Fatty acid composition of microalgae from Adriatic Sea

In this study, the total lipid content and fatty acid composition of four diatom species isolated from Adriatic Sea (Šibenik, Croatia) were examined. Microalgae were isolated by serial dilution and the streaking plate method combined with use of antibiotics and physical separation methods. All four diatom species were screened for their capacity to produce lipids under phototrophic conditions. Moreover, the effect of nitrogen concentration on lipid production of isolated Navicula sp. was investigated under mixotrophic growth conditions with glucose as carbon source. Mixotrophic cultivations were performed at three NaNO3 concentrations: 8, 82 mM (original f/2 medium), 4, 42 mM (nitrogen limitation) and 17, 64 mM (nitrogen excess). In phototrophic cultivation lipid content varied between strains from 24% to 63% of dry weight. Lipid content was 53, 91% in presence of nitrogen excess and it was similar as in original medium where lipid content was 56, 57% of dry weight. The highest lipid content was obtained under nitrogen limitation. The analysis of the fatty acid composition revealed the predominance of C14:0 (11%-21%) and C16:0 (30%-42%) and C16:1 cis 9 (13%-43%) fatty acids in all strains for phototrophic conditions as well as in mixotrophic cultivation for all three nitrogen concentrations. In both experiments polyunsaturated fatty acids 20:5n-3 (EPA) and 20:4n-3 (ARA) comprised a significant proportion of total fatty acid. In phototrophic cultivation the highest content of EPA was obtained in strain S1 (7, 39%) and for ARA in strain S4 (8, 03%). Fatty acid analysis showed significant impact of nitrogen concentration on content of saturated and unsaturated fatty acids. In nitrogen excess conditions content of EPA and ARA was increased from 5, 42 % to 8, 24 % and from 2, 45% to 3, 75% respectively when compared with original medium. Nitrogen limitation resulted in significant reduction of these fatty acids content with obtained values of 2, 58 % for EPA and 0, 4% of ARA. Moreover, growth under nitrogen excess decreased content of saturated fatty acids from 41, 59 % to 34, 77 % comparing to original medium while nitrogen limitation resulted in increment up to 47, 56%. Consequently, content of mono- and poly- unsaturated fatty acid of 65, 54 % achieved in presence of nitrogen excess was higher than in original medium and under nitrogen limitation where their content was 58, 41% and 52, 44%, respectively.