The intended final biofuel product defines successful microalgae cultivation. If biodiesel is the final product, algal strains should be selected and cultured to produce maximal saturated fatty acids. If biocrude is the desired product, high organic content, or a simple abundance of biomass, is required. Whatever the target product, suc­cessful cultivation requires specific environmental condi­tions to drive the production of specific fuel precursors. Major parameters that influence biomass production include adequate light (wavelength and intensity), tem­perature, CO2 concentration, nutrient composition, salinity, contaminants, and mixing conditions.

PHOTOTROPHIC MICROALGAE

Phototrophic microalgae use carbon dioxide (carbon source), sunlight (energy source), and nutrients to prolif­erate. Two properties of light energy are important for algal growth and metabolism: quality of the light spec­trum and quantity of the light photons. As phototrophs, light-harvesting pigments (chlorophyll and carotenoids) absorb light at specific wavelengths to drive the photo­synthetic process. Light absorption, however, is hin­dered both by light scattering through increasing depths of the culture medium and by mutual shading as the culture increases in density. Antenna structures of microalgae are excessively efficient at harvesting light energy, absorbing all the photons that hit them even though only a fraction of those photons are used for photosynthesis. This deprives nearby algae from absorbing photons and consequently leads to low pro­ductivity. Aggressive mixing of the culture mitigates some of these effects, but cannot completely overcome the light penetration limitations inherent in a photosyn­thetic system.

HETEROTROPHIC MICROALGAE

Several wild-type and genetically modified species of microalgae have been reported capable of growing pho — totrophically, heterotrophically or both (mixotrophi — cally). Unlike phototrophic algae that require light energy, heterotrophic algae have no such requirement. Instead, these algae utilize organic carbons supplied in the media to drive cellular proliferation and lipid accu­mulation. Without the limitations imposed by inefficient light harvesting due to mutual shading and light scat­tering in the medium, the densities of heterotrophic cul­tures can far exceed the densities achieved in phototrophic systems. Increased densities can translate to higher biofuel precursor yields. For example, when Chlorella protothecoides was grown heterotrophically us­ing an organic carbon source, oil accumulation far exceeded that seen in corresponding autotrophic cells (Miao and Wu, 2004). Hence, heterotrophic production has several advantages over phototrophic systems including increased densities that eliminate the need for dewatering, and increased process control that facil­itates the maintenance and rapid growth of monocul­tures and the creation of a consistent product. The primary limitation for commercial-scale heterotrophic production of biofuel oils in microalgae is the cost of the organic carbon source. Sugars such as glucose and acetate have been utilized as the primary carbon source at the bench scale, but become cost-prohibitive at pro­duction scale. It is therefore unsurprising that increased efforts to identify microalgal species that can thrive on waste sugars, such as bagasse or cellulosic waste, are underway.