Organic carbon dissolved in sea water is an important component of the global carbon cycle(1). Concentrations of dissolved organic carbon (DOC) in the ocean’s surface mixed layer are at least twice those in the deep sea(2,3), because of the production of soluble carbon compounds by marine algae in the euphotic zone(4,5). But very little is known about the chemical composition of DOC, and the connection between photosynthetic production and DOC accumulation is not well understood(6,7). Here we report the chemical characterization of macromolecular DOC at several sites in the Atlantic and Pacific oceans. Neutral sugars, acetate and lipids show similar distributions, suggesting that these constituents are linked together in a common macromolecular structure. Chemical linkage patterns between the oligosaccharide portions of dissolved organic matter subjected to ultrafiltration are highly specific, with little variation between ocean basins. We show that laboratory culture experiments on the decomposition of algal exudate produce macromolecular organic matter with similar compositions and linkage characteristics. We propose that a significant fraction of DOC in sea surface water consists of structurally related and biosynthetically derived acyl oligosaccharides that persist after more labile organic matter has been degraded.