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.L. Zemke-White et al. J. Exp. Mar. Biol. Ecol. 245 2000 57 –68
1. Introduction
The porosity or lack of porosity of algal cell walls presents special problems for marine herbivorous fish that seek to utilise algae as a food source. It is generally
believed that marine herbivorous fishes lack the endogenous enzymes necessary to break down the cell walls of marine macroalgae Horn, 1998. Therefore, to gain access to the
intracellular polymeric components e.g., starch of their dietary algae, fish enzymes capable of degrading these polymers must be capable of passing through the algal cell
walls. However, Zemke-White et al. 1999 have shown that a-amylases were incapable of entering live algal cells without the algae first being treated in acidic conditions.
Therefore, these fish must possess some mechanism either to break down, or increase, the porosity of algal cell walls in order to gain access to the intracellular nutrients of
their dietary algae. Four methods have been proposed to overcome this problem Horn, 1989: i acid lysis in a thin walled stomach, where the plant or algal cells are said to be
broken down by acidic gastric conditions, ii trituration by pharyngeal jaws, iii trituration in a gizzard like stomach, and iv degradation by enzymes produced by the
resident microflora. It has been pointed out more recently that these mechanisms are not mutually exclusive Choat and Clements, 1998. Approximately 150 species | 33 of
marine herbivorous fish species ostensibly utilise acid lysis to gain access to intracellular algal nutrients Zemke-White et al., 1999.
While the plant plasma membrane is the primary interface which regulates the passage of substances into and out of the cells Lee, 1980, plant cell walls can also act as filters,
restricting the entry or exit of ions or molecules from cells Bacic et al., 1988. Consequently, the porosity of the cell wall can be an important factor in determining the
communication both between individual cells and between cells and their external environment. While there have been a number of studies on the porosity of angiosperm
cell walls reviewed by Read and Bacic, 1996, a literature search revealed no such studies on the porosity of algal cell walls.
Acid lysis of marine macroalgae was first tested by Lobel 1981. We extended this work Zemke-White et al., 1999 by showing that exposing these algae to the pH levels
found within the stomachs of the fishes enabled digestive enzymes to enter rhodophyte and chlorophyte algal cells and hydrolyse the starch contained within. We demonstrated
that the algal plasma membrane was lysed by the low pH conditions, but we did not examine the porosity of the algal cell walls.
Methods that have been used to determine the pore size of plant cell walls include: i microscopic visualisation of cell wall pores; ii a ‘‘solute exclusion’’ technique, in
which cells are placed in hypertonic solutions of solutes of differing molecular sizes and plasmolysis or cytorrhysis is determined; and iii observation of the uptake of tracer
molecules or particles Read and Bacic, 1996. The last of these methods allows an examination of the abilities of different sized molecules to transport across the cell wall
in close to physiological conditions, and can determine the limiting size of the pores. However, it does not resolve variations in pore sizes or reveal whether pores arise
simply as gaps in the polymer lattice of the cell wall or are specifically constructed pores Read and Bacic, 1996.
In this study we extended our work on acid lysis by examining the effect of low pH
W .L. Zemke-White et al. J. Exp. Mar. Biol. Ecol. 245 2000 57 –68
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on the porosity of algal cell walls. We examined four marine macroalgae: the subtidal Ulva rigida Adams Chlorophyta: Ulvaceae and Polysiphonia strictissima C. Agardh
Rhodophyta: Rhodomelaceae, and the intertidal Enteromorpha intestinalis Chloro- phyta: Ulvaceae and Porphyra sp. Tag no. PYGRosP13 5 98, Museum of New
Zealand, Te Papa this genus is currently under taxomonic revision, pers. com., Glenys Knight, Museum of New Zealand, Te Papa, New Zealand. We determined the ability of
the algal cells to take up fluorescein isothiocyanate FITC conjugated to dextrans of different molecular size to: 1 determine the porosity of algal cell walls in their native
state, and 2 determine any changes in that porosity after immersion in seawater with a lowered pH.
2. Methods and materials
