The surface structure of Botryococcus braunii colony prevents the entry of extraction solvents into the colony interior

The green colonial microalga Botryococcus braunii (race B) has attracted considerable attention because it is known to produce many hydrocarbons. The hydrocarbons are present in the extracellular matrix that connects the cells located in the interior of a B. braunii colony; however, the surface of each colony is covered with a retaining wall and a fibrillar colony sheath. This unique colony structure is thought to be associated with hydrocarbon storage and known to affect the extractability of hydrocarbons by using an organic solvent. In this study, we investigated the relation between hydrocarbon extractability and changes in the colony surface structure after two distinct treatments: culture of the algae in brackish medium or thermal pretreatment of the algae before solvent extraction. Both the treatments improved hydrocarbon extractability when non-polar solvents such as n-hexane were used. Thermal pretreatment inhibited the formation of fibrils on the surface of algal colonies, as revealed by rapid-freezing and freeze-substitution electron microscopy. However, when the alga was cultured in brackish medium, the fibrils on the surface shortened and became less dense. Thus, the fibrillar colony sheath that mainly consisted of saccharic components prevented the entry of organic solvents into the colony interior, and thus hydrocarbon extraction.