The first photosynthetic microbe to be isolated and grown in pure culture was the fresh­water microalga Chlorella vulgaris. It is a spherical unicellular eukaryotic green algae that presents a thick cell wall (100-200 nm) as its main characteristic. This cell wall provides mechanical and chemical protection, and its relation to heavy metals resistance is reported, which explains why C. vulgaris is one of the most used microorganisms for waste treatment.

The uptake of carbon by C. vulgaris cells is done through the enzyme carbonic anhydrase, which catalyzes the hydration of CO2 to form HCO3 and a proton. Hirata and collaborators (1996) studied carbon dioxide fixation by this microalga, which showed important variations comparing cultivation under fluorescent lamps and sunlight. In the first case the estimated rate of carbon dioxide fixation was 865 mg CO2 L-1 d-1; in a sunlight regimen the estimated rate achieved 31.8 mg CO2 L-1 d-1. Winajarko et al. (2008) achieved a transferred rate of

441.6 g CO2 L-1 d-1 under the same cultivation conditions as Hirata et al. (1996). According to Sydney et al. (2011), in experiments using classic synthetic media and a 12-h light/dark regimen, C. vulgaris biofixation rate of carbon dioxide is near 250 mg L-1 day-1.

Carbon fixation by Chlorella vulgaris is variable and depends, among other factors, on the concentration of CO2 in the gaseous source. Yun et al (1997) cultivated C. vulgaris in 15% of carbon dioxide and achieved a fixation of 624 mg L-1 day-1; Scragg et al. (2002) achieved a fixation of 75 mg L-1 day-1 under CO2 concentration of 0.03%. In the same study, Scragg tested a medium with low nitrogen and the fixation rate was 45 mg L-1 day-1, suggesting that nitrogen also influences carbon uptake rate.

Some studies (Chinassamy et al., 2009; Morais and Costa, 2007) indicate that the best concentration of CO2 in the gas supplied to C. vulgaris growth is about 6%.