siliculosus rather than a malate synthase pathway as in diatoms. Changes in the concentrations of glycine and serine, genome annotation and targeted expression analysis together suggest the presence of a classical photorespiratory glycolate pathway in E. γ-Aminobutyric acid was only detected in traces. Amino acid profiles were similar to those of C3-type plants, including glycine/sehne accumulation under photorespiration-enhancing conditions. siliculosus cells contain low levels of polyols, organic acids and carbohydrates. Metabolite profiling was performed across two light-dark cycles and under different CO₂ and O₂ concentrations, together with genome and targeted gene expression analysis. The Ectocarpus siliculosus genome now facilitates integrative studies of the molecular basis of primary metabolism in this brown alga. Knowledge about primary metabolic processes is essential for the understanding of the physiology and ecology of seaweeds.