Summary Cytosine methylation has been shown to regulate essential cellular processes and impact biological adaptation. Despite its evolutionary importance, only a handful of bacterial, genome‐wide cytosine studies have been conducted, with none for marine bacteria. Here, we examine the genome‐wide, C 5 ‐Methyl‐cytosine (m5C) methylome and its correlation to global transcription in the marine nitrogen‐fixing cyanobacterium Tric...