Effect of seasonal changes in the photosynthates mannitol and laminaran on maturation of Ecklonia cava (Phaeophyceae, Laminariales) in Nishiki Bay, central Japan

Seasonal changes in the contents of two photosynthates, namely, mannitol and laminaran, were investigated in Ecklonia cava Kjellman with reference to its maturation. The samples were collected from a depth of 8 m in Nishiki Bay, central Japan. The mannitol and laminaran contents in the bladelets were measured. We also determined the content of these photosynthates in the bladelets occupying different positions on the frond and in the sorus and non-sorus portions of the bladelets. The maturation of sori initiated in July and peaked in October. The content of the two photosynthates in the bladelets was low in winter; this coincided with the active formation of new bladelets. Mannitol levels were high during the beginning of the maturation season in summer. Laminaran, in particular, was present in the bladelets only in summer. The laminaran content was higher in the sorus portions than in the non-sorus ones; however, the mannitol content was almost equal in both portions. The laminaran content was high only in the bladelets just prior to maturation and in the mature bladelets at the beginning of the maturation season. These results suggested that mannitol and laminaran were important energy sources for the maturation of E. cava from summer to autumn, and laminaran in particular was closely associated with the maturation of reproductive structures.