Firstly, an investigation of the kinetic accumulation of intracellular neutral lipid of selected strains during the cell growth cycle was conducted. Fig. 5 shows the growth curve
and cellular lipid accumulation of strain 4 i.e. Thalassiosira sp., as a representative strain. NR fluorescence was positively correlated to lipid accumulation (R2 1⁄4 0.84). Neutral lipid content was low (intensity of 0.6e1.4 relative units) for the first 4 days of culture, after which it increased by twelve times to a peak value on day 10 during the stationary phase. During the exponential phase (day 1 to day 4), cell density increased from 1.6 104 to 10.3 104 cell mL1. Fig. 5 shows that during early exponential phase, neutral lipid production was limited, but then increased substantially during the stationary phase. In addition, NR fluorescence of neutral lipids can be utilized to determine the relationship between lipid accumulation and the growth phase of microalgae [12,27e29]. Typically, protein