Abstract
Cladosiphon okamuranus is a major cultivated seaweed in the Ryukyu Islands. There has been a marked fluctuation in annual production in recent years. To resolve the problem, our research group investigated the effect of light irradiance and color on the development of an erect thallus from a discoid germling and nutrient uptake characteristics of the sporophyte. Discoid germlings cultured with rich nutrients showed higher germination rates of erect thalli than those in sterilized seawater. This result indicates that the nutrient is required to develop the erect thallus. Moreover, the low irradiance and blue light conditions might promote the development of an erect thallus. This finding coincides with our past study and the fishermen's rule that the cultivation nets at the nursery stage are installed in deep areas. Uptake rates of NH4+ and PO43- in the sporophytes increased as the substrate concentration increased, whereas NO3- uptake rates increased until about 20 μM and then gradually decreased. The relatively higher uptake rates for NH4+ may indicate an adaptive property for efficient absorption of transient NH4+ supply from groundwater discharge in the subtropical Ryukyu Islands under low nitrate conditions. Collectively, our findings indicate the importance of environmental condition on the development of erect thallus and the nutrient uptake of sporophyte. Therefore, this study has the possibility to contribute for the stabilization of productivity.
