CULTURE OF THE MARINE DIATOM PHAEODACTYLUM-TRICORNUTUM WITH DIFFERENTNITROGEN-SOURCES - GROWTH, NUTRIENT CONVERSION AND BIOCHEMICAL-COMPOSITION

Cultures of the marine diatom Phaeodactylum tricornutum were grown in nitrate, nitrite, ammonia or urea media. Nitrogen transformation effic iencies were close to 100% in cultures with nitrate, nitrite or urea, with productivities in dry biomass between 2.2 g.l(-1) (urea) and 2.8 g.l(-1) (nitrate). However growth in ammonia cultures ceased due to a drop in pH of the medium. The biochemical profile was more affected by the growth phase than by the N source. Cellular contents of chlorophy lls a and c decreased and the chlorophyll ale ratio increased when cul tures entered the stationary phase. Protein and RNA were higher during exponential growth (about 30% and 8% ash free dry weight (AFDW), resp ectively), but carbohydrates increased up to 220% in the stationary ph ase. Although significant differences were observed in cellular conten ts, the biochemical composition (as % AFDW) of the nitrate, nitrite an d urea-grown cells was very similar. The highest total fatty acids and polyunsaturated fatty acids (PUFAs) contents in the stationary phase were obtained in the urea-grown cells, with an eicosapentaenoic acid ( EPA) content of 26.8 mg.g(-1) of dry biomass.