Absorption of Inorganic and Organic Nitrogen by Nannochloropsis sp. in a Practical Culture

Abstract

Relationships between ammonium, nitrate, and organic nitrogen concentration in the culture medium of Nannochloropsis sp. (marine Chlorella) was studied in order to know the depletion of the nutrients by recycling process in an artificial ecosystem culture of rotifer. Desorption rate of ammonia from the culture medium to air due to aeration was also estimated to observe nitrogen budget of the culture. In both Exp. 1 and 2, ammonium sulphate and urea were mixed into 20‰ salinity natural filtered seawater as medium. Whilst, in Exp. 3, shark silage was used as nutrient. Nannochloropsis sp. was inoculated initially at 2×10⁶ cells/ml. Desorption of ammonia was determined at pH values of 6.5, 8, 9, and 10.
The reduction of the ammonium concentration was more rapid than other types of nutrient. Decreasing concentration of organic nitrogen and nitrate were abserved once the ammonium concentration was lower than 100 μg-at/l. The percentage of organic nitrogen increased up to 80-85% of the total nitrogen and became constant on the 5-7th day of the culture period. Desorption rate of ammonia rose exponentially as pH increased. Desorption of ammonia within the first 5 days of the culture experiment of Nannochloropsis sp. was 0.9%in average of the initial nitrogen content of the culture medium. It could be considered that the nitrogen concentration which was required to maintain high nitrogen content of Nannochloropsis sp., was existed.