Structural colouration is widespread in the marine environment. Within the large variety of marine organisms, macroalgae represent a diverse group of more than 24,700 species. Some macroalgae have developed complex optical responses using different nanostructures and material compositions. In this review, we describe the mechanisms that are employed to produce structural colour in algae and provide a discussion on the functional relevance by analysing the geographical distribution and ecology in detail. In contrast to what is observed in the animal kingdom, we hypothesise that structural colour in algae predominantly functions for a non-communicative purpose, most likely protection from radiation damage, e.g. by harmful UV light. We suggest that the presence of structural colour in algae is likely influenced by local factors such as radiation intensity and turbidity of the water.Biotechnology and Biological Sciences Research Council (Grant ID: BBSRC David Phillips, 13 BB/K014617/1), European Research Council (Grant ID: ERC-2014-STG H2020 639088), Department of Chemistry, Cambridge (Philip and Patricia Brown Next Generation Fellowship), National Centre of Competence in Research “Bio-Inspired Materials”, Adolphe Merkle Foundatio
