advection of warm currents into the Jeju Strait, Korea.

Micro/macro plastics are ubiquitous and are emerging agents causing many ecological problems in marine environments. Plastics carry various fouling organisms, including harmful microorganisms, that could potentially have ecological impacts on the marine environment and eventually human health. In this study, marine micro/macro plastics drifting at multiple locations in the Jeju Strait and around Jeju Island, Korea, were collected and analyzed. DNA metabarcoding and scanning electron microscopy were used to characterize the species composition of the attached eukaryotic microbial communities, with a special emphasis on harmful or toxic dinoflagellates, fungi, and parasites. A total of 1,035 eukaryotic microbial operational taxonomic units (OTUs) were identified from plastics and surrounding seawater samples. Two types of samples shared half of the eukaryotic OTUs, and of which approximately 12% were unique to plastic samples. These included 68 Archaeplastida (54.0%), 45 Stramenopiles (35.7%), 12 Opisthokonta (9.5%), and 3 Alveolata (2.4%) OTUs. The harmful or toxic dinoflagellates observed on the plastic surface were Alexandrium, Coolia, Dinophysis, Heterocapsa, Karlodinium, Noctiluca, Ostreopsis, Prorocentrum, Scrippsiella, and Tripos. The most dominant parasite community of the plastisphere was represented by fungi (42% of all parasite OTUs), followed by Oomycota (33%), Stramenopiles (14%), and dinoflagellates (12%). Nonetheless, this study demonstrates that the eukaryotic communities are considerably shared between the plastisphere and seawater. This implies plastics serve as a new ecological habitat in the sea and could function as dispersal vectors that facilitate the spread of harmful eukaryotic species and parasites.

Abstract Cyanobacteria are ubiquitous in marine environments and play an important role as primary producers. Some cyanobacteria, the so-called cyanobionts (cyanobacterial symbionts), have a symbiotic relationship with unicellular organisms. Among these relationships, in particular, the nature (e.g., genetic diversity, host or cyanobiont specificity, and cyanobiont seasonality) of the cyanobiont-dinoflagellate host consortia remains poorly understood. In this study, 16S rDNA of cyanobionts in 138 single host cells isolated over four seasons from temperate waters were sequenced using the MiSeq platform. Genetic analysis of cyanobionts from the dinoflagellate host Ornithocercus revealed that three genetic types of Synechococcales cyanobionts occurred in a wide range of water temperatures (11–24 °C), and their distribution seemed to be closely associated with variations in salinity. Furthermore, a certain degree of host (or cyanobiont) specificity in cyanobionts (or the host) among Ornithocercus species as well as among other dinophysoid species (i.e. Amphisolenia, Citharistes, and Histioneis), was observed. In addition to the Synechococcales cyanobionts, this study identified OTU sequences affiliated with Vampirovibrionales and Chroococcidiopsidales in some Ornithocercus cells, suggesting that Ornithocercus species are an additional habitat for these bacterial groups.

The Raphidophyceae is an ecologically important eukaryotic lineage of primary producers and predators that inhabit marine and freshwater environments worldwide. These organisms are of great evolutionary interest because their plastids are the product of eukaryote-eukaryote endosymbiosis. To obtain deeper insight into the evolutionary history of raphidophycean plastids, we sequenced and analyzed the plastid genomes of three freshwater and three marine species. Our comparison of these genomes, together with the previously reported plastid genome of Heterosigma akashiwo, revealed unexpected variability in genome structure. Unlike the genomes of other analyzed species, the plastid genome of Gonyostomum semen was found to contain only a single rRNA operon, presumably due to the loss of genes from the inverted repeat (IR) region found in most plastid genomes. In contrast, the marine species Fibrocapsa japonica contains the largest IR region and overall plastid genome for any raphidophyte examined thus far, mainly due to the presence of four large gene-poor regions and foreign DNA. Two plastid genes, tyrC in F. japonica and He. akashiwo and serC in F. japonica, appear to have arisen via lateral gene transfer (LGT) from diatoms, and several raphidophyte open reading frames are demonstrably homologous to sequences in diatom plasmids and plastid genomes. A group II intron in the F. japonica psbB gene also appears to be derived by LGT. Our results provide important insights into the evolutionary history of raphidophyte plastid genomes via LGT from the plastids and plasmid DNAs of diatoms.

The phylum Perkinsozoa is known as an exclusively parasitic group within alveolates and is widely distributed in various aquatic environments from marine to freshwater environments. Nonetheless, their morphology, life cycle, the identity of the host, and physiological characteristics remain still poorly understood. During intensive sampling along the west coast of Korea in October and November 2017, a new parasitoid, which shares several characteristics with the extant families Perkinsidae and Parviluciferaceae, was discovered and three strains of the new parasitoid were successfully established in cultures. Cross-infection experiments showed that among the examined planktonic groups, only dinoflagellates were susceptible to the new parasitoid, with infections observed in species belonging to eight genera. Even though the new parasitoid shared many morphological and developmental characteristics with other Perkinsozoan parasites, it differed from them by its densely packed trophocyte structure without a large vacuole or hyaline material during the growth stage. These characteristics are common among Parviluciferaceae members. Furthermore, through palintomic extracellular sporogenesis, it produced characteristic interconnected sporocytes resembling a string of beads. Phylogenetic analyses based on the small subunit and large subunit ribosomal DNA sequences revealed that the new parasitoid was distantly related to the family Parviluciferaceae and was more closely related to the families Perkinsidae and Xcellidae. Morphological, ultrastructural, and molecular data on the new parasitoid raised the need to erect a new family, i.e., Pararosariidae, within the phylum Perkinsozoa with Pararosarium dinoexitiosum gen. et sp. nov. as the type species. The isolation and establishment in culture of the new parasitoid outside the family Parviluciferaceae in the present study would contribute to the better understanding of the diversity of Perkinsozoan parasites and provide useful material for comparisons to other parasite species in the further study.

Marine microbial interactions involving eukaryotes and their parasites play an important role in shaping the structure of phytoplankton communities. These interactions may alter population densities of the main host, which in turn may have consequences for the other concurrent species. The effect generalist parasitoids exert on a community is strongly dependent on the degree of host specificity. Parvilucifera sinerae is a generalist parasitoid able to infect a wide range of dinoflagellates, including toxic-bloom-forming species. A density-dependent chemical cue has been identified as the trigger for the activation of the infective stage. Together these traits make Parvilucifera-dinoflagellate hosts a good model to investigate the degree of specificity of a generalist parasitoid, and the potential effects that it could have at the community level. Here, we present for the first time, the strategy by which a generalist dinoflagellate parasitoid seeks out its host and determine whether it exhibits host preferences, highlighting key factors in determining infection. Our results demonstrate that in its infective stage, P. sinerae is able to sense potential hosts, but does not actively select among them. Instead, the parasitoids contact the host at random, governed by the encounter probability rate and once encountered, the chance to penetrate inside the host cell and develop the infection strongly depends on the degree of host susceptibility. As such, their strategy for persistence is more of a game of Russian roulette, where the chance of survival is dependent on the susceptibility of the host. Our study identifies P. sinerae as a potential key player in bloom community ecology, where in mixed dinoflagellate communities consisting of hosts that are highly susceptible to infection, parasitoid preferences may mediate coexistence between host species, reducing the dominance of the superior competitor. Alternatively, it may increase competition, leading to species exclusion. If, however, highly susceptible hosts are absent from the community, the parasitoid population could suffer a dilution effect maintaining a lower parasitoid density. Therefore, both host community structure and host susceptibility will determine infectivity in the field.