Water Relations of the Intertidal Saccate Alga Colpomenia peregrina (Phaeophyta, Scytosiphonales)

The saccate alga Colpomenia peregrina (Sauv.) Hamel uses seawater held within the central cavity to reduce desiccation. Apoplastic fluorescent tracers have shown that seawater used to fill the cavity can enter freely via pits scattered over the surface of the alga. Importance of the pits is suggested by their density [488 ± 155 (sd) cm"] and the fact that the density increases as a function of body size. Based on their ability to metabolize fluorescein diacetate, cells of hair filaments found in the pits are metabolically active. Fine structural observations show that filaments and medullary cells at the base of filaments contain numerous mitochondria and plasmodesmata between cells. Cortical cells, on the other hand, do not contain many mitochondria, but do possess vacuoles and plasmodesmata, features found only in cells subjected to little or no desiccation. However, an external cuticle-like layer is absent and may help to explain the rapid evaporative water loss during periods of emersion. Osmolality of the retained seawater is in a constant state of flux. Desiccation increased osmolality of the fluid, but to levels significantly lower than expected, suggesting the existence of an osmolality regulating mechanism. After re-submersion osmolality of the retained seawater returned to control levels. n uction cavity readily. Unlike other saccate Seawater held in the central cavity of the intertidal algae, such as Halosaccionglandiforme(Gmel.)Rupr. , saccate alga Colpomenia peregrina (Sauv.) Hamel prothe thallus wall of C. peregrina does not possess holes vides a mechanism to avoid desiccation and enhance to allow entry of seawater. The surface of C. peregrina phptosynthetic production (Oates 1985). During does, however, contain abundant pits filled with colemersion, desiccation is avoided and photosynthesis ourless hair filaments (Fritsch 1945, Parsons 1982). prolonged because water from the cavity replaces that The pits and/or hair filaments may provide a pathway lost from thallus cells. This seawater also supplies for uptake of seawater and nutrients. Conversely, loss nutrients and dissolved inorganic carbon, further enof water from the surface during periods of emersion hancing photosynthetic activity. Additionally, since draws seawater through the thallus wall, reducing cells of C. peregrina are subjected to reduced desicdesiccation and supplying nutrients to photosynthetic cation stress during low tide they do not require a cells. For this adaptation to function effectively searecovery period with the incoming tide. water must be able to move freely down an osmotic „ ^ , . « gradient from inside to outside. The absence of a For Colpomenia peregrina to utilize retained seawater cutide.Hke , coveri ±e surface of c. peregrina as a source of moisture and nutrients, seawater must ^^ m2) $uggests ±at ̂ ̂ does not pOSSess 1 Address for reprints. a structural means for water retention. There is, howBotanica Marina / Vol. 31 / 1988 / Fase, l Copyright © 1988 Walter de Gruyter Berlin · New York 58 Oates: Colpomenia water relations ever, potential for seawater remaining within the cavity to become highly saline. It is important that this seawater does not become hypertonic during emersion as this would result in water loss from the tissues leading to plasmolysis and eventually death. There^ fore, C. peregrina must have a means of stabilizing ion concentrations in the retained seawater. The purpose of this study was to examine the pathways utilized to cycle seawater through the thallus of C. peregrina. Material and Methods