Seasonal Changes of Nitrogen Metabolism in the Sporophyte of Laminaria japonica (Phaeophyceae)

Abstract

The seasonal changes of nitrogen metabolism in the sporophyte of Laminaria japonica (Phae-ophyceae) were investigated under a natural environment. The experiment was conventionally distinguished by concentrations of nitrogen compounds dissolved in seawater, that is, a relatively high NO₂^-+NO₃^- concentration period and a low one. In the high-nitrogen period, the prominent nitrogen metabolism was nitrogen uptake, assimilation, and accumulation. After the ambient nitrite+nitrate was exhausted from the seawater, the nitrogen metabolism was displaced to the utilization of the stored nitrogen. When the nitrogen limitation in the blade was severe about two months after entering the low-nitrogen period, the metabolism was characterized by the utilization of catabolite NH₄+ in apical blade tissues. It was suggested from these results that the nitrogen metabolism was divided into three periods: (i) nitrogen uptake and assimilation, (ii) the utilization of stored nitrogen, and (iii) catabolic and re-assimilation periods, with the change in the nitrogen environment. Furthermore, the critical nitrogen content for maintaining maximum growth and the subsistent nitrogen content were 2.4 and 1.3% (on a dry weight basis), respectively. It is sug-gested that these nitrogen contents are useful for predicting the nitrogen status of the blade and for the effective management of L. japonica cultivation.