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 cells, 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 examined. We tested the effects of low pH on the porosity of cells of four species of dietary algae, Enteromorpha intestinalis and Ulva rigida (Chlorophyta) and Porphyra sp. and Polysiphonia strictissima (Rhodophyta) consumed by herbivorous fishes. The uptake of fluorescein isothiocyanate (FITC)-conjugated dextrans of different molecular sizes was used to determine the cell-wall pore size in these algae. Secondly we tested whether acidic conditions increased the porosity of the algal cell walls by first immersing the algae in 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. Limiting cell-wall pore diameter in E. intestinalis, U. rigida and P. strictissima 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. Porphyra sp. was the most resistant to this low pH treatment, followed by P. strictissima, then E. intestinalis and finally U. rigida. The cell-wall pore size of all algae increased to at least 13.5 nm after 20 min at pH 2.0, and after 60 min at either pH 2.5 or pH 3.0. These findings have important implications for the ability of marine herbivorous fish to digest these algae. 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 under 10 min. α-Amylases have molecular diameters ranging from 6.1 to 6.5 nm, and would require up to 30 min to traverse the algal cell walls. The increase 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 α-amylases, to enter the cells in under 5 min, and is therefore likely to be an important factor in the digestion of intracellular algal nutrients.

[1]  A. Gertler,et al.  Pancreatic proteolytic enzymes from carp (Cyprinus carpio)—I purification and physical properties of trypsin, chymotrypsin, elastase and carboxypeptidase B , 1981 .

[2]  B. Sanwal,et al.  MODERN METHODS OF PLANT ANALYSIS , 1955 .

[3]  W. L. Zemke-White,et al.  Acid lysis of macroalgae by marine herbivorous fishes: myth or digestive mechanism? , 1999 .

[4]  U. Sabapathy,et al.  A quantitative study of some digestive enzymes in the rabbitfish, Siganus canaliculatus and the sea bass, Lates calcarifer , 1993 .

[5]  M. Wells.  Feeding and digestion , 1978 .

[6]  A. Gelman,et al.  Membrane‐linked digestion in fish , 1997 .

[7]  D. Delmer,et al.  Cell Wall Structure in Cells Adapted to Growth on the Cellulose-Synthesis Inhibitor 2,6-Dichlorobenzonitrile : A Comparison between Two Dicotyledonous Plants and a Graminaceous Monocot. , 1992, Plant physiology.

[8]  D. Wingate.  Comparative physiology of the vertebrate digestive system , 1989 .

[9]  L. Sánchez-Chiang,et al.  Gastricsinogens and gastricsins from Merluccius gayi—Purification and properties , 1981 .

[10]  G. Reeck,et al.  Pancreatic enzymes of the African lungfish Protopterus aethiopicus. , 1970, Biochemistry.

[11]  C. Hawes,et al.  Internalisation of fluorescein isothiocyanate and fluorescein isothiocyanatedextran by suspension-cultured plant cells , 1990 .

[12]  A. Usov,et al.  Polysaccharides of Algae XXXIII: Isolation and 13C-NMR Spectral Study of Some New Gel-forming Polysaccharides from Japan Sea Red Seaweeds , 1983 .

[13]  J. Oláh,et al.  Proteolytic digestive enzymes of carnivorous (Silurus glanis L.), herbivorous (Hypophthalmichthys molitrix Val.) and omnivorous (Cyprinus carpio L.) fishes , 1983 .

[14]  S. Read,et al.  Determination of cell‐wall porosity by microscopy: walls of cultured cells and pollen tubes , 1993 .

[15]  D. Delmer,et al.  Determination of the Pore Size of Cell Walls of Living Plant Cells , 1979, Science.

[16]  R. D. Preston,et al.  The physical biology of plant cell walls , 1975 .

[17]  David H. Evans,et al.  The Physiology of Fishes , 1994 .

[18]  R. C. Karn.  The Comparative Biochemistry, Physiology, and Genetics of Animal α-Amylases , 1978 .

[19]  L. Melton,et al.  The galactan sulfate from the edible, red alga Porphyra columbina , 1981 .

[20]  M. Horn.  Biology of marine herbivorous fishes , 1989 .

[21]  C. Araki.  Chemistry and enzymology of marine algal polysaccharides , 1969 .

[22]  Antony Bacic,et al.  8 – Structure and Function of Plant Cell Walls , 1988 .

[23]  E. Percival,et al.  Algal Walls — Composition and Biosynthesis , 1981 .

[24]  B. G. Kapoor,et al.  The Alimentary Canal and Digestion in Teleosts , 1976 .

[25]  P. Lobel.  Trophic biology of herbivorous reef fishes: alimentary pH and digestive capabilities , 1981 .

[26]  A. Bacic,et al.  Cell Wall Porosity and Its Determination , 1996 .

[27]  I. Chakrabarti,et al.  Digestive enzymes in 11 freshwater teleost fish species in relation to food habit and niche segregation , 1995 .

[28]  K. Clements,et al.  VERTEBRATE HERBIVORES IN MARINE AND TERRESTRIAL ENVIRONMENTS: A Nutritional Ecology Perspective , 1998 .

[29]  P. Squire.  Calculation of hydrodynamic parameters of random coil polymers from size exclusion chromatography and comparison with parameters by conventional methods , 1981 .

[30]  J. Barry,et al.  Human Colonic Bacterial Degradability of Dietary Fibres from Sea‐Lettuce (Ulva sp) , 1997 .

[31]  R. Peters.  Fluorescence microphotolysis to measure nucleocytoplasmic transport and intracellular mobility. , 1986, Biochimica et biophysica acta.

[32]  Harold W. Chapman,et al.  Comparative Physiology of the Vertebrate Digestive System, 2nd ed , 1997 .