Acid lysis of macroalgae by marine herbivorous fishes: effects of acid pH on cell wall porosity

It has been demonstrated that treatment of algae at low pH values, similar to those found in the stomachs of herbivorous fishes, damages the algal cel ls, allowing digestive enzymes to enter the cells. However, the effects of the low pH treatment on the porosity of algal cell walls has not been exami ned. We tested the effects of low pH on the porosity of cells of four speci es of dietary algae, Enteromorpha intestinalis and Ulva rigida (Chlorophyta ) and Porphyra sp. and Polysiphonia strictissima (Rhodophyta) consumed by h erbivorous fishes. The uptake of fluorescein isothiocyanate (FITC)-conjugat ed dextrans of different molecular sizes was used to determine the cell-wal l pore size in these algae. Secondly we tested whether acidic conditions in creased the porosity of the algal cell walls by first immersing the algae i n seawater adjusted to a low pH, then used the uptake of the FITC-dextrans into the acid treated cells to measure changes in cell-wall porosity. Limit ing cell-wall pore diameter in E. intestinalis, U. rigida and P. strictissi ma was less than 8.8 nm, and in Porphyra sp. was less than 7.1 nm. The low pH treatment increased the porosity of the cell walls in all four algae. Po rphyra sp. was the most resistant to this low pH treatment, followed by P. strictissima, then E. intestinalis and finally U. rigida. The cell-wall por e size of all algae increased to at least 13.5 nm after 20 min at pH 2.0, a nd after 60 min at either pH 2.5 or pH 3.0. These findings have important i mplications for the ability of marine herbivorous fish to digest these alga e. Fish proteases range in molecular diameter from 4.2 to 5.4 nm and would therefore be able to pass through the cell walls of untreated algae in unde r 10 min. a-Amylases have molecular diameters ranging from 6.1 to 6.5 nm, a nd would require up to 30 min to traverse the algal cell walls. The increas e in algal cell-wall porosity as a result of exposure to low pH conditions in the stomachs of marine herbivorous fishes would allow molecules, similar in size to proteases and a-amylases, to enter the cells in under 5 min, an d is therefore likely to be an important factor in the digestion of intrace llular algal nutrients. (C) 2000 Elsevier Science B.V. All rights reserved.