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When a calcareous red alga, Serraticardia maxima (Ohoshikoro) was illuminated in sea water containing H14CO3 , floridoside, floridean starch and trehalose that are major soluble carbohydrates in this alga were rapidly labelled in this der . During the dark culture following the illumination in sea water free of radiocarbon, the radioactivities and the absolute quantities of floridoside and trehalose remained almost unchanged while those of floridean starch were markedly decreased .. On the other hand, feeding experiments with 14C-floridoside and 14C-trehalose on chopped algal fronds revealed that these carbohydrates were rapidly interconvertible in the intact cells. Moreover, the analysis of Ca14CO3 deposited in cell wall during the feeding of the fronds with 14C-floridoside and 14C-trehalose indicated that these carbohydrates were rapidly consumed by respiration of the cells. From these results, it was concluded that these carbohydrates are storage products and in particular, floridoside and floridean starch seem to play similar physiological roles to sucrose and starch in higher plants, respectively . A small amount of laminitol was foundin this red alga, but the cyclitol appears to be an inert product .