The growth curves of M. aeruginosa were significantly different under fluorescent and blue light. The specific growth rate for M. aeruginosa under fluorescent light (0.33 day− 1) was consistent with the one that Ohkubo et al. (1991) obtained from the same strain cultivated under similar conditions (0.32 day− 1). Compared with the results under the fluorescent light, the growth of M. aeruginosa was inhibited under blue light, and this result was in line with the previous studies (Wyman and Fay 1986; Watanabe et al. 2019; Tan et al. 2020). Luimstra et al. (2018) has reported that when phycobilisome-containing cyanobacteria Synechocystis sp. absorbed blue light and the efficiency of oxygenic photosynthesis and growth declined because of phycobilisomes. They can absorb light in the red range and work as accessory pigments to compensate for the deficiency of photons in photosystem Ⅱ under red LED. However, they cannot absorb light in the blue range, which created an imbalance in the electron transport energy between two photosystems under blue LED. Since M. aeruginosa contains phycobilisomes (Raps et al. 1985), it is considered that its growth was suppressed like Synechocystis sp.
The degree of the inhibition effects on the growth of M. aeruginosa by blue light differed among the studies. As shown in Table 1, the culture conditions were not consistent among each experiment. For instance, Wyman and Fay (1986) and Tan et al. (2020) used BG-11 medium at different concentration, suggesting available nutrient concentrations must have differed among the studies. It is known that the growth rate follows the Droop equation, which is a model that depends on the intracellular content, and the nutrient uptake rate is calculated by the Michaelis-Menten equation which depends on the nutrient concentration in the medium (Ducobu et al. 1998; Mikawa et al. 2016). Since the half-saturation constant for nutrient uptake is usually smaller than the concentration of the medium, it is considered that the growth rate does not differ depending on the type of the medium unless the nutrient is depleted. Li et al. (2014) demonstrated that there was no significant difference in the growth rate even when the medium with different nutrient concentration was used.
Temperature is also known to affect the growth of algae. It has been reported that the growth rate of M. aeruginosa increases with increasing temperature (You et al., 2018; Imai et al., 2009). Li et al. (2014) demonstrated that the growth rate of M. aeruginosa was higher at 25 ℃ than that at 20 ℃. Therefore, the growth rate obtained by Wyman and Fay (1986) is expected to increase further when performed at 25 ℃ (as in this experiment). The light-dark cycle used in the culture is not the same. According to the experimental data by Zevenboom and Mur (1984), The growth rate of M. aeruginosa is about the same in the 12:12 and 24:0 light-dark cycles. Furthermore, the difference in growth rate due to different light-dark cycles is smaller under the light intensity which is insufficient to cause saturation with respect to the growth rate. As shown in Fig. 2, the blue light intensity such as 10, 20, and 32 µmol photons m− 2 s− 1 are not considered to have the intensity to cause saturation. If the relationship between the light-dark cycle and the growth rate is similar to that of fluorescent light, the difference in the growth rate under the blue light at different light-dark cycles is presumed to be small.
The growth rate may vary depending on the strain. Despite being cultivated under the same experimental conditions, which was in BG-11 at 25 ℃ under fluorescent light at 45 µmol photons m− 2 s− 1, the growth rates of M. aeruginosa obtained from Li et al. (2014) and Tan et al. (2020) were different, indicating 0.60 day− 1 and about 0.32 day− 1, respectively. On the other hand, the strain used in this study and Watanabe et al. (2019) were the same. The growth rate under blue light was zero at 20 µmol photons m− 2 s− 1 (Watanabe et al. 2019), while that at 32 µmol photons m− 2 s− 1 was 0.11 day− 1. Since the effect of the light-dark cycle can be assumed to be small, the increase in the growth rate of M. aeruginosa was due to the increase in the light intensity. The growth rate may vary depending on the strain, however, it is expected that the growth rate will increase as the blue light intensity increases. Overall, the observed difference in the growth rate of M. aeruginosa is determined by the light intensity. As shown in Fig. 2, it is considered that the growth rate tends to be at its minimum when the blue light intensity is around 20 µmol photons m− 2 s− 1.
The specific growth rate for N. palea under fluorescent light (0.62 day− 1) was higher than that under blue light (0.36 day− 1). It was previously reported that the specific growth rate under blue light (0.23 day− 1) was higher than under fluorescent light, which was 0.21 day− 1 (Watanabe et al. 2019). The difference of the specific growth rate under each light condition between the previous and current study is likely due to the difference in light intensity. Figure 2 indicates that the increase in the specific growth rate for N. palea in a light intensity was higher in fluorescent light than in blue light. The results showed that N. palea grew under blue light while M. aeruginosa did not, which was the same as the previous study for all conditions (Watanabe et al. 2019). Diatoms have a fucoxanthin which is a natural pigment (Wang et al. 2018). The adsorption of the fucoxanthins is optimal in the range of 480–560 nm, although there is some light absorption in the range of 420–470 nm, which represents the blue wavelength (Papagiannakis et. al. 2005). Similarly, Ohgai et al. (1992) reported diatom Cocconeis sp. grew drastically better in blue fluorescent light. Therefore, the growth of N. palea in blue light was promoted by fucoxanthin. However, the growth rate of N. palea under blue light at 32 µmol photons m− 2 s− 1 did not increase significantly. A fucoxanthin assists the growth of N. palea under blue light, but it may be slow to respond to increased light intensity. It should also be noted that no significant differences were detected in maximum cell yields between fluorescent and blue light conditions.
In order to provide a good environment in which N. palea has a competitive advantage over M. aeruginosa, it is important to irradiate blue light at the intensity that can inhibit the growth rate of M. aeruginosa while promoting that of N. palea. The blue light intensity at 20 µmol photons m− 2 s− 1 is ideal as the growth rate of M. aeruginosa is zero while N. palea can still grow. When fluorescent light was replaced by blue light at 32 µmol photons m− 2 s− 1, the growth rate of M. aeruginosa changed from 0.33 to 0.11 day− 1, meaning that the growth rate dropped by 67 % while that of N. palea dropped by only 42 %. Therefore, it is considered that blue light irradiation was useful in inhibiting the growth of M. aeruginosa, indicating that M. aeruginosa was in relatively disadvantageous conditions.