EFFECT OF IRRADIANCE AND LIGHT WAVELENGTH ON A MIXED MICROALGAL/CYANOBACTERIAL CULTURE

Microalgae has been produced traditionally in monocultures. Mixed cultures of microalgae and other species such as bacteria, including cyanobacteria, have some advantages over single-species culture. Among these advantages are a greater resistance to contamination, symbiotic relation among the cultured species and a diversification of the products that can be obtained from the resulting biomass.

Despite the advantages of mixed cultures, there are some issues that need to be considered. Among these issues is the maintenance of a specific species ratio and composition in the culture, to optimize the target bioproducts production. The species ratio depends on a number of factors. In mixed cultures of photosynthetic species, light is an important parameter to consider for this control.

In this work, two sets of experiments were performed to determine the effect of light irradiance and wavelength distribution on the mixed culture. In the first set of experiments, four wavelength distributions (blue - 467 nm, green - 522 nm, red - 640 nm and white - narrow peak at 450 nm and a broad range with a peak at 550 nm) and two irradiance levels (400 and 800 µmol m^-2 s^-1) were investigated. The wavelength distribution resulting in the highest overall growth rate and productivity of the mixed culture, was used to determine the effect of the irradiance on the growth, productivity and species ratio of the mixed culture. For this second set of experiments, seven irradiance levels (180, 400, 600, 800, 1000, 1200 and 1400 μmol m^-2 s^-1) with the wavelength selected on the first set of experiments and white light as control.

The results show that no photoinhibition was observed with irradiances up to 1200 μmol m^-2 s^-1. Lipid content increased at higher light intensities (irradiance), and decrease when the irradiance was lower than 600 μmol m^-2 s^-1. Red light resulted in the highest productivity, growth rates and photosynthetic efficiency. The culture shifted from a microalgae to a cyanobacteria dominated culture as the light decreased. At 80 μmol m^-2 s^-1this shift was evident after six days of culture. At other irradiances, the shift was observed as the culture density increased. Attenuation curves were prepared to determine the irradiance levels that were related to the shift in speciues composition.