Colony sheath formation is accompanied by shell formation and release in the green alga Botryococcus braunii (race B)

Botryococcus braunii race B is a promising candidate for renewable biofuel production. Its colony organization is unique. Cells are connected by extracellular biopolymers containing hydrocarbons at their basolateral regions, enclosed by a retaining wall, and form a sphere inside a colony sheath composed of polysaccharide fibrils. This unique colony structure is a key for the hydrocarbon production and storage, but how the colony forms remains unknown. We studied ultrastructure of colony sheath and its formation process by rapid freezing and freeze-substitution electron microscopy. Fibrils of colony sheath extend from the retaining wall and are branched, and repel India ink. The sheath formation starts in mitosis and completes during the following daughter cell maturation. In this process, the older retaining wall and thin layers at the cell apex are released as shells and new fibrils formed on the new retaining wall extend into and merge with the pre-existing colony sheath. Its formation also accompanied by the hydrocarbon secretion and accumulation at the basolateral region. The fibrils of colony sheath that extend from a cell wall are replaced by new fibrils after each cell division, while fibrils in the extracellular biopolymers accumulate to form the remaining part of sheath, without replacement.