We now know that the abundance of free viruses in most marine environments is high. There is still, however, a lack of understanding of their occurrence and distribution and of in situ relationships between viral and host communities in natural environments. This may be partly due to methodological limitations. Our main aim was therefore to perform a case study in which a variety of methods were applied in order to give an improved, high-resolution description of the microbial communities in a natural environment. In order to do this we combined light microscopy (LM), transmission electron microscopy (TEM), flow cytometry (FCM), PCR denaturing gradient gel electrophoresis (PCR-DGGE) and pulsed-field gel electrophoresis (PFGE) and studied the diversity and succession of algae, bacteria and viruses in a nutrient enriched seawater enclosure. In the enclosure we experienced a situation where the development of the dominating algal population, which consisted of several flagellate species, was followed by proliferation of several different size-classes of viruses. The total bacterial number decreased markedly during the flagellate bloom but the community composition was maintained and the diversity remained high. Our results indicate a close linkage between various algal, bacterial and viral populations and show that virioplankton do not necessarily terminate algal and bacterial blooms but that they keep the host populations at non-blooming levels.KEY WORDS: Bacteria · DGGE · Diversity · Flow cytometry · Light microscopy · PFGE · Phytoplankton · VirusResale or republication not permitted without written consent of the publisher

Two lytic viruses specific for Chrysochromulina ericina (Prymnesiophyceae) and for Pyramimonas orientalis (Prasinophyceae) were isolated from Norwegian coastal waters in June 1998. The lytic cycle was 14-19 h for both viruses; the burst size was estimated at 1800-4100 viruses per host cell for the Chrysochromulina virus and 800-1000 for the Pyramimonas virus. Thin sections of infected cells show that both viruses replicate in the cytoplasm and that they have a hexagonal cross section, indicating icosahedral symmetry. The Chrysochromulina virus had a particle size of 160 nm and a genome size of 510 kbp; the size of the major polypeptide was 73 kDa. The Pyramimonas virus had a particle size of 220 x 180 nm and a genome size of 560 kbp; the size of the major polypeptide was 44 kDa. The genome sizes of these viruses are among the largest ever reported for viruses and they are larger than the minimum required for cellular life. The Chrysochromulina virus clone CeV-01B and the Pyramimonas virus clone PoV-01B described in this study have several properties in common with other viruses infecting microalgae, suggesting that they belong to the Phycodnaviridae.

Most studies of spring bloom succession in Norwegian waters have employed light microscopy and accounted for species composition of phyto-and zooplankton. Flow cytometry and molecular tools enable us to extend such investigations to include smaller organisms like bacterio-and virioplankton. Here, we describe succession and diversity of algae, bacteria, and viruses in relation to environmental changes from 15 February to 27 April. The spring succession started with an increase in autotrophic picoeukaryotes and Synechococcus sp. The diatoms bloomed around the middle of March and caused nutrient depletion in the upper part of the water column. Upwelling in the beginning of April gave rise to a second bloom, consisting of diatoms and Phaeocystis pouchetii. Numerically, autotrophic picoeukaryotes and Synechococcus sp. dominated the periods between and after these two major blooms. Heterotrophic bacterial abundance increased throughout the experimental period and reached peak values during and after phytoplankton blooms. These bacteria were succeeded by viruses having low DNA fluorescence, whereas viruses with medium DNA fluorescence bloomed during or after blooms of autotrophic picoeukaryotes. High-DNA fluorescence viruses reached maximum concentrations during and after the diatom and Phaeocystis blooms. The diversity of the bacterial community remained relatively stable, whereas viral diversity varied more and increased after major phytoplankton blooms. Our investigation thus demonstrates how virioplankton are important elements of the total microbial diversity and how they are intimately linked to the rest of the microbial community and possibly act as an internal driving force in spring bloom successions.

Several previous studies have shown that Emiliania huxleyi blooms and terminations have been succeeded by an increase in large virus-like particles (LVLP), strongly suggesting the bloom collapse was caused by viral lysis. However, due to methodological limitations, knowledge of how such blooms affect the rest of the microbial community is limited. In the current study we induced a bloom of E. huxleyi in seawater enclosures and applied methods enabling us to describe the algae, bacteria and virus communities with greater resolution than has been done previously. The development of the dominating algal, viral and bacterial populations in the nutrient-amended seawater enclosures was followed by flow cytometry (FCM). Light microscopy (LM), PCR-denaturing gradient gel electrophoresis (PCR-DGGE) and pulsed-field gel electrophoresis (PFGE) were used to describe the changes in community composition in greater detail. The algal community was dominated by E. huxleyi until termination of the bloom by viral lysis. After bloom termination the additional algal populations present in the enclosures increased in abundance. A marked increase in viruses other than the one infecting E. huxleyi was also observed. Total bacterial number and community composition were also greatly influenced by the bloom and its collapse. KEY WORDS: Diversity · Emiliania huxleyi · Microbial community · Viral lysisResale or republication not permitted without written consent of the publisher

The isolation and characterization of a virus (designated EhV) that infects the marine coccolithophorid Emiliania huxleyi (Lohmann) Hay & Mohler are described. Three independent clones of EhV were isolated from Norwegian coastal waters in years 1999 and 2000. EhV is a double‐stranded DNA‐containing virus with a genome size of ∼415 kilo‐base pairs. The viral particle is an icosahedron with a diameter of 160–180 nm. The virus particle contains at least nine proteins ranging from 10 to 140 kDa; the major capsid protein weighs ∼54 kDa. EhV has a latent period of 12–14 h and a burst size of 400–1000 (mean, 620) viral particles per cell. A phylogenetic tree based on DNA polymerase amino acid sequences indicates EhV should be assigned to the Phycodnaviridae virus family and that the virus is most closely related to viruses that infect Micromonas pusilla and certain Chlorella species.