Metagenomic Analysis Of Viral Communities In The North Yellow Sea

Viruses are the most abundant and diverse biological entities in the marine ecosystems. They play an important role in the biogeochemical cycle and horizontal gene transfer, and could significantly influence the phytoplankton dynamics and diversity. In the western Pacific Ocean, the investigation on virome is still scant, especially their seasonal variation and distribution lacking sufficient details. The metagenomic analysis of the viral diversity of three stations in the North Yellow Sea of China during four seasons was conducted, in order to study the seasonal variation of virioplankton communities. Meanwhile, gene functions of viral communities in the North Yellow Sea are annotated in order to explore the functional characteristics of viral communities.Metagenomic studies showed viral metagenomes of North Yellow Sea obtained 239,421 assembled sequences (contigs) and 474,295 open reading frame sequences (ORFs) from the assembled sequences. Assembled contigs and predicted ORFs were compared against the CAMERA viral proteins database, showing that only 23.42%-32.34%and 11.94%-18.24%of metagenomic sequences were similar to known viral sequences, respectively. Most of the virus-like sequences belonged to Caudovirales virus, double-stranded DNA virus, which showed that Caudovirales virus was the dominant group in the North Yellow Sea. Caudovirales includes Myoviridae, Podoviridae, Siphoviridae, among which myoviruses were the most abundant tail viruses. In particular, Myovirus-like sequences were the most abundant (40.89%) in the summer, but least (29.82%) in the winter. Phycodnavridae (infecting algae) was the second-most-abundant taxa. A large number of SARI 1 viral sequences were detected in the viral metagenomes of the North Yellow Sea, including Pelagibacter phage HTVC008M, HTVC011P, HTVC019P and HTVC010P, among which Pelagibacter phage HTVC008M was the most abundant viruses in our metagenomes. In contrast, Pelagibacter phage HTVC010P represented one of the most abundant virus families in marine viral metagenomes from the Pacific Ocean. Pelagibacter phage HTVC008M was greatly underestimated previously. The ecological role of SAR11 virus remains to be further investigated. In addition, phylogenetic analysis of the polB gene for phycodnaviruses indicated that that our sequences represent a new clade with a high internal diversity and 23 new species of phycodnaviridae. In addition, phylogenetic analysis of the MCP gene for virophages showed that two new virophage-like sequences were identified and belonged to the Sputnik virophage group, but NYSVs shared a genetically distant ancestor with Sputnik and Zamilon virophages.Functional analysis showed that the most represented categories were replication, recombination and repair followed by energy production and conversion, amino acid transport and metabolism, cell wall/membrane/envelope biogenesis, transcription, signal transduction mechanisms in the viral metagenomes of the North Yellow Sea. Comparative analysis of viral communities showed that the structural composition of the North Yellow Sea viral communities has significant difference between seasons (F(3,8)= 2.640, P= 0.001), but between any two seasons except spring and summer, there was a significant difference. In addition, the functional composition of the North Yellow Sea viral community has a significant difference between seasons (F (3; 8)= 7.439, P= 0.029), but no significant difference (P> 0.05) between any two seasons. Our results indicate that viral diversity is very high in the North Yellow Sea and the viral communities are different between seasons. In addition, the discovery of new viruses has increased our understanding of the structure and evolution of viral communities.