Abstract Organic carbon fixed through the Calvin Cycle can be diverted towards different metabolic fates within and beyond the plastids of photosynthetic eukaryotes. These include export to the cytoplasm and mitochondrial respiration; gluconeogenesis of storage compounds; and the anabolic synthesis of lipids, amino acids and cofactors via the plastidial pyruvate hub. In plants, pyruvate is principally synthesised via the lower half of glycolysis-gluconeogenesis in the cytoplasm, although a secondary plastid-targeted pathway in non-photosynthetic tissue directly links glyceraldehyde-3-phosphate to the pyruvate hub. Here, we characterize a complete plastidial lower half glycolytic-gluconeogenic pathway in the photosynthetic plastids of diatoms, obligately photosynthetic eukaryotic algae that are important contributors to marine primary production. We show that the two enzymes required to complete plastidial glycolysis-gluconeogenesis, plastidial Enolase and PGAM ( bis -phospho-glycerate mutase), originated through recent duplications of mitochondria-targeted respiratory glycolytic isoforms. Through CRISPR-Cas9 mutagenesis and integrative ‘omic analyses in the diatom Phaeodactylum tricornutum , we present evidence that this pathway functions to divert excess plastidial glyceraldehyde-3-phosphate into diverse fates accessed from the pyruvate hub, and may potentially also function in the gluconeogenic direction to permit more efficient management of cellular carbon. Considering meta-genomic data, we show that this pathway is of greater importance in polar and sub-polar oceans, in which diatoms dominate primary production; and considering experimental data, we show that this principally relates to the elongated photoperiods present at high latitudes. Our data provide insights into the functions of a poorly understood yet evolutionarily recurrent plastidial metabolic pathway, and a further explanation for the success of diatoms in the contemporary ocean.