OPTIMUM GROWTH-CONDITIONS AND LIGHT UTILIZATION EFFICIENCY OF SPIRULINA-PLATENSIS (EQUALS ARTHROSPIRA-FUSIFORMIS) (CYANOPHYTA) FROM LAKE-CHITU, ETHIOPIA

Spirulina platensis (= Arthrospira fusiformis) was isolated from Lake Chitu, a saline, alkaline lake in Ethiopia, where it forms an almost u nialgal population. Optimum growth conditions were studied in a turbid ostat. Cultures grown in modified Zarrouk's medium and exposed to a ra nge of light intensities (20-500 mu mol photons m(-2) s(-1)) showed a maximum specific growth rate (mu(max)) of 1.78 d(-1). Quantum yield fo r growth (Phi mu) was 3.8% at the optimum light for growth of 330 mu m ol photons m(-2) s(-1), and ranged from 2.8 to 9.4%. With increase in irradiance, the chlorophyll a concentration decreased, and the caroten oids/chlorophyll a ratio increased by a factor of 2.4. The phosphorus to carbon ratio (P/C) showed some variation, while the nitrogen to car bon ratio (N/C) remained relatively constant, thus causing fluctuation s in the N:P ratio (7-11) of cells. An optimum N:P ratio of about 7 wa s attained in cells growing at the optimum light for growth. Results f rom the continuous culture experiments agreed well with maximum values of photosynthetic efficiency given in the literature for natural popu lations of S. platensis in the soda lakes of East Africa, Lake Arengua de (Ethiopia), and Lake Simbi (Kenya).