In this research, the effect of solar irradiance on Chlorella vulgaris cultivated in open bioreactors under greenhouse conditions was investigated, as well as of ratio of light intensity in the 420–520 nm range to light in the 580–680 nm range (I_420–520/I_580–680) and of artificial irradiation provided by red and white LED lamps in a closed flat plate laboratory bioreactor on the growth rate and composition. The increase in solar irradiance led to faster growth rates (μ_exp) of C. vulgaris under both environmental conditions studied in the greenhouse (in June up to 0.33 d⁻¹ and in September up to 0.29 d⁻¹) and higher lipid content in microalgal biomass (in June up to 25.6% and in September up to 24.7%). In the experiments conducted in the closed bioreactor, as the ratio I_420–520/I_580–680 increased, the specific growth rate and the biomass, protein and lipid productivities increased as well. Additionally, the increase in light intensity with red and white LED lamps resulted in faster growth rates (the μ_exp increased up to 0.36 d⁻¹) and higher lipid content (up to 22.2%), while the protein, fiber, ash and moisture content remained relatively constant. Overall, the trend in biomass, lipid, and protein productivities as a function of light intensity was similar in the two systems (greenhouse and bioreactor).

The effect of different environmental conditions on the growth rate, biomass production, nutrient composition, and phenolic content of the microalgae species Chlorella vulgaris, Botryococcus braunii, Chlamydomonas reinhardtii, Euglena gracilis, and Nannochloropsis oculata was investigated. The experiments were conducted in open bioreactors in a greenhouse in three different periods (during October, March, and June), and in a controlled environment in a closed plant growth chamber. It was found that the growth rate and production of C. vulgaris and B. braunii was higher during March, C. reinhardtii and N. oculata grew better in June, and the growth of E. gracilis was similar in March and June. The lipid content of the biomass of all five species increased with increasing light intensity and temperature, while the nitrogen free extractable (NFE) content decreased and the protein, fiber, moisture, and ash content remained relatively unaffected. The phenolic content varied from species to species with E. gracilis having the highest and N. oculata the lowest content among the species studied. The results can be taken into account when cultivating the different microalgae studied in full scale applications, such as in open raceway bioreactors, where conditions could be adjusted to obtain the most favorable growth conditions, depending on the particular species cultivated.