Biosynthesis of the cell wall polysaccharide in the red microalga Rhodella reticulata

This research forms part of our ongoing study to elucidate cell wall biosynthesis in red microalgae. Cell wall formation during the cell cycle of the red microalga Rhodella reticulata was followed in cultures synchronized by a regime of dark (12 h), light (12 h), and dilution of the culture thereafter to 1–1.5 × 10⁶cells/ml. Under these conditions, cell number doubled after 24 h, DNA replication occurred between the 6th and 12th hours, and cell division took place between the 8th and 14th hours of the cycle. Cell wall constituents increased only during the light hours, peaking as follows: sulfur at the 2nd hour, protein at the 11th hour, and the various sugars (each at different times) between the 6th and 12th hours of the cycle. Since xylose predominated from the beginning of the cycle, it appears that this sugar was produced first and formed the basic polymer skeleton to which other sugars were attached. Two polymers were produced during the cycle, their sizes (as determined by gel filtration) being 0.5 × 10⁶ and 1.15 × 10⁶ daltons. It thus seems likely that the smaller 0.5 × 10⁶ dalton polymers are produced inside the cells and then excreted into the medium, where they are further polymerized to produce the final-size polymers. The herbicide 2,6-dichlorobenzonitrile (DCB), an inhibitor of cellulose biosynthesis, was previously found to inhibit cell wall formation in red microalgae. When it was added to the cultures at the beginning or at the end of the cycle, no inhibition in cell division was detected, but when it was added at the 8th hour, cell division was significantly inhibited (38%), resulting in an increase in mean cell volume. Addition of DCB did not affect DNA replication or cell wall polysaccharide content or composition, as measured after 24 h of the cycle. It seems that DCB affects an inhibitory phase in cell division and that this inhibition is not necessarily coupled with its inhibition of formation of the sulfated polysaccharide.