Botryococcus braunii

Botryococcus is a colonial microalga that is widespread in fresh and brackish waters of all con­tinents. It is characterized by its slow growth and by containing up to 50% by weight of hydro­carbons. B. braunii is classified into A, B, and L races, mainly based on the difference between the hydrocarbons produced (Metzger and Largeau, 2005). Banerjee et al. (2002) differentiate the races as follows: Race A produces C25 to C31 odd-numbered n-alkadienes and alkatrienes; B race pro­duces polymethylated unsaturated triterpenes, called botryococcenes (CnH2n-10, n = 30-37); and L race produces a single tetraterpene hydrocarbon C40H78 known as lycopadiene.

The cells of B. braunii are embedded in a communal extracellular matrix (or "cup"), which is impregnated with oils and cellular exudates (Banerjee et al., 2002). B. braunii is capable of synthesizing exopolyssaccharides, as reported by Casadevall et al. in 1985. Higher growth and production of EPS, which ranges from 250 g m-3 for A and B races to 1 kg m-3 for the L race, occur when nitrate is the nitrogen source instead of urea or ammonium salts (Banerjee et al., 2002). Phosphorus and nitrogen are also important factors in accumulation of hydrocarbons by the microorganism (Jun et al., 2003).

The metabolic energy devoted to produce such large amounts of hydrocarbons makes this species noncompetitive in open mass cultures, since strains not so burdened can grow much faster and soon dominate an outdoor pond culture (Benemann et al., 2002). B. braunii has been reported to convert 3% of the solar energy to hydrocarbons (Gudin and Chaumont, 1984). Being synthesized by a photosynthetic organism, hydrocarbons from algae can be burned without contributing to the accumulation of CO2 in the atmosphere.

Dayananda et al. (2007) cultivated Botryococcus braunii strain SAG 30.81 in shake flasks and obtained a maximum cell concentration of 0.65 g L-1 under 16:8 light:dark cycle. Experiments with different strains of B. Braunii indicate that the biomass yield is inversely proportional to lipid accumulation. The maximum biomass yield achieved was 2 g L-1 (with 40% of lipids) and the lower was 0.2 g L-1 (with 60% of lipids). Outdoor experiments with this microalga achieved a high biomass yield of 1.8 gL-1 but a very low lipid accumulation. It was also showed by Dayananda and collaborators that exopolyssaccharides production by Botryococcus braunii SAG 30.81 is not affected by light regimen in MBM media, different from lipids and proteins pro­duction. Sydney et al. (2011) carried experiments with this same strain under 12 h light: dark cycle in 5% CO2 enriched air and achieved a high biomass production of3.11gL-1 with 33% lipids in 15 days. Carbon dioxide fixation rate was calculated as near 500 mg L-1 day-1. B. braunii biomass composition also included 39% proteins, 2.4% carbohydrates, 13% pigments, and 7.5% ash.

Marukami and Ikenouochi (1997) achieved a carbon dioxide fixation greater than 1 gram per liter by Botryococcus braunii cultivated for hydrocarbon accumulation.

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