(599by) Effect of Electric Field in Chlorophyll and Carotenoids Production By Chlorella vulgaris Under Photoeterotrophic Culture

Microalgae are a major natural source for a vast array of valuable compounds, including a diversity of pigments, for which these photosynthetic microorganisms represent an almost exclusive biological resource. Chlorella vulgaris has been the oldest commercial application of microalgae. This is a freshwater, fast-growing and versatile green alga with the highest content of chlorophyll found in nature. The content of pigment and another compounds in C. vulgaris, like other microalgae, can be improve by several stress conditions can either be nutritional or physical. In recent years, the application of electric fields (EF) has been used successfully in many areas such as chemical and biomedical engineering, food technology and agriculture; however, few studies with application on microorganism with biotechnological importance have been found. In this work we attempted to investigate the effect of electric field on growth and pigment production by Chlorella vulgaris under photoheterotrophic culture.

The freshwater microalga Chlorella vulgaris was used. The application of EF (2 V for 4 h) at different growth phases of microalga (0, 48, 96 and 144 h) under 12 h light/dark cycles was evaluated. The treatments were conducted under sterile conditions. An experiment without electric field application was used as control. Experiments were carried out in 250 mL Erlenmeyer flask containing 150 mL of artificial freshwater media under photoheterotrophic conditions using glucose as sole carbon source (2 g L^-1) and white cool fluorescent light as energy source (45 μmol photons m⁻² s⁻¹) at 27±2 °C. The final cell concentration was reported as dry weight (g L^-1). Pigments were extracted using dimethylsulphoxide (DMSO) and the OD was measured at 649, 665 and 480 nm to calculate pigment content.

Responses of Chlorella vulgaris to electrical signals depended on the intensity of the electrical field applied. In this study, similar growth profile was observed for culture exposed at 48, 96 and 144 h, showing different behavior for cultures exposed at the beginning of the culture, however, this was not reflected in final biomass production. The maximum pigment content, 15.02 mg L^-1 for total chlorophyll and 3.33 mg L^-1 for carotenoids, was obtained when EF was applied at 96 h of culture.

This result was 36.13% and 28.28% higher than EF exposure at initial time and 47.82% and 28.27% than non-stressed culture for chlorophyll and carotenoids production, respectively. At 96 h C. vulgaris was in exponential phase thus photosynthetic rate was higher and therefore photosynthetic pigments increased. The exposed to EF causes oxidative stress in microalgae cells increasing the content of carotenoids (antioxidant). This might be the reason for no visible variation in final biomass production after the EF exposure in the present study. These results suggested that algae growth phase (96 h) is the optimum period for EF application, resulting in a pigment content increase without affecting final biomass production.