Culture of the marine diatom Phaeodactylum tricornutum with different nitrogen sources: Growth, nutrient conversion and biochemical composition

Cultures of the marine diatom Phaeodaetylum tricornutum were grown in nitrate, nitrite, ammonia or urea media. Nitrogen transformation efficiencies 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.t1 (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 chlorophylls a and c decreased and the chlorophyll a/c ratio increased when cultures entered the stationary phase. Protein and RNA were higher during exponential growth (about 30% and 8% ash free dry weight (AFDW), respectively), but carbohydrates increased up to 220% in the stationary phase. Although significant differences were observed in cellular contents, the biochemical composition (as % AFDW) of the nitrate, nitrite and 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.