INHIBITION BY DCB OF CELL-WALL POLYSACCHARIDE FORMATION IN THE RED MICROALGA PORPHYRIDIUM SP (RHODOPHYTA)

The cell wall of the rhodophyte (red) microalgae is composed of a comp lex amorphous polysaccharide containing xylose, glucose and galactose as the main sugars, together with some minor sugars, sulphate and prot ein. The rigid microfibrillar cellulose layer of red macroalgal walls is absent in red microalgae. However, 2,6-dichlorobenzonitrile (DCB), a herbicide that inhibits cellulose polymerization, was found to inhib it cell wall formation in the red microalga Porphyridium sp. When DCB (200 muM) was added upon inoculation, the algal culture died. However, when added at the stationary phase of growth DCB had no effect on cel l concentration, but it did inhibit the production of the cell wall po lysaccharide. DCB was also found to prevent the regeneration and proli feration of protoplasts of Porphyridium sp. In the presence of DCB the burst rate of protoplasts exposed to hypotonic conditions was higher than that of the control. This indirectly confirmed our observations t hat cell wall formation was inhibited in the presence of DCB. A sponta neous DCB-resistant mutant designated DCB-RP-1 was isolated. The mutan t was morphologically similar to the wild type, but its cell wall comp osition was modified, the main change being in the xylose constituent of the polysaccharide, which was 86.4% in the mutant vs. 41.2% in the wild type.