Effect of N concentration and source on the Chlorella sp. biomass growth, cultivated in medium BG11, NaNO3, 1.5g l-1 (N concentration 0.247 g l-1) (1); in modified medium NaNO3, 0.75 g l-1 (N concentration 0.1235 g l-1) (2) and in medium with urea, 0.1 g l-1 (N concentration 0.0466 g l-1) (3)