Comparative studies on hair cells of two agarophyte red algae, Gelidium vagum (Gelidiales, Rhodophyta) and Gracilaria pacifica (Gracilariales, Rhodophyta)

Abstract

Similarities as well as unexpected differences were noted between unicellular apoplastidic hairs on two agarophytes, Gelidium vagum Okamura and Gracilaria pacifica Abbott. Hairs of both species originate from highly differentiated hair-producing cortical cells (HPCs) and have similar densities (approx. 150 mm−2). Gelidium vagum hairs are localized subapically, whereas those of Gr. pacifica are distributed over almost the entire thallus and are larger. Hair cells of Gr. pacifica increase the thallus surface area by as much as 180% and those on Ge. vagum by 50%. They are involved in active uptake and transport of vital dyes (as nutrient analogues) to HPCs. Cytological features facilitating this transport include: (1) thin walls; (2) metabolically active tips containing numerous mitochondria, ribosomes, ER and Golgi bodies; (3) a central vacuole-like region; and (4) a basal area containing abundant parallel ER longitudinally oriented and appressed to a distinctive asymmetrical pit plug that adjoins the HPC. On the HPC side the plug is flared, with much associated parallel ER. HPCs of Ge. vagum are uninucleate and contain plastids similar to, but smaller than, those of other cortical cells, whereas HPCs of Gr. pacifica are multinucleate with extremely small plastids that lack thylakoids and the ability to autofluoresce. Hair production is initiated as each HPC develops a protuberance devoid of plastids that penetrates the outer wall and is separated from the HPC at cytokinesis. Multinucleate hairs are reported for the first time in any alga–in Gr. pacifica. In addition, both protuberances and hairs ofthis alga are constricted in the wall region distal to the site of cytokinesis. A thick mucilage layer surrounds the thallus of Gr. pacifica only when hairs are present. Hairs of both species were induced in culture using a nitrogen-free medium and a photon flux density >50 μmol m−2 s−1. Those of Ge. vagum also required an extended preconditioning period of short day length and low light, suggesting a seasonal component to the production process. Probable ecophysiological roles for these hairs are discussed.