Algae constitute an immensely diverse group of photosynthetic eukaryotes that span most lineages of the eukaryotic tree of life. They include both simple unicellular and complex multicellular forms. The green, red and brown algae represent three of the six eukaryotic groups where multicellularity evolved independently. As such, algae offer the opportunity to tackle many questions regarding the origin and evolution of eukaryotic traits. In all of these lineages, the transition from cellular autonomy to cellular cooperation intrinsically involved the evolution of an extracellular matrix.

Algal cells are surrounded by an extracellular matrix or cell wall. This major cellular component is structurally and biochemically complex and is central to many essential functions. The biosynthesis of the cell wall often requires significant amounts of the organism's photosynthetically-fixed carbon and employs a notable contribution of the alga's genetic machinery and membrane trafficking networks. Once deposited onto the cell surface, the cell wall often undergoes modulations that are direct responses to internal developmental prompts and environmental stresses. As a consequence of their deep evolutionary history, much diversity has arisen in the biochemistry and architectural design of algal cells walls. In most algal groups, they consist of load bearing fibrillar polysaccharides embedded in a hydrogel matrix of polysaccharides and proteins. In some taxa, the matrix may complex cations to create a hard surface. In marine algae, the ability to synthesize sulfated polysaccharides is a common trait. In other algal groups, the cell wall manifests in distinctly different forms, as exemplified by the silica-based frustule of diatoms, the plate-like coccoliths of haptophytes, and the various scaly coverings of early divergent green algae.

Algal cell walls are profoundly important to the planet's ecology. Their synthesis represents a significant means of sequestering atmospheric carbon dioxide. They often serve as the basis of food chains or the foundations of biological sediments in the planet's oceans. Algal cell walls are also economically important. In addition to being used as direct food sources by humans, specific extracts of brown and red algae are the bases of food and pharmaceutical additives. Fossilized cell walls are used as abrasives, filters and as light-reflective additives to paints. Finally, the cell walls of the evolutionary progeny of charophycean green algae (i.e. land plants) are one of the key products of agriculture.

The study of algal cell walls has recently blossomed, much of which is due to the application of modern molecular, biochemical and cell biology methodologies. This Research Topic is shedding new light on the biosynthesis and subcellular processing of cell walls as well as their evolutionary origins. Specifically, we encourage submission of Original Research, Reviews, Methods, Perspectives, and Opinions covering the following topics:

- Advances in evolutionary aspects of algal extracellular matrix or cell wall structure.
- Cell wall polysaccharide structure and common structures among different taxonomic groups of algae.
- New methods for studying cell walls.
- Development of new uses for algal cell wall components.
- Biosynthesis of algal extracellular matrixes.

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