(6eq) Efficient Accumulation of Carbonhydrate in Microalgae and It's Utilization

Efficient Accumulation of Carbonhydrate in Microalgae and It’s Utilization

Xu Jingliang, Guo Ying，Zhou Weizheng， Li Xiekun， Yuan Zhenhong

Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, P.R. China 

xjl@ms.giec.ac.cn     http://people.ucas.ac.cn/~xjl?language=en

Abstract:  As efficient photosynthetic microorganism, microalgae can convert solar energy and CO₂ into biomass, such as starch and cellulose, which is the desired feedstock for bioethanol production. As bioethanol feedstock, microalgae bioethanol is known as the third generation bioethanol because of many advantages, such as high photosynthetic efficiency, no farmland occupation, can be cultivated during all year, can absorb CO₂, high carbohydrate content and easy to be degraded. In present work, a high carbohydrate content microalgae strain was identified. Nitrogen stress was performed on microalgae cultivation. Simulated flue gas was used as carbon source to study the effect of flue gas on microalgae cell components. Liquid hot water was applied in the microalgae pretreatment process to increase the enzyme hydrolysis efficiency.

Based on the morphological characteristics and phylogenetic analysis, microalgae WZKMT was identified as Scenedesmus raciborskii WZKMT. Five BG11 cultures with different NaNO₃ concentrations (0, 0.5, 1.0, 1.5, 2.0 g L^-1) were designed for microalgae culture, biomass yield, total sugar and starch content was determined every 24h. The results showed that the growth of microalgae was inhibited severely under nitrogen-free condition, while there was no difference on biomass yield under other four nitrogen concentrations, the maximum of which can attain to 4.0 g L^-1. The content of starch under nitrogen deficiency condition (0 g L^-1, 0.5 g L^-1) was much higher than that under the other three nitrogen conditions in the first 6 days. After day 8, the starch content showed no obvious distinction under all nitrogen conditions, which indicated that starch accumulated as short-term stress response under nitrogen deficiency condition. For long-term cultivation, the nitrogen content in culture can be economized to reduce cost. The total sugar and starch content of WZKMT can attain to 68.95% and 56.27%, respectively and is the highest starch content report in current reported articles, which indicates that S. raciborskiiWZKMT is a desired feedstock candidate for bioethanol production.

Still the growth parameters, biomass yield, total sugar, starch, lipid composition, protein, chlorophyll of Scenedesmus raciborskii WZKMT cultivated with simulated flue gas (15% of CO₂, 200 ppm SO₂, 100 ppm NO with N₂ balance) and 7% CO₂ (as control) were investigated to study the effects of flue gas on the cellular components. Dissolution of SO₂ and NO in simulated flue gas led to pH decrease and toxicity to microalgae cells, resulting in the decrease of buffer capacity of culture. Microalgae growth and starch accumulation was inhibited. with simulated flue gas, the maximum biomass yield and starch content were 2.14 g/L and 36.23%，while with 7% CO₂, those were 3.25 g/L and 53.16%, respectively. Microalgae cells produced more chlorophyll, protein and lipid when simulated flue gas was used as the carbon source. Fatty acids composition analysis indicated that there was no significant distinction on fatty acids relative content (fatty acid/TFA) between cells aerated using simulated flue gas and 7% CO₂.

 Liquid hot water (LHW) pretreatment on the carbonhydrate-rich microalgae S. raciborskiiWZKMT was performed to enhance glucose recovery and the glucose concentration in enzymatic hydrolysate was determined to study the effect of LHW pretreatment on enzyme hydrolysis. Temperature, time,  ratio of solid to liquid in pretreatment process were analyzed. The optimal temperature (140℃), time (40 min), the ratio of solid to liquid (1:15) were determined by single-factor experiments. The optimization pretreatment conditions analyzed by response surface methodology were as follows: ratio of liquid to solid 13:1, temperature 147℃, heating time 41 min. Under the conditions, the glucose concentration was 14.223 g/L, the glucose recovery was 89.32%, which was up to 5-fold higher than the one of the sample without LHW pretreatment (17.91%). SEM images of microalgae after LHW pretreatment showed that cells were broken and fibre bundles on cell surface were more exposed.

Key Words: microalgae, nitrogen stress, simulated flue gas, liquid hot water, starch