Floridoside as a carbon precursor for the synthesis of cell-wall polysaccharide in the red microalga Porphyridium sp. (Rhodophyta)

Although red algae are known to be obligatory photoautotrophs, the red microalga Porphyridium sp. was shown to assimilate and metabolize floridoside. A pulse-chase experiment with [¹⁴C]floridoside showed that at the end of a 240-min pulse, 70% of total ¹⁴C-uptake by the cells remained in the floridoside fraction. To evaluate the assimilation of floridoside by Porphyridium sp. cells, we exposed Porphyridium sp. not only to [¹⁴C]floridoside but also to its constituents, [¹⁴C]glycerol and [¹⁴C]galactose, as compared with [¹⁴C]bicarbonate. The extent of incorporation of [¹⁴C] galactose by the Porphyridium sp. cells was insignificant (50-80 dpm.mL^-1), whereas uptake of ¹⁴C from [¹⁴C]glycerol into the algal cells was evident (2.4 × 10³ dpm.mL^-1) after 60 min of the pulse. The pattern of ¹⁴C distribution among the major constituent sugars, xylose, glucose and galactose, of the labeled soluble polysaccharide was dependent on the ¹⁴C source. The relative content of [¹⁴C]galactose in the soluble polysaccharide was highest (28.8%) for [¹⁴C]floridoside-labeled culture and lowest (19.8%) for the [¹⁴C]glycerol-labeled culture. Upon incubation of [¹⁴C]floridoside with a crude extract of a cell-free system prepared from nonlabeled cells of Porphyridium sp., the label was indeed found to be incorporated into the sulfated polysaccharide. Our results suggested that the carbon metabolic pathway in Porphyridium sp. passes through the low molecular weight photoassimilatory product-floridoside-toward sulfated cell-wall polysaccharide production.