Ecology of viruses which lyse the photosynthetic marine picoflagellate Micromonas pusilla

Clonal isolates of viruses which cause lysis of the photosynthetic marine picoflagellate Micromonas pusilla (Prasinophyceae) were isolated from the coastal waters of New York, Texas, California and British Columbia, as well as the oligotrophic waters of the central Gulf of Mexico. The viruses are large polyhedrons (ca. 115 nm dia.) lacking tails, and are morphologically similar to previously described Micromonas pusilla virus (MpV). Each of the clones were genetically different. In field samples the concentrations of viruses causing lysis of M. pusilla were found to be spatially and temporally variable, ranging from {dollar}<{dollar}20 to 1.3 {dollar}times{dollar} 10{dollar}sp8{dollar} infective units I{dollar}sp{lcub}-1{rcub}{dollar}.; The genetic similarity among 8 clones of MpV isolated from 5 geographic locations was measured using DNA hybridization. The similarity among MpV isolated from a single geographic location, 34 {dollar}pm{dollar} 12.6% (mean {dollar}pm{dollar} SE, n = 4), was not significantly different from the similarity among MpV isolated from geographically distant locations, 26.6 {dollar}pm{dollar} 2.7% (mean {dollar}pm{dollar} SE, n = 24) (P = 0.92, Mann-Whitney U test). If MpV within a geographic location share genetic characteristics that they do not share with MpV from geographically distant locations, this was not reflected in the overall similarity of their genomes.; The kinetics of MpV adsorption to host cells, the in situ decay rate of MpV infectivity, and the temporal change in the abundance of MpV were used to infer the impact of MpV on Micromonas pusilla populations. The abundance of infective MpV in inshore water of the Texas coast of the Gulf of Mexico decreased from 1.3 {dollar}times{dollar} 10{dollar}sp5{dollar} ml{dollar}sp{lcub}-1{rcub}{dollar} in January 1993 to 2.1 {dollar}times{dollar} 10{dollar}sp3{dollar} ml{dollar}sp{lcub}-1{rcub}{dollar} at the end of April. The fraction of the M. pusilla population lysed per d was estimated to range from 9 to 51% d{dollar}sp{lcub}-1{rcub}{dollar} (average 22% d{dollar}sp{lcub}-1{rcub}{dollar} at a volume clearance rate of 7.03 {dollar}times{dollar} 10{dollar}sp{lcub}-9{rcub}{dollar} ml min{dollar}sp{lcub}-1{rcub}{dollar} (all contacts yield adsorption) and from 2 to 10% d{dollar}sp{lcub}-1{rcub}{dollar} (average = 4.4% d{dollar}sp{lcub}-1{rcub}){dollar} at a volume clearance rate of 1.40 {dollar}times{dollar} 10{dollar}sp{lcub}-9{rcub}{dollar} ml min{dollar}sp{lcub}-1{rcub}{dollar} (rate for MpV-SP1 and M. pusilla strain Plymouth 27). The abundance of M. pusilla required to support the turnover of MpV was estimated to range from 9,200 to 52,000 ml{dollar}sp{lcub}-1{rcub}{dollar} and the infection rate was estimated to equal 100 to 570 cells ml{dollar}sp{lcub}-1{rcub}{dollar} d{dollar}sp{lcub}-1{rcub}{dollar}.