As a typical marine ecosystem, coral reefs possess huge carbon reserves and strong carbon sequestration potential. However, the net carbon capacity (carbon release and carbon uptake) of coral reefs remains controversial, mainly due to the diversity and complexity of carbon metabolism of the coral symbiont. Coral reefs release carbon dioxide into the atmosphere during biological calcification and respiration. They can also bury carbon during biosynthesis and deposition. Therefore, their identity as a net carbon source or sink remains unclear. The study of carbon metabolism in coral reef ecosystems is a frontier subject under the umbrella of carbon neutrality studies. Existing studies have demonstrated that symbionts could directly promote coral reef absorption of atmospheric CO₂ through carbon metabolism and exhibit ecosystem connectivity functions that improve the carbon sink functions of salt marsh vegetation, seagrass bed, and marine phytoplankton. To deepen our understanding of coral reef carbon source and sink functions, this paper reviews the carbon metabolism characteristics of coral symbionts, identifies key ecological processes involving carbon in symbionts (migration of organic carbon, transformation of inorganic carbon, and occurrence status), summarizes the roles of bacteria-zooxanthellae-viruses in symbiont carbon metabolism, and reviews factors the cause controlling coral reef carbon source and sink functions.In addition, this paper summarizes relevant information regarding bacterial community composition, functional differences, and metabolic pathways that can all affect coral carbon metabolism. Deep-sea cold-water corals use bacteria as a carbon source and viruses can change the metabolic rate and utilization efficiency of coral reef organic carbon flow by switching strategies. Photosynthesis and respiration of zooxanthellae affect coral carbon metabolism, and the influence of temperature and light on these two metabolic processes has been extensively studied. The purpose of the paper is to deepen our understanding of the carbon metabolism of coral symbionts, in addition to seeking an optimal carbon sinking technology based on coral reefs. It calls for further exploration of the assessment technology of coral reef carbon sink function and ways to increase the sink in future studies, so as to form the coral protection and restoration technology based on carbon increment, and enhance the contribution of coral reefs to the blue carbon ecosystem. This paper provides a helpful progressive summary covering carbon metabolism characteristics of coral symbionts, analysis of the source sink paradox, and finally coral reefs' negative carbon emission potential, to further our understanding of the carbon source sink model of coral reef ecosystems, and to gather scientific evidence for its entry into the "blue carbon club".