In vivo measurements to estimate culture status and neutral lipid accumulation in Nannochloropsis oculata CCALA 978: implications for biodiesel oil studies

The development of effi cient, rapid and species-specifi c techniques is indispensable for assessing growth and neutral lipid yield in microalga cultures for biodiesel oil production. Nannochloropsis oculata is a small microalgae with a thick cell wall. In vivo techniques to es- timate cell density, chlorophyll a and neutral lipids are reported. A calibration curve of cell density versus optical density was obtained and validated at 540 nm, under different growth phases. Intracellular neutral lipid storage was evaluated with fl uorometry and epifl uorescent microscopy employing fl uorochrome Nile Red. The addition of 5 % dimethyl sulfoxide en- hanced 12.5 times the fl uorescence signal effi ciency. In situ fl uorescence measurements al- lowed estimating the neutral lipid content (NR-FI). Besides, no signifi cant differences were found in the lipid neutral content between gravimetric and triolein methods. The relationship between NR-FI and chlorophyll fl uorescence signals was used as a neutral lipid accumulation index, which is useful in order to establish the optimum harvesting time. Thus, these procedures may be applied for a better monitoring mode of growth and neutral lipid accumulation in N. oculata's cultures at commercial scale.