Ultrastructure of the gland cells of the red alga Asparagopsis armata (Bonnemaisoniaceae)

Localization of natural products in the gland cells of the tetrasporophyte of Asparagopsis armata Harvey was examined using light microscopy, epifluorescence microscopy, and TEM. A. armata produces a range of halogenated metabolites that deter herbivores and inhibit bacterial fouling. The halogenated metabolites accumulate as a refractile inclusion inside specialized gland cells and this inclusion was no longer produced when the alga was cultured without bromine. Gland cells are formed soon after the apical division and can occupy a large portion of the algal volume, up to 10% of some parts of the filament. TEM was carried out on cryofixed and freeze-substituted samples. Ultrastructure studies revealed that gland cells are positioned inside the pericentral cell, originating from the axial cell wall. The refractile inclusion of these gland cells is comprised of numerous electron-translucent vacuoles enclosed by an electron-opaque matrix. Some contents of the inclusion autofluoresced under UV excitation by epifluorescence microscopy. Light microscopy further revealed that stalk-like structures connected the gland cell to the outer wall of the pericentral cell. These stalk-like structures may provide the mechanism for metabolite transfer to the algal surface. Gland cell walls are relatively thin, which in turn would aid the transfer of metabolites to the stalk-like structure. These features of the gland cells provide essential clues to the production and storage of the halogenated metabolites in A. armata and offer new insights into a possible mechanism for their release.