Isolation and preliminary characterisation of a small nuclear inclusion virus infecting the diatom Chaetoceros cf. gracilis

A novel virus (Chaetoceros nuclear inclusion virus: CspNIV) causing lysis of a culture of the diatom Chaetoceros cf. gracilis was isolated from the Chesapeake Bay, USA, in April 2003. Transmission electron microscopy Of ultrathin sections of infected C. cf. gracilis revealed that CspNIV proliferates within the nucleus and forms paracrystalline arrays corresponding to the alignment of icosahedral viral particles of about 25 nm diameter. CspNIV shows some strong similarities with Heterosigma akashiwo nuclear inclusion virus (HaNIV) (cf. Lawrence et al. 2001; J Phycol 37: 216-222). The latent period of CspNIV is <24 h. The most widespread occurrence of Chaetoceros viruses in Chesapeake Bay was recorded in April 2003, ca. 1 mo after the winter-spring Chaetoceros bloom. However, results indicate that CspNIV remains infectious in surface water of the bay no longer than 1 mo after the disappearance of its host. Thus, our results reinforce the idea that microalgae are also sensitive to viruses other than those belonging to the family Phycodnaviridae. Furthermore, discovery and initial description of the infection process and ecology of CspNIV expands the breadth of phytoplankton shown to be susceptible to viral attack to include a ubiquitous diatom genera.