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Dissolved organic carbon (DOC) is the largest pool of exchangeable organic carbon in the ocean. However, less than 10% of DOC has been molecularly characterized in the deep ocean to understand DOC's recalcitrance. Here we analyze the radiocarbon (C-14) depleted, and presumably refractory, low molecular weight (LMW) DOC from the North Central Pacific using atomic force microscopy to produce the first atomic-resolution images of individual LMW DOC molecules. We evaluate surface and deep LMW DOC chemical structures in the context of their relative persistence and recalcitrance. Atomic force microscopy resolved planar structures with features similar to polycyclic aromatic compounds and carboxylic-rich alicyclic structures with less than five aromatic carbon rings. These compounds comprise 8% and 20% of the measurable molecules investigated in the surface and deep, respectively. Resolving the structures of individual DOC molecules represents a step forward in molecular characterization of DOC and in understanding its long-term stability.

Plain Language Summary The marine dissolved organic carbon (DOC) reservoir is similar in size to that of atmospheric carbon dioxide and plays an important role in the ocean carbon cycle. DOC comprises a vast mixture of diverse molecules, the majority of which remain structurally uncharacterized. Identification of DOC molecular structures will aid our understanding of DOC cycling. Here, we present the first images of individual DOC molecules using atomic force microscopy. We compare images of low molecular weight DOC molecules extracted from samples collected at different depths in the North Central Pacific. Our results help unravel the molecular architecture, formation processes, and cycling of marine DOC in the ocean. This work demonstrates that atomic force microscopy can facilitate structure determination in oceanic mixtures and provide an additional complimentary toolbox for analytical techniques in geophysical relevant environments.