# Influence of Medium Frequency Light/Dark Cycles on the Cultivation of _Auxenochlorella pyrenoidosa_
## 1 Introduction  
Photosynthetic microorganisms have received growing attention, especially for mass cultivation of microalgae, due to their possible production of health foods, biofuels, and also for carbon dioxide reduction. Light attenuation in microalgae cultivation systems has been reported as a major bottleneck in microalgal production. In general, light attenuation leads to the light zone, a well-illuminated volume of the culture near the light source, and the dark zone, a self-shaded zone of culture [1; 2]. Light condition is one of the main factors that affects the microalgae photosynthesis kinetics. Numerous studies have indicated that light intensity and light wavelength change the results in increasing or decreasing the biomass and metabolite yields [3; 4]. [PERSON] et al. [5] reported that the energy associated with photons with a wavelength of 680 nm is the exact energy level required by chlorophyll to initiate photosynthesis. [PERSON] et al. [6] summarized previous studies that have suggested that a light source with narrower spectral outputs close to the photosynthesis absorption spectrum (680 nm) are photosynthetically more efficient. Hence, a red light source results in higher biomass accumulation compared with the other light wavelength. When the light intensity is insufficient, it has been reported that the blue light can also be absorbed by chlorophyll because of the blue light contains 40% more energy than the other light wavelength [5; 7; 8]. For light intensity, the optimal light intensity for microalgae cultivation ranges from 2000 to 10000 lux [9]. The excessive and insufficient light intensity result in decreases to the biomass productivity. Due to the light attenuation in microalgae cultivation systems, an efficient light energy supply has been reported as the greatest scientific and technological challenge in the research and the development of the cultivation of photosynthetic microorganisms [10]. Regarding the effect of the light/dark cycle, [PERSON] [11] reported that microalgae growth can be influenced by three ranges of light/dark cycle: (1) high frequency, namely, the flashing light effect; (2) medium frequency, fluctuation cycles of seconds to minutes; and (3) low frequency, fluctuation cycles of hours to days and years. It was reported that proper flashing light under high cell concentrations will enhance microalgae growth rate and metabolite productivity [6; 12], while a reduction in biomass production with a decrease in the duration of light periods was evident in low frequency [13; 14]. For medium frequency light/dark cycles, conflicting results have been reported in the literature. [PERSON] et al. [15] reported that the red microcalga _Porphyridium_ sp. has a maximal growth rate at cycle times of 27 s, 60 s, and 110 s, when the maximal dark period reaches about 5.6 s and cool white fluorescent light reaches an intensity of 300 \(\mu\)E m\({}^{-2}\) s\({}^{-1}\). It clearly indicates that the longer light period could not lead to a higher growth rate. In contrast, [PERSON] and [PERSON] [16] reported that the duration of the dark was 9.2 s (with a total cycle time of 40 s) when the specific growth rate of _Chlorella vulgaris_ (_Chlorophyta_) was at its maximum value, but the increase in the dark period leads to a decrease in the growth rate. The same results were reported by [PERSON] [17] at a light/dark cycle from 4.1 s to 50 s and light intensity of 800 \(\mu\)E m\({}^{-2}\) s\({}^{-1}\), where the growth rate of _C. vulgaris_ depends on the light/dark ratio; the larger the light/dark ratio, the more productivity and photosynthetic efficiency. In addition, [PERSON] et al. [18] investigated the effect of light/dark ratio on the growth rate of _Porphyridium_ sp. at a low concentration of \(1.0\times 10^{7}\)-\(1.20\times 10^{8}\) cell mL\({}^{-1}\) with three intensities of 110, 220, and 550 \(\mu\)E m\({}^{-2}\) s\({}^{-1}\).