ABSTRACT

The true abundance and ecological importance of viruses in marine ecosystems and global processes has only been elucidated in the last few decades. Discoveries have led to the emergence of virus ecology as one of the most exciting and fastest developing branches of marine science. The world’s oceans are estimated to contain more than 10 viruses—they are the smallest and most abundant biological entities in marine ecosystems. Virus particles, termed “virions,” are composed of nucleic acid surrounded by a protein coat. A fundamental difference in the makeup of viruses compared with cells is that viruses contain only one type of nucleic acid, either DNA or RNA, whereas cells contain both. As in other microbes with extensive genetic diversity, the concept of a “species” of virus is problematic. The International Committee on Taxonomy of Viruses provides a formal definition of a viral species as a monophyletic group whose properties can be distinguished from those of other species by multiple criteria.