Isolation and Characterization of a Single-Stranded RNA Virus Infecting the Marine Planktonic Diatom Chaetoceros tenuissimus Meunier

Abstract

Diatoms are important components of the biological community and food web in the aquatic environment. Here, we report the characteristics of a single-stranded RNA (ssRNA) virus (CtenRNAV01) that infects the marine diatom Chaetoceros tenuissimus Meunier (Bacillariophyceae). The ca. 31-nm virus particle is icosahedral and lacks a tail. CtenRNAV01 forms crystalline arrays occupying most of the infected host's cytoplasm. By growth experiments, the lytic cycle and the burst size were estimated to be <24 h and ϳ1 ؋ 10 4 infectious units per host cell, respectively. Stationary-phase C. tenuissimus cultures were shown to be more sensitive to CtenRNAV01 than logarithmic-phase cultures. The most noticeable feature of this virus is its exceptionally high yields of ϳ10 10 infectious units ml ؊1 ; this is much higher than those of any other algal viruses previously characterized. CtenRNAV01 has two molecules of ssRNA of approximately 8.9 and 4.3 kb and three major proteins (33.5, 31.5, and 30.0 kDa). Sequencing of the total viral genome has produced only one large contig [9,431 bases excluding the poly(A) tail], suggesting considerable overlapping between the two RNA molecules. The monophyly of CtenRNAV01 compared to another diatom-infecting virus, Rhizosolenia setigera RNA virus, was strongly supported in a maximum likelihood phylogenetic tree constructed based on the concatenated amino acid sequences of the RNA-dependent RNA polymerase domains. Although further analysis is required to determine the detailed classification and nomenclature of this virus, these data strongly suggest the existence of a diatom-infecting ssRNA virus group in natural waters.

First evidence for the existence of pennate diatom viruses

“…The lower bands were considered to be components of the viral genome, but not revealed in this study. Similar results were reported on ssRNA centric diatom virus CtenRNAV (Shirai et al, 2008).…”

Rapid quantification of viable cells of the planktonic diatom <i>Chaetoceros tenuissimus</i> and associated RNA viruses in culture

“…Generally, defective virions are often produced in host cells during the proliferation processes, e.g., deletion and recombination of its genome. CtenRNAV virus particles accumulate in host cells, forming structures similar to crystalline arrays (Shirai et al 2008, and small microalgal viruses (< 40 nm) may often form aggregates in lysates (Tomaru & Nagasaki 2007). Thus, the lower numbers of MPNs relative to the qPCR results were expected.…”

“…The axenic, clonal, algal strains used in this study, C. tenuissimus strain 2-10 ( Shirai et al 2008), were isolated from Harima-Nada, Seto Inland Sea, Japan on 10 August 2002. C. tenuissimus is a cosmopolitan marine planktonic diatom and often produces blooms in western Japanese coastal waters during early summer to autumn.…”

“…C. tenuissimus is a cosmopolitan marine planktonic diatom and often produces blooms in western Japanese coastal waters during early summer to autumn. C. tenuissimus strain 2-10 was used previously for isolation of the viruses CtenRNAV type I (Shirai et al 2008) and Cten-RNAV type II . Algal cultures were primarily maintained in SWM-3 medium (salinity: 30) enriched with 2 nM Na 2 SeO 3 (Imai et al 1996), under a 12/12-h light/dark cycle with approximately 130-150 µmol photons·m −2 s −1 using cool white fluorescent illumination at 20°C.…”

Bridging the Gap between Knowing and Modeling Viruses in Marine Systems—An Upcoming Frontier

“…So far viruses have been reported to infect a wide variety of phytoplankton groups including diatoms (Shirai et al, 2008), dinoflagellates (Yuji et al, 2004), prasinophytes (Derelle et al, 2008), and cyanobacteria (Ortmann et al, 2002), among others.…”