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.

In eastern Hokkaido, 60 to 80 kilometers above a subducting oceanic plate, tidal mudflats changed into freshwater forests during the first decades after a 17th-century tsunami. The mudflats gradually rose by a meter, as judged from fossil diatom assemblages. Both the tsunami and the ensuing uplift exceeded any in the region's 200 years of written history, and both resulted from a shallow plate-boundary earthquake of unusually large size along the Kuril subduction zone. This earthquake probably induced more creep farther down the plate boundary than did any of the region's historical events.

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Diatoms are a major phytoplankton group that play important roles in maintaining oxygen levels in the atmosphere and sustaining the primary nutritional production of the aquatic environment. Among diatoms, the genus Chaetoceros is one of the most abundant and widespread. Temperature, climate, salinity, nutrients, and predators were regarded as important factors controlling the abundance and population dynamics of diatoms. Here we show that a viral infection can occur in the genus Chaetoceros and should therefore be considered as a potential mortality source. Chaetoceros salsugineum nuclear inclusion virus (CsNIV) is a 38-nm icosahedral virus that replicates within the nucleus of C. salsugineum. The latent period was estimated to be between 12 and 24 h, with a burst size of 325 infectious units per host cell. CsNIV has a genome structure unlike that of other viruses that have been described. It consists of a single molecule of covalently closed circular single-stranded DNA (ssDNA; 6,005 nucleotides), as well as a segment of linear ssDNA (997 nucleotides). The linear segment is complementary to a portion of the closed circle creating a partially double-stranded genome. Sequence analysis reveals a low but significant similarity to the replicase of circoviruses that have a covalently closed circular ssDNA genome. This new host-virus system will be useful for investigating the ecological relationships between bloom-forming diatoms and other viruses in the marine system. Our study supports the view that, given the diversity and abundance of plankton, the ocean is a treasury of undiscovered viruses.

BackgroundDiatoms belong to the stramenopiles, one of the largest groups of eukaryotes, which are primarily characterized by a presence of an anterior flagellum with tubular mastigonemes and usually a second, smooth flagellum. Based on cell wall morphology, diatoms have historically been divided into centrics and pennates, of which only the former have flagella and only on the sperm. Molecular phylogenies show the pennates to have evolved from among the centrics. However, the timing of flagellum loss – whether before the evolution of the pennate lineage or after – is unknown, because sexual reproduction has been so little studied in the ‘araphid’ basal pennate lineages, to which Pseudostaurosira belongs.Methods/Principal FindingSexual reproduction of an araphid pennate, Pseudostaurosira trainorii, was studied with light microscopy (including time lapse observations and immunofluorescence staining observed under confocal scanning laser microscopy) and SEM. We show that the species produces motile male gametes. Motility is mostly associated with the extrusion and retrieval of microtubule-based ‘threads’, which are structures hitherto unknown in stramenopiles, their number varying from one to three per cell. We also report experimental evidence for sex pheromones that reciprocally stimulate sexualization of compatible clones and orientate motility of the male gametes after an initial ‘random walk’.Conclusions/SignificanceThe threads superficially resemble flagella, in that both are produced by male gametes and contain microtubules. However, one striking difference is that threads cannot beat or undulate and have no motility of their own, and they do not bear mastigonemes. Threads are sticky and catch and draw objects, including eggs. The motility conferred by the threads is probably crucial for sexual reproduction of P. trainorii, because this diatom is non-motile in its vegetative stage but obligately outbreeding. Our pheromone experiments are the first studies in which gametogenesis has been induced in diatoms by cell-free exudates, opening new possibilities for molecular ‘dissection’ of sexualization.

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.KEY WORDS: Diatom · Virus · Chaetoceros · DNA · Viral ecology Resale or republication not permitted without written consent of the publisher

Diatoms are one of the most significant primary producers in the ocean, and the importance of viruses as a potential source of mortality for diatoms has recently been recognized. Thus far, eight different diatom viruses infecting the genera Rhizosolenia and Chaetoceros have been isolated and characterized to different extents. We report the isolation of a novel diatom virus (ClorDNAV), which causes the lysis of the bloomforming species Chaetoceros lorenzianus, and show its physiological, morphological, and genomic characteristics. The free virion was estimated to be ϳ34 nm in diameter. The arrangement of virus particles appearing in cross-section was basically a random aggregation in the nucleus. Occasionally, distinctive formations such as a ring-like array composed of 9 or 10 spherical virions or a centipede-like array composed of rod-shaped particles were also observed. The latent period and the burst size were estimated to be <48 h and 2.

Recent studies have led to a rapid increase in knowledge of auxospore formation in diatoms. However, these studies have been limited to centric and raphid pennate diatoms, and there is still very little information for the araphid pennate diatoms. Using LM and SEM, we studied the development of the auxospore and the initial cell of the marine epiphytic diatom Gephyria media Arnott. Auxospores were bipolar and curved in side view, as in many other pennate diatoms. SEM revealed many transverse perizonial bands, all of which were incomplete rings. There was an elongate, sprawling, silicified structure beneath the ventral suture of the transverse perizonial bands. This structure is presumably equivalent to the longitudinal perizonial band in other pennate diatoms, although we could not determine the homologous relationship between the two features. Scales were found both in the inner wall of the perizonium and around the primary perizonial bands. The presence or absence of scales may be of phylogenetic significance in diatoms, only during the final stages of auxospore formation because scales are found in early spherical stages. The distinctive finger-like structures observed throughout all stage of G. media have not been observed before in the other diatom taxa.