Our paper presents the results of a study in which we used whole genome bisulfite sequencing (WGBS), RNA-Seq (i.e. transcriptomics), high-CO2 physiology experiments, and spatiotemporally separated samples isolated in situ (i.e. directly from the ocean) to examine the metabolic potential of genome-wide cytosine (5mC) methylation (i.e. epigenomics), its potential impacts to transcriptional dynamics under both present-day and future ocean acidification conditions, and its biogeographic conservation in the globally-significant, biogeochemically-critical marine cyanobacterium Trichodesmium. Overall design: Characterize the 5mC cytosine methylation landscape of Trichodesmium erythraeum IMS101 and examine its relationship to its transcriptome under both 380 and 750 µatm CO2, respectively. WGBS of other T. erythraeum isolates (IMSB and 2175) as well as samples from natural populations isolated in situ in order to biogeographically characterize the Trichodesmium cytosine methylome.