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Fig. 4 Dynamics of in vivo chlorophyll fluorescence (Fo) and photosynthetic efficiency (Fv/Fm) of Phaeocystis globosa during viral infection as assessed by fluorometry. Open symbols represent uninfected cultures, while the filled symbols represent virally infected P. globosa. Maximum fluorescence (Fm) was obtained after addition of the photosystem II inhibitor DCMU (20 pM final concentration). Fv equals Fm-Fo. Data are expressed in relative units (r.u.)

The strong dynamics of the Phaeocystis viruses indicate that, besides substantial production, the viral particles are also lost. Ways of removal can include passive adsorption of viruses onto the abundant organic aggregates (TEP) that are grazed upon, or onto inorganic colloids (clay, sand) that are removed from the euphotic zone by sinking (Kapuscinski and Mitchell 1980; Bruss-aard et al. 2005b). Other factors affecting the loss of the virus particles or infectivity are grazing by protozoa, enzymatic hydrolysis, and UV radiation as it damages the viral nucleic acids (Kapuscinski and Mitchell 1980; Suttle and Chen 1992; González and Suttle 1993; Noble and Fuhrman 1997; Jacquet and Bratbak 2003). Despite the dynamic nature and the substantial losses of PgV, pheno-typic characterization and molecular analysis of PgV isolates collected one year apart from the same area revealed identical sequences, indicating considerable stability of these PgV populations

(Brussaard et al. 2004; Baudoux and Brussaard 2005). Isolation of PgV during periods with very low to undetectable P. globosa host abundance, furthermore, suggests this robust group of viruses has a constant presence in the water column in these coastal areas where Phaeocystis occurs.

Diversity of Phaeocystis viruses and its ecological role

To achieve successful infection a virus depends on the encounter rate and thus on the abundance of its host species. The Phaeocystis virus isolates have, however, not only a species-restricted host ranges but most often also a strain-specific spectrum of infection (Jacobsen et al. 1996; Baudoux and Brussaard 2005). Thus, not all strains of a Phaeocystis species (e.g., P. globosa) will become infected, even when coexisting in the same water mass. Factors influencing this are the ability of the virus to bind optimally to a proper host cell, as well as the sensitivity of the host to infection.

Based on the structural capsid protein composition, 12 PgV isolates that originate from the southern North Sea could be divided in two groups (Baudoux and Brussaard 2005). The proteins on the surface of free viruses serve as a means of virus-to-cells attachment and allow transfer of viral genomes into the host cells. Any changes in the composition of these structural proteins between viruses may affect the binding to the host cell's receptors, selecting for host range restriction.

Considering that some of these different PgV isolates and their algal host strains originate from the same water sample, the ecological impact of such virus-host diversity is intriguing. Infection by a specific PgV does not act merely at the total host species population level, but rather on the subpopulation level. Essential advantages of such virus diversity could be the promotion of coexisting P. globosa strains to guarantee the availability of algal hosts. If one (dominating) strain of P. globosa gets infected and undergoes lysis, another resistant P. globosa strain that otherwise might be less fit for competition for nutrients, for example, can fill the niche. The lysis of the infected P. globosa strain

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