Effects of light-emitting diode on the growth and biochemical composition of Chlorella vulgaris

Abstract

Microalgae play a crucial role in marine ecosystems by synthesizing organic matter using light, water, and carbon dioxide, thus providing primary production. Unlike terrestrial photosynthetic plants, microalgae contain a balanced composition of carbohydrates, proteins, and lipids, and produce a variety of useful substances. As photosynthetic organisms, they fix carbon dioxide and utilize various nutrients for cell growth, aiding in environmental solutions like water purification and heavy metal removal. Efficient light sources are vital for effective microalgae cultivation. The recently developed LED light source, with its small size, excellent energy efficiency, and the ability to irradiate specific wavelengths. In particular, recent studies have shown that by irradiating specific light intensity appropriately, wavelength, and pulse of LED according to the cultivation stage, the synthesis of useful physiologically active substances such as photosynthetic pigments, antioxidants, and lipids can be promoted. This study aims to identify the optimal wavelength for the growth of Chlorella vulgaris and the effective wavelength for promoting useful substances using LED. In this study, the C. vulgaris strains used were wild type PKVL7422 and a genetically modified strain, PKVL7422, engineered to insert the flounder growth hormone gene for enhanced growth hormone production. Under a single wavelength, the maximum growth rate μmax of the wild type was high in both blue and red wavelengths, with a threefold lower Ks in the red wavelength compared to other wavelengths. The growth rate of the genetically modified strain was highest in red and blue wavelengths, similar to the wild type. Under mixed wavelengths, the growth rate of C. vulgaris showed no significant difference in wavelength ratio for both strains. The highest growth rate under light-dark cycles was observed with an optimal adjustment of 12 hours light (L) and 12 hours dark (D). The carbohydrate content of the wild type under a single wavelength was highest in the green wavelength at a normal light intensity of 10 μmol/m2/s, correlating with a lower growth rate. The transformed strain showed a carbohydrate content approximately 3.6 times lower than that of the wild type and did not show a significant difference by wavelength. Also, proteins and lipids appeared highest in the blue wavelength of normal light intensity 100 μmol/m2/s for both wild type and transformed strains, and in particular, the lipid content of the transformed strain showed a content about 1.4 times higher than that of the wild type. The pigment content also peaked in the blue wavelength at 100 μmol/m2/s for chlorophyll-a, chlorophyll-b, lutein, zeaxanthin, violaxanthin, and neoxanthin in both strains. In a two-stage cultivation approach, cells were irradiated with the red wavelength until the late exponential phase to promote cell density, was irradiated until the late exponential phase, and the blue wavelength was irradiated from the normal phase to promote useful substances, the carbohydrate content of the wild type did not show a statistically significant difference (P<0.05). In the case of proteins and lipids, they showed a content 1.5 times higher than that of the red single wavelength, but lower than that of the blue single wavelength. The pigment content was 4.4 times higher in violaxanthin compared to fluorescent lamps, but lower for other pigments than under the blue single wavelength. The results of changes in biochemical composition according to the control of the light-dark cycle showed that most of them reached their peak when the light-dark cycle was set to 12L:12D, and the differences were not significant. The pigment content also appeared highest in chl-a, chl-b, and lutein, zeaxanthin, violaxanthin, and neoxanthin when the light-dark cycle was 12L:12D. In mixed wavelength conditions, where red and blue wavelengths were combined at specific ratios, the carbohydrate content of the wild type showed no statistically significant difference (P<0.05), while the transformed strain's content was about 1.1 times lower than that of the wild type. The protein content was highest at a 7:3 ratio favoring the blue wavelength for both strains. In the case of the wild type, this ratio resulted in a protein content 2.1 times higher than under the blue single wavelength, and the transformed strain showed a content 2.4 times higher. Lipids also reached their highest content at the 7:3 ratio, with the lipid content of the transformed strain being 3 times higher than under the blue single wavelength. Additionally, the pigment content of both strains was highest at the 7:3 ratio for chl-a, chl-b, lutein, zeaxanthin, violaxanthin, and neoxanthin, with chl-a, chl-b, and lutein content in the transformed strain being 1.3, 1.5 and 1.2 times higher, respectively. The experimental results demonstrate that the red wavelength serves as an economical light source to promote cell density in C. vulgaris, while the blue wavelength is effective in enhancing valuable biochemical substances such as proteins, lipids, and pigments. Notably, experiments with mixed wavelengths, combining the growth-promoting red and the substance-enhancing blue wavelengths, revealed that a higher ratio of the blue wavelength in this mix leads to more effective accumulation of these substances. Therefore, employing a mixed wavelength with a predominant blue wavelength ratio in the construction of a photobioreactor (PBR) could yield economic benefits and increased productivity.