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A hallmark of the SUP05 clade of marineGammaproteobacteriais the ability to use energy obtained from reduced inorganic sulfur to fuel autotrophic fixation of carbon using RuBisCo. However, some SUP05 also have the genetic potential for heterotrophic growth, raising questions about the roles of SUP05 in the marine carbon cycle. We used genomic reconstructions, physiological growth experiments and proteomics to characterize central carbon and energy metabolism inCandidatusThioglobus singularis strain PS1, a representative from the SUP05 clade that has the genetic potential for autotrophy and heterotrophy. Here, we show that the addition of individual organic compounds and 0.2 μm filtered diatom lysate significantly enhanced the growth of this bacterium. This positive growth response to organic substrates, combined with expression of a complete TCA cycle, heterotrophic pathways for carbon assimilation, and methylotrophic pathways for energy conversion demonstrate strain PS1's capacity for heterotrophic growth. Further, our inability to verify the expression of RuBisCO suggests that carbon fixation was not critical for growth. These results highlight the metabolic diversity of the SUP05 clade that harbours both primary producers and consumers of organic carbon in the oceans and expand our understanding of specific pathways of organic matter oxidation by the heterotrophic SUP05.