A MODIFIED CELL-WALL MUTANT OF THE RED MICROALGA RHODELLA-RETICULATA RESISTANT TO THE HERBICIDE 2,6-DICHLOROBENZONITRILE

The rigid component of the cell walls of red macroalgae, cellulose, is lacking in the red microalgae. Instead, the cells are encapsulated within an amorphous polysaccharide. These complex sulfated polysaccharides are composed of at least 10 different sugars, but their structure is not known. When the herbicide 2,6-dichlorobenzonitrile (DCB), a compound that specifically inhibits cellulose biosynthesis, was applied to cultures of the red microalga Rhodella reticulata upon inoculation, growth was inhibited. When added during the stationary phase of growth (after cell division had ceased), DCB did not affect cell number but it did inhibit polysaccharide production. A spontaneous mutant resistant to DCB was selected, it had physiological characteristics similar to those of the wild-type parent. The composition of the cell wall polysaccharide of the mutant was totally modified, being composed almost entirely (98% of its dry matter, as compared to 2.9% in the wild type) Of methyl galactose, but retaining the same sulfate content. The molecular mass of the mutant polysaccharide was, however, similar to that of the wild-type parent (approximately 6 x 10(6) daltons), although its viscosity was significantly lower.