Light intensity and duration of irradiation determine the growth rate and production yield, which is limited by the enzymatic mechanisms of the microorganism. This limit, referred to as light saturation point, is between 5 and 10 klux (60-120 pmol photons m-2 s-1) according to Balloni et al. [4].

Vonshak et al. [110] showed that the light/dark cycle, to which cells in the ponds are exposed during the day, is an important factor that influences the growth rate and photosynthetic efficiency and it can be completed in seconds. The light regimes to which the cultures are submitted are considered to be an important factor in the productivity and yield of photosynthetic reactions [94, 105]. Several studies have been carried out focusing on the effect of different photosynthetic photon flux den­sities (PPFDs) incident on photobioreactors, but there are few reports focusing on the effects of the duration of the day and night cycles [66, 78].

The dark/light regime in the cells is influenced by the agitation, turbulence, and cell density in the ponds [82]. When light intensity is very low, cell growth is limited or there is no cell growth [4] . On the other hand, high light intensity results in increasing cell growth up to a light intensity at which it stops with increasing light intensity. The light intensity at which cell growth begins is known as the light limit­ing region, while the light intensity at which no further increase in growth takes place with increasing light intensity is known as the light saturation point. Further increase in light intensity neither increases the specific growth rate, nor hinders growth. The point at which increased light intensity decreases the specific growth rate is the point where photoinhibition begins [53]. A high-intensity light can induce photo-oxidative stress, resulting in photo-inhibition of photosynthesis, destruction of photosynthetic pigments and cell death [64] .

Photo-inhibition is a reduction of the photosynthetic activities caused by the exposure to high PPFDs [119]. When the flux of photons absorbed by chloroplasts is too high, the concentration of high-energy electrons inside the cell is excessive, and they cannot be consumed through the Calvin cycle. These excessive electrons lead to the formation of H2O2 , which can damage cell structures [20]. Even in densely populated outdoor cultures of Spirulina spp., photo-inhibition can be observed when light intensity is 60-70% of full sunlight [111].

Light intensity has a strong influence on cell growth, nitrogen-to-cell conversion factor and cell productivity, as well as biomass composition. According to Rangel — Yagui et al. [80], the best S. platensis growth carried out in 5-L open tanks was observed with 500 mg L-1 urea, added for 14 days by a exponentially feeding pro­tocol, at a light intensity of 5,600 lux (67.2 mmol photons m-2 s-1), whereas the highest concentration of chlorophyll in the biomass was observed at a light intensity of 1,400 lux (16.8 mmol photons m-2 s-1). The best chlorophyll productivity was observed with 500 mg L-1 urea at a light intensity of 3,500 lux (42 mmol photons m-2 s-1), providing the optimal balance between cell growth and biomass chloro­phyll content. Under the best conditions for cell growth, maximum cell concentra­tion using urea as a nitrogen source was higher than that obtained with the use of KNO3 [ irrespective to the cultivation process (batch or fed-batch) used for cell growth with the latter nitrogen source. As expected, these findings highlight that the fed-batch process conducted properly is not hampered by light intensity. Using a different strategy, Danesi et al. [34[ investigated the influence of light intensity reduction on S. platensis cultivation, using urea and KNO[ as nitrogen sources applying fed-batch and batch processes, respectively, reducing the light intensity from 5 to 2 klux (60-24 mmol photons m-2 s-1) on the 9th and the 13th day of culti­vation. Increases of up to 29% in total chlorophyll production were observed for the cultivations with light intensity reduction, in comparison with the cultivations car­ried out at fixed light intensities. Irrespective of the time for light reduction, the use of urea as nitrogen source by fed-batch process led to higher cell growth, obtaining maximum cell concentration of about 1,800 mg L-1.

Soletto et al. [96] showed that the photosynthetic efficiency of S. platensis in a bench-scale helical photobioreactor reached its maximum value (PE = 9.4%) at a PPFD of 125 mmol photons m“2 s-1 . The photo-inhibition threshold appeared to strongly depend on the CO2 feeding rate: at high PPFD, an increase in the amount of fed CO2 delayed the inhibitory effect on biomass growth, whereas at low PPFD, excessive CO2 addition caused the photosynthetic microorganism to stop growing.