Cultivation of the brown alga Hizikia fusiformis (Harvey) Okamura: controlled fertilization and early development of seedlings in raceway tanks in ambient light and temperature

Commercial cultivation of the dioecious brown macroalga Hizikia fusiformis (Harvey) Okamura in East Asia depends on the supply of young seedlings from regenerated holdfasts or from wild population. Recent development of synchronized release of male and female gametes in tumble culture provides a possibility of mass production of young seedlings via sexual reproduction. In this paper, we demonstrate that controlled fertilization can be efficiently realized in ambient light and temperature in a specially designed raceway tank in which the sperm-containing water has been recirculated. The effective fertilization time of eggs by sperm was found to be within six hours. Fast growth and development of the young seedlings relied on the presence of water currents. Velocity tests demonstrated that young seedlings of 2–3 mm in length could withstand a water current of 190 cm s⁻¹ without detachment. Culture experiments at 24 h postfertilization showed that elongation of both the seedlings and their rhizoids were not hampered by high irradiance up to 600 μmol photons m⁻² s⁻¹. However, growth was slightly retarded if cultured at a temperature of 16 ^∘C compared to other culture temperatures of 22, 25 and 29 ^°C. No seedling detachment was observed after transfer of the young seedlings to raft cultivation in the sea after one and 1.5 months post-fertilization, indicating the feasibility of obtaining large quantity of seedlings in such a system.