The Marine Roseobacter Clade (MRC) is a numerically and biogeochemically significant component of the bacterioplankton. Annotation of multiple MRC genomes has revealed that an abundance of carbon monoxide dehydrogenase (CODH) cox genes are present, subsequently implying a role for the MRC in marine CO cycling. The cox genes fall into two distinct forms based on sequence analysis of the coxL gene; forms I and II. The two forms are unevenly distributed across the MRC genomes. Most (18/29) of the MRC genomes contain only the putative form II coxL gene. Only 10 of the 29 MRC genomes analysed have both the putative form II and the definitive form I coxL. None have only the form I coxL. Genes previously shown to be required for post-translational maturation of the form I CODH enzyme are absent from the MRC genomes containing only form II. Subsequent analyses of a subset of nine MRC strains revealed that only MRC strains with both coxL forms are able to oxidise CO.