Dynamics of lytic virus infecting the photosynthetic marine picoflagellate Micromonas pusilla

The impact of Micromonas pusilla virus (MpV) on Micromonas pusilla was inferred from measurements of the abundance of MpV, the kinetics of MpV adsorption to host cells, and the estimated in situ decay rate of MpV infectivity. The viral production rate was calculated to balance the estimated in situ decay rate of MpV infectivity. In inshore water of the Texas coast, the abundance of infective MpV was high and decreased from 1.3 × 105 ml−1 in January 1993 to 2.1 × 103 ml−1 at the end of April 1993. Decay rates of MpV infectivity in seawater incubated in the dark ranged from 0.06 d−1 at 4°C to 0.09 d−1 at 25°C. In unattenuated sunlight, decay rates of infectivity were much higher, ranging from 6.9 to 7.1 d−1. Sunlight-mediated decay rate of viral infectivity was depth-dependent, with an attenuation coefficient estimated to equal 0.73 m−1. The MpV production rate was 0.79 d−1, equal to a turnover time of 1.3 d. MpV abundance changed slowly relative to its turnover time, suggesting a stable coexistence of M. pusilla and the lytic virus. The adsorption coefficient for MpV-SP1 and host strain Plymouth 27 was 1.40 × 10−9 ml min− 1. Using this coefficient, we calculated that from 2 to 10% of the M. pusilla population was lysed per day (avg, 4.4% d−1). These results suggest that lysis of phytoplankton by viruses is a process that needs to be incorporated into models of nutrient and energy cycling in aquatic food webs.