Kinetic model for effects of simulated flue gas onto growth profiles of Chlorella sp. AE10 and Chlorella sp. Cv.

Microalgae are potential candidate for biofuel production as alternative one for fossil fuels. CO2 in flue gas is available carbon source to support microalgae growth. In this study, the effects of different concentrations of the simulated flue gas onto algal growth and photosynthetic activity were evaluated for both Chlorella sp. AE10 and Chlorella sp. Cv. The growth profiles were correlated by a simple kinetic model. It was indicated that the simulated flue gas led to low pH and the photosynthetic activity was partially destroyed. Chlorella sp. Cv can tolerate full simulated flue gas, 10% CO2  + 200 ppm NOx  + 100 ppm SOx . The pH in medium maintained at 6 and the photosynthetic activity was more than 0.6 at the first 6 days. If the concentration of NOx was more 100 ppm and that of SOx was more than 50 ppm, the pH was declined to 4 at day 2 or 3 for Chlorella sp. AE10. At the same time, the related photosynthetic activities of Chlorella sp. AE10 were less than 0.4, which was not suitable for algal growth. It was shown that Chlorella sp. Cv could be used for CO2 fixation from the simulated flue gas.

Effect of light quality on growth rate, carbohydrate accumulation, fatty acid profile and lutein biosynthesis of Chlorella sp. AE10.

Microalgae are feedstocks for multiple product development based on algal biorefinery concept. The effects of light quality (white, red and blue light emitting diodes) and macro-element starvations on Chlorella sp. AE10 were investigated under 20% CO2 and 850 µmol m-2 d-1. Nitrogen and phosphorus starvations had negative effects on its growth rate. The biomass productivities were decreased from day 1 and the highest one was 1.90 g L-1 d-1 under white light conditions. Phosphorus starvation promoted carbohydrate accumulation under three LED light sources conditions and the highest carbohydrate content was 75.9% using red light. Blue light increased lutein content to 9.58 mg g-1. The content of saturated fatty acids was significantly increased from 37.51% under blue light and full culture medium conditions to 77.44% under blue light and nitrogen starvation conditions. Chlorella sp. AE10 was a good candidate for carbohydrate and lutein productions.

Adaptive evolution and carbon dioxide fixation of Chlorella sp. in simulated flue gas.

Carbon dioxide and other greenhouse gas emissions leads to global warming. Biological capture through microalgae is a potential approach for solving this environmental problem. It is still a technical challenge to enhance the tolerance of microalgae to flue gas if CO2 is fixed from flue gas directly. A new strain, Chlorella sp. Cv was obtained through adaptive evolution (46 cycles) against simulated flue gas (10% CO2, 200 ppm NOx and 100 ppm SOx). It was confirmed that Chlorella sp. Cv could tolerate simulated flue gas conditions and the maximum CO2 fixation rate was 1.2 g L-1 d-1. In a two-stage process, the biomass concentration was 2.7 g L-1 and the carbohydrate content was 68.4%. Comparative transcriptomic analysis was performed for Chlorella sp. Cv under simulated flue gas and control conditions (10% CO2). These responses against simulated flue gas uncovered the significant difference between the evolved strain and the original strain. The metabolic responses to flue gas were explored with focus on various specific genes. Upregulation of several genes related to photosynthesis, oxidative phosphorylation, CO2 fixation, sulfur metabolism and nitrogen metabolism was beneficial for the evolved strain to tolerate the simulated flue gas. The upregulation of genes related to extracellular sulfur transport and nitrate reductase was essential to utilize the sulfate and nitrate from dissolved SOx and NOx. The results in this study are helpful to establish a new process for CO2 capture directly from industrial flue gas.

Exploring stress tolerance mechanism of evolved freshwater strain Chlorella sp. S30 under 30 g/L salt.

Enhancement of stress tolerance to high concentration of salt and CO2 is beneficial for CO2 capture by microalgae. Adaptive evolution was performed for improving the tolerance of a freshwater strain, Chlorella sp. AE10, to 30 g/L salt. A resulting strain denoted as Chlorella sp. S30 was obtained after 46 cycles (138 days). The stress tolerance mechanism was analyzed by comparative transcriptomic analysis. Although the evolved strain could tolerate 30 g/L salt, high salinity caused loss to photosynthesis, oxidative phosphorylation, fatty acid biosynthesis and tyrosine metabolism. The related genes of antioxidant enzymes, CO2 fixation, amino acid biosynthesis, central carbon metabolism and ABC transporter proteins were up-regulated. Besides the up-regulation of several genes in Calvin-Benson cycle, they were also identified in C4 photosynthetic pathway and crassulacean acid metabolism pathway. They were essential for the survival and CO2 fixation of Chlorella sp. S30 under 30 g/L salt and 10% CO2.

Enhancing Carbohydrate Productivity of Chlorella sp. AE10 in Semi-continuous Cultivation and Unraveling the Mechanism by Flow Cytometry.

Accumulated carbohydrate in microalgae is promising feedstock for bioethanol fermentation. Selection of suitable cultivation conditions in semi-continuous cultivation is critical to achieve a high carbohydrate productivity. In the current study, the effects of macro-nutrient (nitrogen, phosphorus, and sulfur) limitations and light intensity were evaluated for the carbohydrate accumulations of Chlorella sp. AE10 under 10% CO2 conditions. It was shown that nitrogen limitation and high light intensity were effective for improving carbohydrate productivity. The average carbohydrate and biomass productivity in semi-continuous cultivation with 1/4 N medium and 1000 µmol photons m-2 s-1 was 0.673 and 0.93 g L-1 day-1, respectively. Sulfur and phosphorus limitations could improve the carbohydrate content but they could not enhance the carbohydrate productivity. The cell cycle progression and chlorophyll a were investigated using flow cytometry (FCM). The results showed that macro-nutrient limitation and high light intensity indeed influenced cell cycle progression and led to the formation of polyploid cells along with the carbohydrate accumulation in a certain range. FCM was rapid and accurate method to investigate the operation conditions why 1/4 N, 2 days as a cycle, and high light intensity were optimal ones. In addition, the remaining high level of photosynthesis activity was also important for achieving a high carbohydrate productivity. Dynamic tracking of carbohydrate accumulation is helpful for establishment of a semi-continuous cultivation for enhancing carbohydrate productivity in microalgae.

Comparative transcriptomic analysis reveals phenol tolerance mechanism of evolved Chlorella strain.

The growth of microalgae is inhibited by high concentration phenol due to reactive oxygen species. An evolved strain tolerated to 500mg/L phenol, Chlorella sp. L5, was obtained in previous study. In this study, comparative transcriptomic analysis was performed for Chlorella sp. L5 and its original strain (Chlorella sp. L3). The tolerance mechanism of Chlorella sp. L5 for high concentration phenol was explored on genome scale. It was identified that the up-regulations of the related genes according to antioxidant enzymes (SOD, APX, CAT and GR) and carotenoids (astaxanthin, lutein and lycopene) biosynthesis had critical roles to tolerate high concentration phenol. In addition, most of genes of PS I, PS II, photosynthetic electron transport chain and starch biosynthesis were also up-regulated. It was consistent to the experimental results of total carbohydrate contents of Chlorella sp. L3 and Chlorella sp. L5 under 0mg/L and 500mg/L phenol.

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

BACKGROUND: Microalgae are highly efficient cellular factories that capture CO2 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 CO2, 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 CO2 tolerance strain, Chlorella sp. AE10, was investigated with a two-stage process. The CO2 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% CO2, a photon flux of 100 µmol m-2 s-1, and 1.5 g L-1 NaNO3. It was cultivated under 10% CO2, 1000 µmol m-2 s-1, and 0.375 g L-1 NaNO3 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-1 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-1 day-1 and the total carbohydrate productivity was 0.421 g L-1 day-1 in 6 days. CONCLUSIONS: In this study, a novel two-stage process was proposed for improving carbohydrate and starch accumulation in Chlorella sp. AE10. Despite cell dilution at the beginning of the second stage, environmental stress conditions of high concentration of CO2, 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 CO2 capture to biofuel production by microalgae.