Improving carbohydrate and starch accumulation in sp. AE10 by a novel two-stage process with cell dilution.

Background: Microalgae are highly efficient cellular factories that capture CO and are also alternative feedstock for biofuel production. Carbohydrates, proteins, and lipids are major biochemical components in microalgae. Carbohydrates or starch in microalgae are possible substrates in yeast fermentation for biofuel production. The carbon partitioning in microalgae could be regulated through environmental stresses, such as high concentration of CO, high light intensity, and nitrogen starvation conditions. It is essential to obtain carbohydrate-rich microalgae via an optimal bioprocess strategy.

Results: The carbohydrate accumulation in a CO tolerance strain, sp. AE10, was investigated with a two-stage process. The CO concentration, light intensity, and initial nitrogen concentration were changed drastically in both stages. During the first stage, it was cultivated over 3 days under 1% CO, a photon flux of 100 μmol m s, and 1.5 g L NaNO. It was cultivated under 10% CO, 1000 μmol m s, and 0.375 g L NaNO during the second stage. In addition, two operation modes were compared. At the beginning of the second stage of mode 2, cells were diluted to 0.1 g L and there was no cell dilution in mode 1. The total carbohydrate productivity of mode 2 was increased about 42% compared with that of mode 1. The highest total carbohydrate content and the highest starch content of mode 2 were 77.6% (DW) and 60.3% (DW) at day 5, respectively. The starch productivity was 0.311 g L day and the total carbohydrate productivity was 0.421 g L day in 6 days.

Conclusions: In this study, a novel two-stage process was proposed for improving carbohydrate and starch accumulation in sp. AE10. Despite cell dilution at the beginning of the second stage, environmental stress conditions of high concentration of CO, high light intensity, and limited nitrogen concentration at the second stage were critical for carbohydrate and starch accumulation. Although the cells were diluted, the growths were not inhibited and the carbohydrate productivity was improved. These results were helpful to establish an integrated approach from CO capture to biofuel production by microalgae.