Combined, these characteristics allowed classification of the MpVs into two groups.Exposure to chloroform led to loss of infectivity by all MpVs in one group, which suggests the presence of an outer lipid envelop. In addition, all except one of the members in that group formed a monophylogenetic clade that was distinct from all 35 other MpV isolates. The distinctive characteristics of the two MpV groups suggest different infection strategies, which may have important implications for the ecology of both host and virus populations in the environment. Knowledge gained from our study adds value to the MpV isolates as a scientific resource as it will aid in developing and testing in the laboratory new hypotheses about the ecological and 40 biogeochemical implications of M. pusilla viral infection in the environment.-3 -