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 complex amorphous polysaccharide containing xylose, glucose and galactose as the main sugars, together with some minor sugars, sulphate and protein. 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 inhibit 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 cell concentration, but it did inhibit the production of the cell wall polysaccharide. DCB was also found to prevent the regeneration and proliferation 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 that cell wall formation was inhibited in the presence of DCB. A spontaneous DCB-resistant mutant designated DCB-RP-1 was isolated. The mutant was morphologically similar to the wild type, but its cell wall composition 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.