BIOSYNTHESIS OF THE CELL-WALL POLYSACCHARIDE IN THE RED MICROALGA RHODELLA-RETICULATA

This research forms part of our ongoing study to elucidate cell wall b iosynthesis in red microalgae. Cell wall formation during the cell cyc le of the red microalga Rhodella reticulata was followed in cultures s ynchronized by a regime of dark (12 h), light (12 h), and dilution of the culture thereafter to 1-1.5 x 10(6)cells/ml. Under these condition s, cell number doubled after 24 h, DNA replication occurred between th e 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 11 th hour, and the various sugars (each at different times) be tween the 6th and 12th hours of the cycle. Since xylose predominated f rom the beginning of the cycle, it appears that this sugar was produce d first and formed the basic polymer skeleton to which other sugars we re attached. Two polymers were produced during the cycle, their sizes (as determined by gel filtration) being 0.5 x 10(6) and 1.15 x 10(6) d altons. It thus seems likely that the smaller 0.5 x 10(6) dalton polym ers are produced inside the cells and then excreted into the medium, w here they are further polymerized to produce the final-size polymers. The herbicide 2,6-dichlorabenzonitrile (DCB), an inhibitor of cellulos e 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 significantl y inhibited (38%), resulting in an increase in mean cell volume. Addit ion of DCB did not affect DNA replication or cell wan polysaccharide c ontent or composition, as measured after 24 h of the cycle. It seems t hat DCB affects an inhibitory phase in cell division and that this inh ibition is not necessarily coupled with its inhibition of formation of the sulfated polysaccharide.