Abstract
Numbering in excess of 10 million per milliliter of water, it is now undisputed that aquatic viruses are one of the major factors shaping the ecology and evolution of Earth’s microbial world. Nonetheless, environmental viral diversity and roles remain poorly understood. Here we report the first thorough characterization of a virus (designated TsV) that infects the coastal marine microalga Tetraselmis striata. Unlike previously known microalgae-infecting viruses, TsV is a small (60 nm) DNA virus, with a 31 kb genome. From a range of eight different strains belonging to the Chlamydomonadaceae family, TsV was only able to infect T. striata. Gene expression dynamics revealed an up-regulation of viral transcripts already 1 h post-infection (p.i.). First clear signs of infection were observed 24 h p.i., with the appearance of viral factories inside the nucleus. TsV assembly was exclusively nuclear. TsV-N1 genome revealed very different from previously known algae viruses (Phycodnaviridae). Putative function and/or homology could be resolved for only 9 of the 33 ORFs encoded. Among those was a surprising DNA polymerase type Delta (only found in Eukaryotes), and two genes with closest homology to genes from human parasites of the urogenital tract. These results support the idea that the diversity of microalgae viruses goes far beyond the Phycodnaviridae family and leave the door open for future studies on implications of microalgae viruses for human health.
Highlights
The acknowledgement of the importance of viruses in the marine environment started in the late1970s, with the first successful cultivation of a virus infecting a microalga and the first estimates of their extreme abundance [1,2]
Host infectivity range was tested on eight microalgae strains of five different genera of Chlorophyta
The isolation of this virus had been reported [38] as TvV but genetic characterization of the host has identified it as T. striata
Summary
The acknowledgement of the importance of viruses in the marine environment started in the late1970s, with the first successful cultivation of a virus infecting a microalga (eukaryotic phytoplankton) and the first estimates of their extreme abundance [1,2]. Viruses are the most abundant biological entities in the ocean, amounting to an average of 107 viruses per mL of seawater They outnumber bacteria by 10 to 1, and are responsible for an estimated 1023 infections per second, killing about 20%. Viral lysis diverts the flow of organic carbon passed up to higher levels of the food web toward heterotrophic microbes (a process called the viral shunt), and stimulates primary production by releasing growth-limiting nutrients such as nitrogen and phosphorus [5]. These two processes in turn affect the efficiency of the biological carbon pump. The massive mortality impacts population dynamics and community composition in the marine environment, as exemplified by the implication of viruses in the control and termination of various microalgal blooms (extensive proliferation of microalgae) [5,6]
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