Microalgae strains with potentially high lipid content (e. g., as determined by Nile red staining) need to be cultured to increase biomass and directly compared to each other in larger cultivation systems to assess their poten­tial as biodiesel feedstock. Initial tests of the most promising algae strains usually are carried out at laboratory-scale using culturing flasks and other vessels, such as hanging bags, under well-defined growth conditions. The test should follow a standard protocol over a certain growth period to allow direct comparisons between strains in terms of growth rate and lipid ac­cumulation (“lipid productivity”). It should be noted though that a standard assay does not take into consideration the potential of certain microalgae under carefully optimized conditions. An example of this assay is the fol­lowing that is routinely used by our laboratory to compare lipid productiv­ity: Pure (but not axenic) algae strains are cultured in F/2 medium (fresh or seawater) until near stationary growth occurs (less than 20% growth in 24 h as determined by cell counts). An inoculum of 5 mL of this culture is then used to start growth in 20 mL fresh F/2 medium exactly at 2 h after the start of the light cycle. The culture is then grown and monitored by cell counting for 7 days after which medium is replaced with nutrient-free water. Nutrient starvation is conducted after that for another 2 days of cultivation to test the potential for rapid TAG accumulation. In addition, biomass is collected at the end of the experiment for lipid content analysis. This assay is useful for screening of growth and lipid producing capacity of microalgae, leading to selection of potentially useful strains. The best candidate strains with po­tential for biodiesel production should then be used to optimize parameters for rapid growth, lipid induction, harvesting/dewatering and oil extraction. While most of these parameters are typically optimized under small-scale laboratory conditions, it seems advisable to move towards larger size out­door cultivation conditions as soon as possible, as these are typically quite different. Parameters, such as salinity, nutrient composition, pH and cell density can be controlled to some extent, but other factors such as tempera­ture, irradiation and the co-cultivation of other organisms are much harder to control under outdoor conditions.

In summary, it is advantageous to isolate and screen a large number of local microalgae strains and test these under mid-scale outdoor condi­tions as soon as possible to be close to conditions that maybe expected for large-scale cultivation. Figure 2 provides a step-by-step overview of how microalgae may be rapidly isolated and selected for larger scale biodiesel production.