Abstract text: Brown algae have evolved multicellularity independently of plants or animals. A cornerstone of this acquisition was the evolution of an extracellular matrix (ECM) providing strength and suppleness to tissues and facilitating cell-to-cell communication. In brown algae, the ECM is composed primarily of the anionic polysaccharide alginate. An essential feature of this ECM is its ability to be degraded and remodelled to support algal development. However, little is known about the metabolic pathways of ECM degradation in brown algae. Thanks to the recent generation of multiple brown algal reference genomes through the Phaeoexplorer project, and expert annotation of the corresponding carbohydrate-active enzymes (CAZymes), we have identified two CAZyme families likely to be involved in alginate degradation in brown algae and therefore likely to play a crucial role in ECM remodelling. These gene families have shown marked amplifications within the brown algal lineage and present a modular structure with known and putative carbohydrate-binding domains and other uncharacterised domains. Our work now focuses on heterologous expression of these CAZymes in order to characterise their biochemical activity and validate their role in alginate degradation. We are attempting several strategies to overcome the challenges of recombinant expression of these difficult-to-express proteins.