Nitrogen uptake and growth responses of seedlings of the brown seaweed Sargassum hemiphyllum under controlled culture conditions

For the purpose of indoor mass culture of seedlings of Sargassum hemiphyllum during spring and summer, a series of experiments were designed to evaluate nitrogen (N) uptake, growth performance, and N accumulation of seedlings. Transient uptake rates by N-limited seedlings for nitrate (NO3−), ammonium (NH4+), and urea were more rapid than uptake rates by N-replete seedlings. NO3− uptake revealed a rate-saturating uptake kinetic for both N-limited and N-replete seedlings. Uptake rates of NH4+ and urea for N-limited seedlings were linear, whereas the N-replete seedlings displayed Michaelis-Menten curves. The slope of the linear models or equivalently affinity calculated from Michaelis-Menten curves was highest for NH4+ and lowest for urea, indicating that seedlings were more efficient at taking up NH4+. Tissue N content in seedling increased with increasing N concentrations, with the highest values with all three N sources at 150 μM. The N requirement to support maximum growth varied 2.2-fold from 0.39 ± 0.15% dry weight (DW) in NH4+ to 0.84 ± 0.09% DW in NO3−, and hence, seedlings became more easily N limited at low NO3− availability. The results showed that seedlings cultured in NO3− enrichment (50–100 μM) may be better able to meet their maximum growth than at high NH4+ availability, although seedlings have higher NH4+ uptake capacities.