Carbon fixation is central to the marine carbon cycle. While photoautotrophy and chemolithoautotrophy receive most attention, anaplerotic pathways also fix inorganic carbon in both autotrophic and heterotrophic organisms. Anaplerosis has generally been considered a minor contributor to oceanic carbon fixation. Recent studies, however, indicate its potential importance across diverse oceanic regions. To address this knowledge gap, we quantified the contribution of anaplerosis to total dark dissolved inorganic carbon (DIC) fixation. We first screened publicly available multi-omics datasets, which identified Alteromonas, an obligate heterotrophic genus lacking genes for canonical autotrophic carbon fixation, as a key player for anaplerotic pathways. We then combined laboratory cultivation, flux modeling, and targeted ¹⁴C-bicarbonate tracer assays across the Pacific and Atlantic Ocean, showing that Alteromonas actively assimilates DIC via phosphoenolpyruvate carboxylase to sustain their high heterotrophic growth. Our field-based analysis reveals that anaplerotic carbon fixation accounts for approximately 17% of total dark DIC fixation in the global ocean. These results indicate that anaplerosis is a quantitatively important component of the deep-sea carbon budget, expanding its recognized role from a basic cellular function to a major biogeochemical process in the deep ocean.