TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Method: The diatom produces at least one highly unsaturated fatty acid (HUFA) chosen from eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and arachidonic acid (ARA), or omega -3 fatty acid. The diatom biomass contains the HUFA level of at least 2%, preferably at least 3% dry cell weight of the biomass, or the EPA at level of at least 2% dry cell weight of the biomass. The diatom biomass comprises mixture of the EPA and DHA at level of at least 2% dry cell weight of the biomass. The volumetric production rate of the biomass is at least 25 g dry weight/l/day, or at least 20 g dry weight/l/day for at least 72 hours. The mean volumetric production rate of the biomass is at least 20 g dry weight/l/day for at least 1 month, or at least 2 months. The method does not include perfusion culture of the cultured diatoms. The diatom is chosen from Nitzschia sp., Cyclotella sp., and Phaeodactylum sp.. The diatom is cultivated heterotrophically. The source of silicate is added continuously during course of the culture. The culture pH is maintained at 7-9 and 15-30 degrees C. The agitation and/or back pressure are provided to maintain dissolved oxygen level at or above 10% of air saturation. The method involves performing replacement of at least 50 vol.%, 10 vol.%, 5 vol.% or 0.2 vol.% culture with fresh medium for every 24 hours, 4 hours, 2 hours, or 5 minutes. The method utilizes at least 500 l culture medium. The method further involves separating the biomass from the culture and concentrating the biomass to produce biomass concentrate. The step (iii) is done in one portion, or multiple portions total to a substantially equivalent volume to the portion removed in the step (ii). The steps (ii) and (iii) are repeated so that at least 30 g dry cell weight of biomass/l active culture collected in 24 hours. At least 20g dry cell weight of biomass is collected per liter active culture in each day for at least 7 consecutive days. The steps (ii) and (iii) are repeated at least 5 times. The density of biomass in the active culture is at least 20 g/l. The steps (ii) and (iii) are performed in sequential manner. The method involves removing portion of the active culture in step (ii) and adding the fresh culture medium in the step (iii) such that there is replacement of at least 50% active culture volume for every 24 hours. The HUFA is in its native form, preferably triglyceride, phospholipid, or glycolipid. The extracted HUFA in its native form is subsequently esterified in presence of ethanol to form fatty acid ethyl ester (FAEE) of the HUFA. The FAEE of the HUFA is purified by chromatography, solvent partitioning, short path distillation, molecular distillation, HPLC and their combinations. EXAMPLE - No suitable example given.