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Despite their abundance and ecological importance, little is known about the diversity of marine viruses, in part because most cannot be cultured in the laboratory. Here, we used high-throughput viral metagenomics of uncultivated viruses to investigate the dynamics of DNA viruses in tropical seawater sampled from Chuuk State, Federated States of Micronesia, in March, June, and December 2014. Among the identified viruses, 71–79% were bacteriophages belonging to the families Myoviridae, Siphoviridae, and Podoviridae (Caudoviriales), listed in order of abundance at all sampling times. Although the measured environmental factors (temperature, salinity, and pH) remained unchanged in the seawater over time, viral dynamics changed. The proportion of cyanophages (34.7%) was highest in June, whereas the proportion of mimiviruses, phycodnaviruses, and other nucleo-cytoplasmic large DNA viruses (NCLDVs) was higher in March and December. Although host species were not analysed, the dramatic viral community change observed in June was likely due to changes in the abundance of cyanophage-infected cyanobacteria, whereas that in NCLDVs was likely due to the abundance of potential eukaryote-infected hosts. These results serve as a basis for comparative analyses of other marine viral communities, and guide policy-making when considering marine life care in Chuuk State. [ABSTRACT FROM AUTHOR]

Microbes in the coral holobiont play important roles in nitrogen fixation, carbon supply, antibiotic production, mucus recycling, and food supply to maintain homeostasis in corals. However, microbes can also induce coral diseases in response to environmental changes under non-optimal conditions. Therefore, studies of microbial communities are needed to understand the health statuses of corals in response to environmental changes. In this study, we performed 16S rDNA metabarcoding to investigate the bacterial communities in two healthy alcyonacean soft coral species (Sinularia sp. and Litophyton sp.) inhabiting the coast of Weno Island (Chuuk, Micronesia) and in ambient seawater. We identified 18 bacterial phyla, 24 classes, 54 orders, 109 families, and 222 genera associated with the two corals and seawater. The bacterial communities differed in the corals and seawater. The bacterial community in Sinularia sp. was dominated by the genus Spirochaeta in Spirochaetaceae (63.9% relative abundance), followed by Endozoicomonas (10%). In Litophyton sp., the bacterial community also contained Spirochaeta (19.5%) and Endozoicomonas (4.7%), although Cellvibrionaceae (23.7%) was dominant and other groups such as Rhizobiales (11.5%) and Rhodospirillales (8.7%) were evenly distributed. In ambient seawater, the predominant bacteria were Pelagibacter (29.2%), Rhodobacteraceae (15.5%), Prochlorococcus (11.3%), and Vibrio (5.8%), which are distinct from the species in the two coral species. The microbial communities between the two alcyonacean soft corals and seawater were different, and the microbial community differences were coral species-specific. [ABSTRACT FROM AUTHOR]