Morphological and chemotaxonomic studies on Laurencia composita and L. okamurae (Ceramiales, Rhodophyta)

Abstract

Two red algal species, Laurencia composita Yamada and L. okamurae Yamada (Rhodomelaceae, Ceramjales), that grow in Japanese waters have often been confused with each other being similar in gross morphology. They can be morphologically distinguished by branching pattern (polystichous in L. composita and tristichous in L. okamurae) and the frequency of lenticular thickenings in the walls of medullary cells (rare in L. composita and frequent in L. okamurae). The two species have different ecological preferences: L. composita grows in the upper to mid-intertidal zones exposed to higher levels of desiccation, while L. okamurae grows in the lower intertidal and upper subtidal zones. The two species can also be distinguished by their characteristic secondary metabolites: L. composita produces chamigrane-type sesquiterpenoids such as prepacifenol, pacifenol, prepacifenol epoxide and johnstonol, whereas L. okamurae produces cyclolaurane-type sesquiterpenoids such as laurinterol and debromolaurinterol. Furthermore, the production by L. composita of laurencenynes, precursors of many C15 non-terpenoids containing a terminal acetylenic or bromine group, may be a sign of diversification in secondary metabolite synthesis of this species.