Ammonium, nitrate, and urea play different roles for lipid accumulation in the nervonic acid-producing microalgae Mychonastes afer HSO-3-1

摘要

Producing valuable coproducts from oleaginous microalgae is an option to reduce the total cost of biofuel production. Here, the influence of nitrogen sources on biomass yield and lipid accumulation of a newly identified oleaginous green microalgal strain, Mychonastes afer HSO-3-1, was evaluated. Carbon assimilation and the following lipid biosynthesis of M. afer were inhibited to some extent under weak acidic conditions (6 & pH & 7) and any of the tested nitrogen source. The highest lipid productivity of 50.7 mg L-1 day(-1) was achieved with a 17.6 mM nitrogen supplement in the form of urea. The cell polar lipid content was significantly higher than triacylglycerol (TAG), and saturated palmitic acid (C16:0) occupied a dominant position in the fatty acid profiles while culturing M. afer in acidic medium with NH4 (+) as the nitrogen source. Under neutral conditions, the lipid productivities of M. afer cultivated in media containing 17.6 mM of NaNO3, NH4Cl, and NH4NO3 were 76.2, 77.5, and 79.0 mg L-1 day(-1), respectively. The greatest TAG content (58.56%) of total lipids was obtained when NaNO3 was used as the nitrogen source. There was no significant difference in the fatty acid composition of M. afer cells when they were cultivated in neutral media supplemented with NaNO3, urea, NH4Cl, and NH4NO3. Therefore, NH4 (+) was not a suitable nitrogen source for M. afer cultivation due to the additional labor, working procedures, and alkali required to adjust the medium pH. Considering that using urea as nitrogen source could reduce the cost of nutrient salts substantially and urea can be taken up and utilized by most microalgae, it is a preferred nitrogen source. The major properties of biodiesel derived from M. afer HSO-3-1 met biodiesel quality, and nervonic acid concentrations remained at approximately 3.0% of total fatty acids.