INFLUENCE OF IRRADIANCE ON VIRUS-ALGAL HOST INTERACTIONS(1).

The effect of different irradiance levels on the interactions between the algal host and its virus was investigated for two marine phytoplankton, Phaeocystis globosa Scherff. and Micromonas pusilla (Butcher) Manton et Parke. The algal cultures were acclimated at 25, 100, and 250 μmol photons · m(-2)  · s(-1) (LL, ML, and HL, respectively), after which they were infected with a lytic virus (PgV-07T and MpV-02T) and monitored under the appropriate irradiance and in darkness. The effect of irradiance levels on the host-virus interactions differed for the two algal host-virus systems examined. For P. globosa, the LL-acclimated cultures showed a 4 h prolonged latent period (11-16 h), which may be related to the subsaturated growth observed at this irradiance. The burst size was reduced by 50% at LL and HL compared to ML (525 PgV · cell(-1) ). The fraction of infectious viruses, however, remained unchanged. Viral replication was prevented when the LL P. globosa cultures were kept in darkness (up to 48 h) but recovered when placed back into the light. PgV-07T still replicated in the dark for the ML- and HL-acclimated cultures, but viral yield was reduced by 50%-85%. For M. pusilla, the burst size (285-360 MpV · cell(-1) ), the infectivity, and the latent period of MpV-02T (7-11 h) remained unaffected by the incident light. Conversely, darkness not only inhibited MpV replication but also resulted in substantial cell lysis of the noninfected cultures. Our study implies that irradiance level is an important factor controlling algal host-virus interactions and hence the dynamics of phytoplankton populations.