At larger scale, two popular cultivation systems have been used for mi­croalgal biomass and lipid production: open raceway ponds and closed photobioreactors. At present, open pond systems, especially large raceway ponds, are much more widely used, but bear the risk of attracting compet­ing algae, grazers or viruses [38]. Although minimizing the cost of algae farming is one of highest priorities to achieve commercial algal biodiesel production, both systems require optimization of complex factors that sat­isfy high level production cultivation.

For example, these include irradiation, nutrients, temperature, dis­solved O2 and CO2 contents, pH, salinity, water quality, mixing efficiency and harvesting ability. Culturing and environmental conditions affect algae productivity, lipid yield and fatty acid compositions. For example, a pilot study showed that high growth rates and lipid accumulation of Chlorella sp. could be achieved primarily by increasing nitrogen concentrations and nitrogen starvation, respectively [39]. Similarly, growth of hydrocarbon- producing Botryococcus braunii was strongly dependent on light, tem­perature, salinity [40], nutrient quantity and composition [41]. Total lipid, carotene and chlorophyll contents of Navicula sp. increased with increas­ing salinity of the medium from 0.5 to 1.7 M NaCl [42]. In some cases, wastewater (e. g., municipal wastewater) can be used as a nutrient source of microalgae growth. Pulz suggested that productivity of 60-100 mg dry weight L-1 d-1 or a biomass concentration of 1 g L-1 is achievable in open pond systems [43]. Table 2 lists some of the desirable traits for the selec­tion process of microalgae with potential for biodiesel production.

FIGURE 2: Suggested 5-step protocol for rapid selection of microalgae for biodiesel production. Step 1: Local sampling sites should be chosen where microalgae frequently undergo adverse conditions; Step 2: Dilution series in growth medium provide the simplest, most cost-effective and fastest method; Step 3: Nile red staining of near stationary cultures followed by visual inspection provides a simple and rapid screening for algae with high lipid accumulation ability; Step 4: A standardized growth assay in the laboratory can provide comparative data on lipid productivity; Step 5: Parameters can be directly optimized under outdoor conditions using mid-scale cultures as these are often very different to small-scale laboratory conditions.