Application of chemostat culture to nutrient uptake rate measurements by the macroalgae Saccharina japonica var. religiosa (Phaeophyceae) and Ulva australis (Ulvophyceae)

In this study, we applied a chemostat culture method, for the first time, to measure the nutrient uptake rate of macroalgae. We examined two methods of measuring the nutrient uptake rate of two macroalgae, Saccharina japonica var. religiosa and Ulva australis, by comparing nutrient uptake kinetics between the chemostat culture and batch culture. In the chemostat culture, the nutrient concentration was kept constant by monitoring the change in nutrient concentration using an Auto Analyzer in real time and adding nutrients to compensate for the macroalgae's nutrient consumption. The nutrient uptake in the chemostat culture could be best fitted to the Michaelis–Menten saturation kinetics. In the batch culture, the nutrient concentration decreased with time, either constantly or exponentially due to a rapid uptake of nutrients by the macroalgae. The nutrient uptake rate in the batch culture generally showed a scattered relationship with nutrient concentration, with a weak fitting to the Michaelis–Menten saturation kinetics. This discrepancy seemed to be partly because the change in nutrient concentration was large between the sampling intervals in the batch culture. Determining an appropriate sampling interval for detectable concentration change is difficult unless the nutrient concentration is measured in real time. Therefore, the application of the chemostat culture method to the measurement of the uptake rate by macroalgae could greatly improve our understanding of nutrient uptake kinetics.