Circadian rhythm response of Chlorella pyrenoidosa under autotrophic and mixotrophic cultivation

Abstract:

[Objective] To study the effect of autotrophic and mixotrophic cultivation on the growth, division and biochemical components accumulation of Chlorella pyrenoidosa, then to explore the response mechanism on circadian rhythm under artificial cultivation and optimize the corresponding technology. [Methods] BG11 medium was used for autotrophic cultivation of Chlorella pyrenoidosa and BG11 medium supplemented with glucose of different concentrations (1, 5, 10, 20 g/L) for mixotrophic cultivation. The algae cell number was determined by a hemocytometer, biomass was measured by dry weight, and the size and division of algae cells were observed by a microscope. We used staining method for quantification the total lipids of Chlorella pyrenoidosa. Chlorophyll, protein and starch contents were determined by ultraviolet spectrophotometry. [Results] Glucose had a significant growth promoting effect on Chlorella pyrenoidosa, and the optimum concentration was 10 g/L. Under a circadian rhythm, about 76.00% of new daughter cells in the autotrophic cultivation group were proliferated under dark period on the second and fifth day, whereas 67.50% of the new daughter cells were generated under light period in the mixotrophic group. The silent period of the algae in the autotrophic group was 8 hours, but that in the mixotrophic condition was only 4 hours on the second day. On the eighth day, the contents of total lipids and chlorophyll of algae cells kept relatively stable under a light-dark period, the protein and starch content of that reached their respective peak after 8 and 12 hours in light. [Conclusion] Mixotrophic cultivation with glucose can increase the biomass of Chlorella pyrenoidosa notably. The growth, proliferation and the biochemical components accumulation of Chlorella pyrenoidosa were regulated by circadian rhythm, the algae in autotrophic culture mainly grow during day in light and undergo cell proliferation in dark. The machanism of mixotrophic cultivation increasing the biomass was shortening the growth silence period of algae cells, and increasing the proportion of larger cells for passing through commitment point of cell-cycle, it was especially obvious during light period. The protein and starch contents of Chlorella pyrenoidosa showed obvious circadian rhythm, their best harvest time were 8 and 12 hours after in light, respectively.