Isolation and characterization of a new single-stranded DNA virus infecting the cosmopolitan marine diatom Chaetoceros debilis

The genus Chaetoceros (Bacillariophyceae) is the dominant diatom in the world's oceans.Various physical, chemical and biological factors affect depletion of diatom populations, for which the importance of viruses as a potential mortality source has only recently been found.We isolated and characterized a new Chaetoceros-infecting virus causing the lysis of the cosmopolitan species C. debilis Cleve.The virus infectious for C. debilis was isolated from both water and sediment samples collected in western Japan.We show the physiologic, morphologic and genomic characteristics of one virus clone among many isolates.We first described the C. debilis DNA virus (Cdeb-DNAV), a 30 nm icosahedral virus accumulating in the cytoplasm of C. debilis.Host specificity of CdebDNAV is strain-specific and its infectivity is stably maintained through a wide temperature range (between 20 and -196°C).The latent period and the burst size were estimated to be 12 to 24 h and 55 infectious units per host cell, respectively.CdebDNAV harbors a single-stranded DNA (ssDNA) genome whose structure is unknown.Sequence comparisons showed that the partial sequence of CdebDNAV is highly similar (E value = 10 -56 ) to the putative replicase gene of the C. salsugineum nuclear inclusion virus (CsNIV) that harbors a covalently closed circular, partially doublestranded ssDNA genome.This discovery of CdebDNAV may aid in further understanding the ecological dynamics of C. debilis population blooms in nature.