4.2 Light/Dark Cycle
The quality, intensity and light period are very important parameters in microalgae
production (Cardinale 2011). In outdoor cultivation systems, solar radiation is the
only source of light and its availability is therefore dependent on the geographical
location, climate and seasonality (Novoveská et al. 2016). Light regimes (to which
cultures are submitted) are considered an important factor in the productivity and
efficiency of photosynthetic reactions (Sicko-Goad and Andresen 1991; Toro
1989).
Lee and Lee (2001) evaluated the effect of the light/dark cycle in the treatment of
wastewater by Chlorella kessleri. This study showed that the amount of nitrate
removed was 31.6 mg NO 3 /L after 3 days under continuous light conditions, and
14.0 mg NO 3 /L with the light/dark cycle of 12:12. However, the removal of
organic carbon and phosphate was higher for the culture under conditions of light/
dark cycle of 12:12. Therefore, C. kessleri could grow heterotrophically during the
dark periods, once the microalgae are able to metabolize the organic carbon to their
growth without photosynthesis. With regard to the species (C. vulgaris), Santos
et al. (2009) studied the effect of two light/dark cycles: 24:0 and 12:12. This study
demonstrated that the cycle of continuous light presented a higher growth rate.
Depending on the types of effluent (primary, secondary or tertiary), the light/dark
cycle may have different impacts on the treatment efficiency, since the primary and
secondary effluents have high concentrations of organic carbon. On the other hand,
the tertiary effluent has low amounts of organic carbon, which can limit the heterotrophic growth of microalgae and, consequently, the treatment efficiency.
4.3 Competition with the Microflora Present in Wastewater
The coexistence of microalgae and bacteria is a biological process that occurs by
the interaction of two distinct processes: photosynthesis of microalgae and bacterial
Fig. 4 Nutrient exchange in microalgae-bacteria consortium (adapted from Anbalagan 2016)
48
A. P. de Carvalho Lopes et al.
The quality, intensity and light period are very important parameters in microalgae
production (Cardinale 2011). In outdoor cultivation systems, solar radiation is the
only source of light and its availability is therefore dependent on the geographical
location, climate and seasonality (Novoveská et al. 2016). Light regimes (to which
cultures are submitted) are considered an important factor in the productivity and
efficiency of photosynthetic reactions (Sicko-Goad and Andresen 1991; Toro
1989).
Lee and Lee (2001) evaluated the effect of the light/dark cycle in the treatment of
wastewater by Chlorella kessleri. This study showed that the amount of nitrate
removed was 31.6 mg NO 3 /L after 3 days under continuous light conditions, and
14.0 mg NO 3 /L with the light/dark cycle of 12:12. However, the removal of
organic carbon and phosphate was higher for the culture under conditions of light/
dark cycle of 12:12. Therefore, C. kessleri could grow heterotrophically during the
dark periods, once the microalgae are able to metabolize the organic carbon to their
growth without photosynthesis. With regard to the species (C. vulgaris), Santos
et al. (2009) studied the effect of two light/dark cycles: 24:0 and 12:12. This study
demonstrated that the cycle of continuous light presented a higher growth rate.
Depending on the types of effluent (primary, secondary or tertiary), the light/dark
cycle may have different impacts on the treatment efficiency, since the primary and
secondary effluents have high concentrations of organic carbon. On the other hand,
the tertiary effluent has low amounts of organic carbon, which can limit the heterotrophic growth of microalgae and, consequently, the treatment efficiency.
4.3 Competition with the Microflora Present in Wastewater
The coexistence of microalgae and bacteria is a biological process that occurs by
the interaction of two distinct processes: photosynthesis of microalgae and bacterial
Fig. 4 Nutrient exchange in microalgae-bacteria consortium (adapted from Anbalagan 2016)
48
A. P. de Carvalho Lopes et al.