Sporopollenin in the cell wall of Chlorella and other algae: Ultrastructure, chemistry, and incorporation of ¹⁴C-acetate, studied in synchronous cultures

Cells of Chlorella fusca var. vacuolata (Cambridge 211/8p) resisted efforts aimed at producing naked protoplasts by enzymatic degradation of the cell wall, and a study of the development and composition of the wall was therefore undertaken. 1. After cytokinesis has produced naked autospores within the mother cell wall, cell wall formation commences outside the autospore plasma membrane with the appearance of small trilaminar plaques. These enlarge while inter-autospore granular material diminishes in quantity, and they eventually fuse to produce a complete trilaminar sheath around each autospore. 2. A microfibrillar, cellulase digestible, layer is deposited between the trilaminar component and the plasma membrane. Meanwhile the corresponding microfibrillar component of the mother cell wall is digested leaving only its resistant trilaminar component. 3. The trilaminar component includes a substance considered to be the polymerized carotenoid, sporopollenin, on the basis of its resistance to extreme extraction procedures including acetolysis, and its infra red absorption spectrum. 4. Two phases of sporopollenin biosynthesis were detected by means of pulse and pulse-chase treatments with ¹⁴C-acetate at intervals during the cell cycle in synchronous cultures. One coincides with the formation of the sporopollenin-containing trilaminar wall component, and the other is 6-8 hours earlier while the cells are in karyokinesis. The former yields labelled sporopollenin directly and the latter probably represents formation of a precursor. 5. Of five other strains of Chlorella tested, only one possesses sporopollenin, and so does one Scenedesmus and two out of three strains of Prototheca. 6. Examination of the wall structure of the above algae suggest a relationship between the presence of sporopollenin and the development of an outer, trilaminar wall component. 7. A survey of the literature gives support to this hypothesis and further suggests that the ability to synthesise sporopollenin is related to the ability to produce secondary carotenoids. 8. The significance of the findings is discussed.