Marine dissolved organic matter (DOM) is a major carbon reservoir, containing as much carbon as the atmosphere. Two decades ago, a controversy arose about its role in the ocean's biogeochemical cycle, challenging previous assumptions about its uniform distribution and biological refractoriness. Initial studies by Sugimura and Suzuki suggested higher DOM concentrations than previously thought, prompting extensive research to verify these findings. Improved analytical methods confirmed earlier estimates, leading to a better understanding of DOM's role in the biological pump and deep ocean sinks. DOM is primarily produced by photosynthetic plankton in the surface ocean and serves as a substrate for microbial populations and a source of nutrients for autotrophs. It is exported through ocean water column overturning, contributing to the biological pump that sequesters carbon in the deep ocean. Despite its large global inventory, DOM concentrations in the open ocean are low, ranging from 34 to 80 µmol kg⁻¹. Recent high-resolution data from the CLIVAR Repeat Hydrography project have revealed significant variability in DOM distribution, showing a 29% decrease along the deep global thermohaline circulation path. DOC export through overturning and meridional circulation leads to long-term sequestration of biogenic carbon. Semi-labile DOC, which is more biologically labile, is rapidly processed, while refractory DOC, more resistant to degradation, accumulates in the deep ocean. DOC concentrations in the deep ocean range from 34 to 50 µmol kg⁻¹, with significant gradients along intermediate and deep ventilation pathways. These gradients are influenced by biotic and abiotic processes, including microbial remineralization and particle interactions. DOC removal in the deep ocean is attributed to both biotic and abiotic processes, such as microbial remineralization and photochemical degradation. Abiotic interactions with particles, such as organic gel formation and adsorption, contribute to DOC removal. The residence time of refractory DOC in the deep ocean is estimated to be around 12,500 years, with a significant portion of DOC removed through abiotic interactions with particles. The study highlights the importance of DOM in the ocean carbon cycle, emphasizing the need for further research to understand its molecular composition and the mechanisms of its removal. The new data challenge previous assumptions about DOM's invariance, revealing dynamic processes in the deep ocean. These findings underscore the ongoing efforts of the marine biogeochemistry community to uncover the complexities of DOM's role in the global carbon cycle.Marine dissolved organic matter (DOM) is a major carbon reservoir, containing as much carbon as the atmosphere. Two decades ago, a controversy arose about its role in the ocean's biogeochemical cycle, challenging previous assumptions about its uniform distribution and biological refractoriness. Initial studies by Sugimura and Suzuki suggested higher DOM concentrations than previously thought, prompting extensive research to verify these findings. Improved analytical methods confirmed earlier estimates, leading to a better understanding of DOM's role in the biological pump and deep ocean sinks. DOM is primarily produced by photosynthetic plankton in the surface ocean and serves as a substrate for microbial populations and a source of nutrients for autotrophs. It is exported through ocean water column overturning, contributing to the biological pump that sequesters carbon in the deep ocean. Despite its large global inventory, DOM concentrations in the open ocean are low, ranging from 34 to 80 µmol kg⁻¹. Recent high-resolution data from the CLIVAR Repeat Hydrography project have revealed significant variability in DOM distribution, showing a 29% decrease along the deep global thermohaline circulation path. DOC export through overturning and meridional circulation leads to long-term sequestration of biogenic carbon. Semi-labile DOC, which is more biologically labile, is rapidly processed, while refractory DOC, more resistant to degradation, accumulates in the deep ocean. DOC concentrations in the deep ocean range from 34 to 50 µmol kg⁻¹, with significant gradients along intermediate and deep ventilation pathways. These gradients are influenced by biotic and abiotic processes, including microbial remineralization and particle interactions. DOC removal in the deep ocean is attributed to both biotic and abiotic processes, such as microbial remineralization and photochemical degradation. Abiotic interactions with particles, such as organic gel formation and adsorption, contribute to DOC removal. The residence time of refractory DOC in the deep ocean is estimated to be around 12,500 years, with a significant portion of DOC removed through abiotic interactions with particles. The study highlights the importance of DOM in the ocean carbon cycle, emphasizing the need for further research to understand its molecular composition and the mechanisms of its removal. The new data challenge previous assumptions about DOM's invariance, revealing dynamic processes in the deep ocean. These findings underscore the ongoing efforts of the marine biogeochemistry community to uncover the complexities of DOM's role in the global carbon cycle.