To maximize biomass production and the accumula­tion of fuel precursors, algal cultures must be supplied with various concentrations of macronutrients, vita­mins, and trace elements depending on species require­ments. While there are limited reports on optimal levels of nutrients required for mass algal cultures, it is gener­ally accepted that required macronutrients are nitrogen and phosphorus (Brzezinski, 1985; Harrison and Berges,

2005) . Trace elements such as cobalt, copper, molybde­num, zinc, and nickel are likewise necessary, and in some species are considered to be effective in hydrogen production (Ramachandran and Mitsui, 1984). There ap­pears to be no consensus on the optimal ratios for these nutrients, even for specific species grown successively in the same system. Therefore, nutrients are often added in excess to avoid nutrient limitations (Richmond, 1999; Sanchez et al., 1999; Acien Fernandez et al., 2001).

One strategy to reduce costs associated with adding excess nutrients involves culturing microalgae in reclaimed water or wastewater blends. The use of algae to absorb nutrients in the wastewater processing stream has been widely employed by water treatment facilities (Megharaj et al., 1992; Tredici et al., 1992; Nurdogan and Oswald, 1995; Kaya and Picard, 1995; Craggs et al., 1995). The green microalga Scenedesmus obliquus has demonstrated vitality in urban wastewaters, registering growth rates similar to those reported for a complete syn­thetic medium. This freshwater alga tolerates a wide range of temperature and pH, making it versatile for water puri­fication (Kessler, 1991). Similar findings for other algal spe­cies continue to emerge, along with the energy return on investment analyses that confirm the utility of coupling scaled algal (EROI) production with nutrient reclamation from waste streams, resulting in decreased costs for both algal growth and water treatment (Beal, 2012b).

CONTAMINATION

Another barrier to the large-scale production of algae biofuels is the maintenance of axenic or nearly axenic cul­tures. In particular, cultivation systems that are open to the environment (e. g. open ponds) are easily susceptible to contamination by unwanted species if extreme care is not taken. A new open pond is typically inoculated with the desired strain of microalgae with the hope that the algae will aggressively proliferate and dominate the pond flora. Over time, it is likely that undesired species will be introduced, which may graze on the algae or outcompete the inoculated species and lead to severely reduced yields. Once a competitor has taken residence in a pond, it is extremely difficult to eradicate (Schenk et al., 2008). It is therefore crucial to aggressively monitor cultures to identify and eradicate contaminates as soon as possible. A number of strategies have been employed to minimize culture contaminations. Cultivating algal extremophiles that tolerate and outcompete invasive spe­cies in particular environments (e. g. pH and salinity) fa­cilitates open-pond production. High bicarbonate concentrations allow Spirulina to be grown in open ponds with few invasive algae, and high-saline environments allow marine algae like Dunaliella salina to be grown in "relative pure cultures" (Anderson, 2005). Another popu­lar strategy involves shortening the longevity of the cul­ture; cultures are scaled and harvested before major contamination can occur (Benemann, 2008). Cultivation of microalgae in closed photobioreactors (PBRs) offers another level of protection against predators. Occasion­ally, cultures can be treated with antibiotics and antifun — gals to eliminate bacteria and fungi, but this practice can lead to microbial resistance and render the treatment ineffective. Predator ciliates can be treated with dioctyl sulfosuccinate, which is used to eliminate ciliates in the udders of milking cows (Abou Akkada, 1968) with mini­mal harm done to the algae.

MIXING

At high algae concentrations, a thin top layer of cells absorbs nearly all light—this phenomenon can be avoided by proper mixing. Mixing must sufficiently keep algae cells in suspension, aid distribution of CO2 and O2, and provide uniform exposure of light to all cells. Mixing also decreases the boundary layer around cells, which facilitates increased uptake of metabolic products (Molina Grima et al., 1999).