Annual net primary production in global oceans is estimated at 50 petagrams (50 * 1015 g) of carbon1. The metabolite glycolate is commonly secreted by marine phytoplankton as a side product of photosynthesis, resulting in an estimated annual flux of one petagram2, making it an abundant carbon source in the ocean. Here we show Here we show that ubiquitous marine Proteobacteria assimilate 23 glyoxylate, the oxidation product of glycolate, via the beta-hydroxyaspartate cycle (BHAC). This cycle represents the most efficient route for glyoxylate assimilation described to date, allowing for the direct conversion of this C2 compound into the tricarboxylic acid cycle intermediate oxaloacetate. We identified the gene cluster encoding for the BHAC and characterized its four enzymes, including a previously unknown primary imine reductase. Furthermore, we determined crystal structures of the two key enzymes of the BHAC, investigated the phylogenetic distribution of the pathway and demonstrated presence and active expression of its genes during phytoplankton blooms in the North Sea. Our results add a new facet to the trophic interactions of photosynthetic primary producers and heterotrophic bacterioplankton in the global oceans, closing an overlooked gap in the marine carbon cycle. This proteomic data set compares the Paracoccus denitrificans proteome under glyoxycolate and succinate growth.