Factors affecting the production of biomass by a nitrogen-fixing blue-green alga in outdoor culture

Abstract

The effectiveness of an aeration-shaking (air-lift) system for outdoor biomass photoproduction by the N 2-fixing filamentous blue-green alga Anabaena variabilis was evaluated and the influence of relevant factors on the productivity of the system was assessed. Air at a flow rate of 60 liters per liter of cell suspension per h was enough, by itself, to promote adequate turbulence and to supply the gaseous nutrients (CO 2 and N 2) needed for maximal productivity. The addition of either or both, CO 2 and combined nitrogen (as KNO 3 or NH 4Cl), did not result in any increase in productivity. In summer and winter, optimal cell density for a suspension depth of 25 cm was 0·2–0·3 g (dry weight) liter −1 and 0·1–0·2 g (dry weight) liter −1 respectively. Reciprocally, optimal suspension depth for a cell density of 0·2 g (dry weight) liter −1 was 20–25 cm in summer and below 15 cm in winter. Optimal values for pH and temperature were 8·2–8·4 and 30–35°C, respectively. Under optimal conditions, mean productivity values were about 13 g (dry weight) m −2 day −1 in summer and 6 g (dry weight) m −2 day −1 in winter. Net protein content of A. variabilis cells was higher than 50%, and nitrogen accounted for 10% of the dry biomass. From the productivity and nitrogen content data, the N 2 fixation rate in outdoor cultures of A. variabilis can be estimated to be higher than 1 g N m −2 day −1, i.e. more than 3 t N per hectare per year when values are extrapolated both in time and area.