The prasinophyte genus Micromonas is comprised of picoeukaryotic (cell diameter = 2 µm) primary producers that prosper in diverse marine environments, ranging from polar to low latitude systems. Micromonas pusilla CCMP1545, Micromonas commoda RCC299 and other prasinophytes in the order Mamiellales harbor a large chromosomal region (big outlier chromosomal region – BOC) of unusually low GC content. In addition, the smallest chromosome (small outlier chromosome – SOC) also has GC content lower than the genomic average. Despite their presence in all the sequenced Mamiellales members, the functional significance, evolutionary history and genomic basis of their maintenance within an otherwise high GC genomic context remain an open question. Using high resolution RNA-seq data from a synchronized mid-exponential growth M. pusilla CCMP1545 over the light:dark cycle, we studied the genomic basis of transcriptional program in Micromonas pusilla CCMP1545.