EFFECT OF LIGHT-INTENSITY ON THE PROXIMATE BIOCHEMICAL AND FATTY-ACID COMPOSITION OF ISOCHRYSIS SP AND NANNOCHLOROPSIS-OCULATA FOR USE IN TROPICAL AQUACULTURE

The total protein, carbohydrate, lipid and ash compositions, and fatty acid contents of two species of marine microalgae, the eustigmatophyte Nannochloropsis oculata (formerly 'Chlorella sp., Japan') and the chrysophyte Isochrysis sp. (Tahitian) used in tropical Australian mariculture, were studied. The microalgae were grown under a range of culture conditions (4 1 and 60 1 laboratory culture, 300 1 bag culture, and 8000 1 outdoor culture) and four light regimes (100 to 107-mu-E m-2 s-1, 240 to 390-mu-E m-2 s-1, 340 to 620-mu-E m-2 s-1, and 1100 to 1200-mu-E m-2 s-1 respectively) to determine the effect of light intensity on the chemical composition of large scale outdoor cultures. Laboratory and bag cultures were axenic and cultured in Walne medium while outdoor cultures were grown in a commercial medium designed for optimum nutrition in tropical outdoor aquaculture operations. Change in growth medium and photon flux density produced only small changes in the proximate biochemical composition of both algae. N. oculata and Isochrysis sp. both showed a trend towards slightly lower carbohydrate and higher chlorophyll a in shaded outdoor culture. Isochrysis sp. showed significant concentrations of the essential polyunsaturated fatty acid 22:6(n-3) (docosahexaenoic acid) from 5.3 to 10.3% of total fatty acid, and 20:5(n-3) (eicosapentaenoic acid) ranged from 0.6 to 4.1%. In contrast, N. oculata had high concentrations of 20:5(n-3) (17.8 to 39.9%) and only traces of 22:6(n-3). The fatty acid composition of Isochrysis sp. grown at high photon flux density (1100-1200-mu-E m-2 s-1) under outdoor culture showed a decrease in the percentage of several highly unsaturated fatty acids, including 20:5(n-3), and an increase in 22:6(n-3). N. oculata showed a similar decrease in the percentage of 20:5(n-3). High light intensity caused a decrease in the ratio of total C16 unsaturated fatty acids to saturated 16:0 in N. oculata, and a decrease in the ratio of total C18 unsaturated fatty acids to saturated 18:0 together with a decrease in the ratio of total unsaturated fatty acids to total saturated fatty acids in both microalgae.