The influence of culture conditions on the biosynthesis and degradation of polysaccharides (starch and carrageenan) in the red alga Solieria chordalis was investigated. Following 1 week of incubation in the dark, the alga synthesized floridean starch at two irradiances, 50 and 480 μmol photons m−2 s−1 (400–700 nm), in nutrient-unenriched seawater. Upon transfer of these cultured algae to nutrient-enriched seawater at 50 μmol photons m−2 s−1, the stored floridean starch was found to break down. 14C labelling revealed that during floridean starch breakdown the tracer was transferred to carrageenan. These results are interpreted as carbon transfer from stored floridean starch for use in carrageenan biosynthesis.