Brown Algae And Chromophyte Phylogeny

Abstract This chapter examines the evidence for phylogenetic relationships between brown algae (Phaeophyceae) and other chromophytes. Brown algae include the largest and most complex multicellular algal thalli, and they are widely considered as a highly evolved group of protists. They are distinguished by the possession of alginates as a major component of cell walls, and phlorotannins which occur mainly in physodes located in the cytoplasm or vacuole. On the basis of cytological characteristics brown algae show closest affinities with chrysophytes. Flagella, found only on the reproductive cells of brown algae, are basically similar to those of other heterokonts, except that they lack a transitional helix. Ultrastructural and biochemical characteristics of brown algal chloroplasts closely resemble those of chrysophytes and diatoms. Brown algal cells have, in addition, several other features of ultrastructure and organization in common with chrysophytes, and in particular the marine order Chrysomeridales. Brown algae are commonly regarded as having an ancient origin, but the fossil evidence is equivocal and a study of rRNA sequences suggests that they may be much younger than previously thought.