Phycoremediation by High-Rate Algal Ponds (HRAPs) - Biotechnological Applications of Microalgae - page 189

Phototrophic cultivation uses sunlight and CO 2 as an inorganic carbon source
for energy production and growth (Mata et al . , 2010). Phototrophic cultivation is
less prone to contamination than other types of cultivation. Heterotrophic growth
occurs in the absence of light using organic carbon sources such as glucose, acetate,
glycerol, fructose, sucrose, lactose, galactose, and mannose (Amaro et al . , 2011).
Organisms that are able to undergo mixotrophic growth have the ability to photosyn-
thesize or use organic substrates as a carbon source. Mixotrophic production reduces
photo-inhibition and decreases the loss of biomass due to dark-phase respiration
(Brennan and Owende, 2010; Pittman et al . , 2011). Organic carbon sources in waste-
water allow microalgae to undergo mixotrophic growth followed by phototrophic
growth. This effectively removes nutrients while improving biomass and potential
lipid productivity (Feng et al . , 2011).
The efficiency of nutrient removal depends on the species of algae cultivated and
has been shown to be influenced positively by the cultivation of algal strains that are
tolerant to certain extremes, such as extreme temperatures, quick sedimentation, or
the ability to grow mixotrophically (Olguin, 2003). Choosing a strain for cultivation
in HRAPs should preferentially (1) have a high growth rate, (2) have a high protein
concentration when grown under nutrient-limited conditions, (3) be used for animal/
fish feed, (4) have the ability to tolerate high nutrient levels, (5) produce a value-added
product, (6) be able to grow mixotrophically, and (7) be easily harvested (Sheehan
et al . , 1998; Olguin, 2003; Rawat et al., 2011). Chlorella vulgaris, Haematococcus
pluvialis, and Arthrospira (Spirulina) platensis, among others, are examples of spe-
cies that can grow under photo-autotrophic, heterotrophic, and mixotrophic condi-
tions (Amaro et al . , 2011).
Microalgal wastewater treatment has the potential to significantly reduce the
costs of treatment when compared to conventional chemical methods; this is par-
tially achieved by negation of the requirement for mechanical aeration as microalgae
produce oxygen via the process of photosynthesis (Pittman et al . , 2011). The simul-
taneous treatment of wastewater and production of biomass reduces the cost of both
processes (Brennan and Owende, 2010; Christenson and Sims, 2011). Furthermore,
the production of biofuel in conjunction with wastewater treatment has been put for-
ward as the most viable method for biofuel production from microalgae in the near
future (Brennan and Owende, 2010).
Several studies have proven the potential for nutrient removal from synthetic waste-
water by microalgal biomass production. Phosphorus removal of 98% and total ammo-
nia removal has been achieved by (Martinez et al. l . , 2000) using Scenedesmus obliquus .
Boelee et al . (2011) demonstrated simultaneous removal of nitrate and phosphate to
2.2 mg L −1 and 0.15 mg L −1 , respectively, using microalgal biofilms. Su et al. (2012)
reported phosphorus removal efficiency of algae to be 89%. Certain photosynthetic
bacteria and green microalgae such as Rhodobacter sphaeroides and Chlorella soro-
kiniana can, under heterotrophic conditions, remove high concentrations of organic
acids (>1,000 mg L −1 ) and ammonia (400 mg L −1 ) (Olguin, 2003). The bacterial
removal of substances such as polycyclic aromatic hydrocarbons, organic solvents,
and phenolic compounds may be assisted by the use of microalgae that produce the
oxygen required for bacterial action. Heavy-metal biosorption may be achieved by
microalgae grown under phototrophic conditions (Brennan and Owende, 2010).