The ocean carbon cycle spans multiple scales and reservoirs, challenging efforts to build a coherent picture and fostering misconceptions or fragmented narratives in science and public discourse. Common examples include the belief that the biological processes control the ocean's carbon sink (i.e., fraction of human CO2 emissions absorbed by ocean), that restoring coastal ecosystems is highly effective at mitigating climate change, or that whales substantially contribute to carbon sequestration. We provide a comprehensive review of living and nonliving ocean carbon stocks and fluxes-from plankton to mangroves, whales, fish, and plastics-and an integrated perspective on global ocean carbon cycling, disentangling well-supported insights from misconceptions. This synthesis reaffirms the ocean's key role as a physics- and chemistry-driven carbon sink, while clarifying the limited contribution of coastal and open-ocean ecosystems to carbon sequestration and climate mitigation. We caution against frameworks that justify marine conservation through climate mitigation-a narrative useful to draw attention, but not always robust and unnecessary, since marine biodiversity is worth preserving regardless of its impact on carbon.
