Biochemical characterization of Rhodomonas sp. Hf-1 strain (cryptophyte) under nitrogen starvation

Rhodomonas sp. Hf-1 strain (cryptophyte) has been known as an excellent feed for some marine animals in aquaculture. In this study, the Hf-1 strain was cultured under different nitrogen concentrations to evaluate the effect of N-starved conditions on its growth and biochemical profile. Two nitrate concentrations of 0.9 mM (Low-N) and 3.5 mM (High-N) were supplemented to the media. The cell suspensions were sampled on day 3 and 7 and analyzed for photosynthesis pigments, protein and relative fatty acids contents. The nitrate in Low-N and High-N cultures was exhausted by day 3 and 7, respectively, and growth in Low-N culture was significantly lower than in High-N culture. Phycoerythrin (PE) content decreased in both cultures compared to chlorophyll (Chl) a and c, and a cell-color change was only observed in Low-N culture, indicating that PE was preferentially degraded compared with Chl a and c when nitrogen in the medium was limiting. The intercellular carbon and hydrogen content in the Hf-1 cells cultured in both media showed similar changes throughout the experiment, hence C/H ratios did not differ in between. On the other hand, the intercellular nitrogen content in the Hf-1 cells cultured with High-N medium increased from 5.6 to 10.3% from day 3–7. However, Low-N culture showed a decrease in nitrogen content from 8.1 to 4.1%. Consequently, C/N ratio in Low-N and High-N culture showed entirely different tendency. Similarly, the protein content in the Hf-1 cell cultured with High-N medium increased during 4 days. In contrast, Low-N culture showed a decrease by 30% in protein content. Relative fatty acids content in the Hf-1 cell was lower in Low-N culture than High-N culture through the experiment. The dominant fatty acids in Low-N culture were poly-unsaturated fatty acids (PUFA) on day 3, which accounted for 37.4% of total fatty acid. However, the relative content of PUFA reduced to 24.7% of total fatty acid by the end of the experiment, and the dominant fatty acids shifted to saturated fatty acids (SFA). These data indicate that nitrogen starvation in the medium induced a simultaneous decrease in protein and relative fatty acids contents with the cell-color change for the Hf-1 cell, suggesting the possibility that this behavior on the cell-color change can be used as a valid indicator to determine the harvest timing of Rhodomonas species visually.