Effects of different light qualities on the growth of C. closterium
The growth of C. closterium on groups GL and YL was greater than the other groups. The growth of C. closterium on groups WL and BL was moderate and the lowest growth was observed on the group RL. These results may be due to differences in spectral coverage, photon energy and luminous power among light qualities. Li reported that blue light could significantly promote the growth of Chaetoceros gracilis, while red light could cause growth inhibition . Our results also showed that the main effect of the group RL was a decreasing trend in the growth rate of C. closterium.
Previous studies have suggested that mixed light such as white has more advantages light rather than the single wavelength for enhancing the production of microalgae [19, 20]. WL has a wide range of wavelength containing various wavelength signals and energy, which can meet the requirements of the growth of microalgae. In the present study, group WL grew poorer than groups GL and YL, possibly due to the low proportion of certain important wavelengths. Compared with the single wavelength, there was a higher proportion of unsuitable wavelength in group WL than in the single wavelength groups and the more suitable wavelength may have been less absorbed by microalgae. Some studies demonstrated that the experiments on the effect of different ratios of red-blue mixed light on the growth of Spirulina had also achieved good results . Other studies also showed that the experiment on the effect of different the ratio of red-blue mixed light on the growth of Spirulina has also achieved good results . The effects of C. Closterium with different mixes of suitable wavelength is worthy of further research.
Effects of various light qualities on chlorophylls content in C. closterium
Photosynthesis occurs only in the chlorophyll-containing cells of green plants and algae, as it plays a key role in capturing photoelectrons and acts as an effective catalyst for photoreaction . The observations of this experiment provided the evidence of significant effects of various light qualities on Chla and Chlc contents of C. closterium, and GL was the most suitable light source among these light qualities, which displayed the same trend on the growth of C. closterium. The key to the metabolism of microalgae lies in the conversion of light energy to chemical energy, and the photosynthetic pigment molecules carried by themselves can solve this problem very well . Some studies have indicated that Chlorophyta and Cyanobacteria showed a preference for short wavelength, which may be related to their pigment composition [23, 24]. The light energy absorption mode of Chlorophyta is mainly chlorophyll a and b, which is more efficient for red light such as long wavelength, and the light system of cyanobacteria based on phycobilin can effectively absorb red light and blue light [25, 26].
Chla and Chlc, the main photosynthetic pigments of C. closterium, directly indicates the speed of the photosynthetic rate and is related to the growth of algal cells . C.closterium to generally snuff-colouredcolored, rich in diadinoxanthin, lutein and other carotenoids, can greatly improve the efficiency of photoelectron capture and is important to assisting photocatalytic water splitting . Consequently, there were favorable advantages of using the short wavelength light. The use of short-wave lengths, such as GL and YL, prevailed for C. closterium. In addition, photosynthetic pigments have different light absorption with different wavelengths, which can affect the formation of photosynthetic pigments . Kana indicated that environmental factors could affect the concentration and composition of photosynthetic pigments in microalgae in a dynamic manner . Prasiola crispa, an aerial green-algae, has the long wavelength chlorophylls that can absorb considerable at approximately 710 nm .
Effects of various light qualities on lipid contents of C. closterium
Photons are one of the important factors affecting biochemical components in cultures, and various light quality conditions will have a significant impact on lipid synthesis [31, 32]. The results showed that in comparison to the other light qualities, the WL, GL and YL cultivation were all conducive to the synthesis and accumulation of total lipids for C. closterium. Because of short-term stimulation of algal cultivation with light quality, RL stimulation did not damage algal cells; compared to growth experiments with long-term adaptation, those with RL stimulation could still result in growth. Hultberg found that Chlorella vulgaris with red and green light enhanced biomass production and fatty acid contents . The methods of blue light use and nutrient starvation were also proved to be highly efficient in promoting the production of Chlorella sp. AE10 . As mentioned above, certain studies have suggested that the total lipids of microalgal cells have distinct responses under different light quality treatments. Various light qualities have different effects on the activities of crucial enzymes in the lipid synthesis pathway, leading to total lipid content variation in microalgae [9, 34].
In this study, in comparison to the other light quality, group BL had no significant effects on lipid content and growth, which was not in agreement with the previous conclusions that BL would increase growth and lipid content [23, 25]. These results seemed to suggest that BL, the shortest wavelength among the five kinds of light quality studied, had the highest energy with light irradiance and may cause high-energy damage to algal cells, which can be confirmed by the determination of antioxidant enzyme activity in algal cells in future experiments .
Effects of various light qualities on fatty acid profiles of C. closterium
Various light quality affected the fatty acid composition of C. closterium and influenced their growth and development. SFAs and MUFAs were negatively correlated with growth, while PUFAs were positively correlated with growth (Fig. 4). The PCA analysis of the fatty acid profile also showed several distinct differences under different light quality conditions (Fig. 5).
The main components of SFAs were myristic acid (C14: 0), palmitic acid(C16:0) and stearate acid (C18: 0) of C. closterium, and this was highest in group RL and lowest in group YL. Radakovits found that the accumulation of SFAs may be related to the expression level of acyl-ACP thioesterases in genetically engineered Phaeodactylum tricornutum . Some studies also found that light quality can affect the activities of enzymes related to fatty acid synthesis, but whether red light can affect the activities of thioesterase and other related enzymes in SFA metabolic pathway . We only confirmed that RL promoted the accumulation of much SFAs, which leads to a decrease in the proportion of PUFA, and so is the performance of group YL. However, which plays a major role in the synthesis of unsaturated fatty acids and saturated fatty acids needs to be further studied.
Some studies have shown that the synthesis of palmitoleic acid was correlated with substrate concentration of its palmitic acid (C16: 0), but the current results did not support this view. Because group BL with the lowest MUFAs also had a high content of palmitic acid (21.33%). However, there is a positive correlation between MUFA and growth, he relationship between growth performance and palmitoleic acid is still unclear. Other studies have pointed out that the existence of PUFA and MUFA is necessary for microalgae to maintain the stability of cellular membrane structure . As the most important lipid reservoir of lipid droplets, the stability of the outer membrane structure of lipid droplets is related to whether it can effectively accumulate more fatty acids .
PUFAs with 18 and 20 carbon are vitally important substrates in the synthesis of long-chain unsaturated fatty acids . The content of 16-carbon PUFAs (including C16: 2(n-6) and C16: 3(n-4)) in was significantly higher the YL treatments than in all the other light treatments, and the high content of 16 carbon PUFAs suggested that YL may play a key role in inducing fatty acid biosynthesis-related regulation of the enzyme activities of prokaryotic plastids in algal cells.The WL treatment resulted in a greater increase in the 18 carbon (C18: 2(n-6) and C18: 3(n-6)) and 20 carbon (C20: 4(n-6), C20: 5(n-3) and C22: 6(n-3)) PUFAs contents; specifically, the content of ARA (2.89%) and C18:2(n-6) (2.34%) were significantly higher in the WL treatment than in the other treatments, which suggested that the composition of fatty acids in the WL treatment was consistent with the substrates of long-chain PUFA synthesis.
Although SFAs proportion limited the production of PUFAs under group BL, the EPA content still maintained the highest level compared that with in other treatments. EPA involved in the synthesis of phospholipids to form the cell membrane, can adjust the membrane fluidity to adapt to the stressful environment . Accompanied by growing experiments, these observations may show that in group BL, C. closterium may be induced to produce light damage. Relative to selecting the optimal light intensity, BL possesses powerful emission energy and can activate the antioxidant system of algal cells. Nonetheless, in terms of light quality regulation, whether BL and RL were able to regulate the lipid metabolism process of diatom cells, the significant levels of C16:2(n-6) and C16:3(n-4) in group YL showed that light quality also plays a regulatory role in the lipid metabolism process of diatoms. There was no significance or advantage in increasing the yield of fatty acids among the groups GL and WL. The process through which energy and information regulation played the main role should be determined by determining the key enzyme that regulates information and the chlorophyll light energy quenching experimental results of photosynthesis [42, 43].