Secondly, to isolate candidate strain(s) that accumulate high neutral lipid, NR was used to stain selected strains in stationary phase. The NR fluorescence of neutral lipid content of selected strains is shown in Fig. 6. Most diatoms responded rapidly to NR staining, where emission of NR fluorescence was representative of total intracellular neutral lipid [12,31]. Ach- nanthes sp. (strain 40) had the highest NR fluorescence inten- sity which corresponded to total lipid, at a relative intensity of 32 au. and 44.5%, respectively. However, Nannochloropsis sp. was not efficiently stained using the conventional NR fluo- rescence method. Thus, the level of total lipid and neutral lipid did not correspond, where the fluorescence of stained cells was weak (see Fig. 6) but gravimetric total lipid content in stationary phase was high (see Table 2). Inefficient NR staining of Nannochloropsis strain has been previously reported by Sheehan et al. [32], and is likely due to cell wall properties of Nannochloropsis sp. which resist dye absorption. In order to address this problem, a modified Nile red staining procedure for Nannochloropsis is required. In this study, therefore only gravimetric lipid content was used to select the most favour- able Nannochloropsis strains.