Growing microalgae for biolipid production usually involves a lag phase of growth followed by a stationary phase induced by some sort of "stress" This "stress", often nitrogen depletion, induces a switch in the meta­bolism of the microalgae, which encourages the produc­tion of storage lipids in the form of triacylglycerides (TAGs) rather than cell division (Meng et al., 2009; Widjaja et al., 2009). Currently microalgae can be grown at industrial scale autotrophically in open raceway ponds (Sapphire Energy, 2013) or closed photobioreactor (PBR) systems (Solix BioSystems, 2013). In addition, many microalgae species have the ability to grow heterotrophically, in closed fermenters, given a suitable carbon source (Solazyme Inc., 2013). Open culture sys­tems, such as race way ponds, are significantly lower cost in terms of capital expenditure. They require greater land area than closed systems and are more prone to contamination by invasive species. Water loss due to evaporation can also be a significant problem when compared to closed systems (Chisti, 2007; Pulz, 2001; Sheehan et al., 1998). Closed systems, on the other hand, such as PBRs or fermenters are by their nature closed and thus less likely to be contaminated. Nutrient concentration can be more easily controlled and water loss through evaporation is negligible. However, some have argued that loss of cooling water, used to control temperature, negates any savings made from using a closed culture system. The tighter control over culture conditions facilitated by a closed culture system, along with more sterile cultures, results in PBRs producing much greater levels of microalgae biomass, when compared to raceway ponds. However, the increased production capability must be offset against the much larger capital cost involved in commissioning and main­taining a closed culture system (Carvalho et al., 2006; Pulz, 2001; Ugwu et al., 2008). Hybrid systems have also been proposed whereby a closed system is used for the log phase production of biomass and the nutrient depleted lag phase is allowed to occur in large raceway ponds. It is hoped that the relatively concentrated inoc­ulation of the raceway ponds will not allow any invasive species to become established (Greenwell et al., 2010; Huntley and Redalje, 2007; Rodolfi et al., 2008).

Microalgae present significant potential as a source of biolipids for bioenergy over more traditional sources of biolipids such as palm, soya or Jatropha for a number of reasons. Firstly, the oil content of microalgae as a percentage of the dry weight, shown in Table 12.3, is generally in the range of 20—70%, although levels above 40% are rarely observed (Borowitzka, 1988). Similarly, the potential yield of biolipids and derived biodiesel from microalgae per area far outweighs that of any current oilseed crop. For example, one of the best available studies of large-scale algae cultivation produced 0.1 g/l day or 20—23 g dry weight/m2 day. A conservative lipid content of 30% could therefore yield 24,000 l biodiesel/ha year (Moheimani and Borowitzka, 2006; Schenk et al., 2008). This compares extremely favorably with both Jatropha (18921 biodiesel/ha year) and oil palm (5950 l biodiesel/ha year) (Schenk et al., 2008).

The high potential yield of biodiesel from microalgae — derived biolipids is due to a number of factors including the growth rate of microalgae (Scott et al., 2010) all year round production capability (Schenk et al., 2008) and the higher photon conversion efficiency compared to terres­trial plants (Melis, 2009). Unlike algae-derived biofuels, first-generation biofuels directly competed with food crops for arable land sparking the "Food vs Fuel" debate (Gui et al., 2008). Although second-generation fuel crops such as Jatropha can grow on marginal land (Francis et al., 2005), microalgae are capable of growing on nonarable land ensuring competition for land with food crops is significantly reduced. Similarly, in terms of other resource demands, 1 kg of algae biomass requires 1.83 kg of CO2 to grow (Chisti, 2007) and much research has investigated the potential of indus­trial flue gases as a source of this CO2 (Bilanovic et al.,

2009) . This possibility of both sequestering excess CO2 from flue gases that would otherwise be released into the atmosphere, while also increasing the growth rate of microalgae to be used for bioenergy, offers both environmental and economic advantages (Pires et al., 2012; Yun et al., 1997). More recently, the apparent "peak phosphorus" problem has been identified whereby phosphorus will become a limiting resource in agriculture. As a result, the potential industrial scale culture of microalgae, which requires a phosphorus

and nitrogen source for growth, would also be affected (Cordell et al., 2009). Both phosphorus and nitrogen are available in plentiful supply within waste water streams (Sawayama et al., 1995; Yun et al., 1997).

Commercial harvesting of algae blooms from waste­water has already been demonstrated in New Zealand (Aquaflow, 2013) and the use of wastewater streams as a nutrient source in large-scale cultivation of microalgae
has been well studied and implemented. Similarly, in terms of water usage, microalgae cultivation, particu­larly in closed cultivation systems, demonstrates signif­icant water savings when compared to traditional biofuel crops. Many microalgae species are also capable of growing in brackish water most notably Dunaliella salina (Weldy and Huesemann, 2007).