MANIPULATION OF THE BIOCHEMICAL-COMPOSITION OF THE EICOSAPENTAENOIC ACID-RICH MICROALGA ISOCHRYSIS-GALBANA IN SEMICONTINUOUS CULTURES

The marine microalga Isochrysis galbana was cultured semicontinuously with two nutrient concentrations [2 and 4 mmol of nitrogen (N)/litre] and five rates of daily renewal of culture medium (from 10 to 50%) in order to optimize the productivity of the valuable polyunsaturated fat ty acids eicosapentaenoic acid (EPA, C-20:5,C-n-3) and docosahexaenoic acid (DHA, C-22:6,C-n-3) The N conversion rate and chlorophyll conten t indicated that a nutrient concentration of 2 mmol of N . I-1 was lim iting, except for a renewal rate of 50%, while cultures maintained wit h 4 mmol of N . I-1 were nutrient-limited only up to a renewal rate of 20%, Carbohydrates, but not lipids, were stored by I. galbana under n utrient deficiency, but the composition of the fatty acid fraction was strongly affected by culture conditions, The degree of unsaturation o f fatty acids increased with increasing renewal rate, mainly at the ex pense of C-16:0 and C-16:1 fatty acids, which decreased under the same conditions, EPA increased from 9% of total fatty acids with 2 mmol of N . I-1 and a renewal rate of 10% to values around 25% of total fatty acids achieved in nutrient saturated cultures for both nutrient conce ntrations, On the other hand, the percentage of DHA was not strongly a ffected by either renewal rate or nutrient concentration, which was re flected in very little variation in the productivity, which remained i n the range 0.9-1.3 mg.day(-1).l(-1). Owing to the decrease of cell de nsity with increasing renewal rates, maximal EPA productivity, 4.6 mg. day(-1).l(-1), was achieved with 4 mmol of N . I-1 and intermediate re newal rates.