Many microalgae are capable of accumulating a large amount of lipids in the cells [10]. On average, the lipid contents typically range from 10 to 30% of dry weight (Table 3). Depending on the specific algae species and their cultivation conditions, however, microalgal lipid production may range widely from 2 to 75% [2]. In some extreme cases, it can reach 70%-90% of dry weight [4,5]. For instance, the freshwater green alga Botryococcus braunii can produce oil (including hydrocarbons) up to 86% of its dry cell weight [44]. This species is being considered as a possible source for biodiesel production in the near future [4], but has the major disadvantage of slow growth rates and a low tolerance for contamination. As a result, lipid productivities (lipid production per area or volume) of other microalgae, such as Nannochloropsis, Chlorella, Tetraselmis and Pavlova sp. are typically much higher [39,45]. Lipid productivity can be dramatically increased by external application of stress factors and is con­sidered a survival strategy for microalgae under adverse conditions. Most notably these include nutrient deprivation, exposure to chemicals, changes in salinity, temperature, pH and/or irradiation [4,39,46]. The composition of fatty acids-containing lipids differs widely among species, but, as men­tioned above, generally includes structural unsaturated polar lipids, as well as neutral storage lipids, mostly in the form of TAG. Significant fatty acids used for biodiesel include saturated fatty acids and polyunsaturated fatty acids (PUFAs) containing 14-18 carbon molecules, such as C14:0, C16:0, C16:1, C18:0, C18:1, C18:2, C18:3 fatty acids [41]. According to European requirements for biodiesel standards, some fatty acids should be excluded because of undesirable properties. For instance, methyl lino — lenate and fatty acid methyl esters with more than four double bonds are limited to 12% due to oxidation properties [47].

It is expected that microalgae that offer a multiple product portfolio as part of a biorefinery, will be most applicable to large-scale commercial cultivation. In a microalgae screening process, besides fatty acids with properties relevant for biodiesel production, some high value products such as protein-rich biomass, omega-3 fatty acids, sterols, antioxidants, vitamins and pigments should also be taken into account. In particular, omega-3 fatty acids from microalgae have received significant attention as a high-value add product, as the current sources of fish oil are unsustain­able due to depleting global fish stocks. A comparison of omega-3 fatty acid contents of different microalgae shows that these differ considerably between species (Table 4).