The biodiesel prepared from the SCH-derived algal oil by transesterification is mainly composed of 9-octadecenoic acid methyl ester (C₁₉H₃₆O₂), 9,12-octadecadienoic acid methyl ester (C₁₉H₃₄O₂), and hexadecenoic acid methyl ester (C₁₇H₃₄O₂). This result shows that the main components of biodiesel produced from algae grown with SCH are similar to those of biodiesel produce from algae grown with glucose, and are accordant with biodiesels produced from crop oils.

Most algae are photoautotrophic, and grow using CO₂ as the carbon source and sunlight as the energy supply. However, both biomass productivity and oil content of photoautotrophic cultures can be low. By contrast, heterotrophic fermentation allows algae to accumulate a much higher proportion of oil within less time and the scale-up is much easier.

(Heterotrophs obtain their carbon and energy for growth from organic compounds; autotrophs use CO₂ as their sole carbon source and obtain their energy from light or from the oxidation of inorganic compounds. Algal fuels company Solazyme, earlier post, is using an optimized heterotrophic fermentation pathway with genetically modified algae.)

The researchers note that from the aspect of energy saving and greenhouse effect relief, the photoautotrophic alternative may seem to be more economical and preferred. However, they wrote, “biofuel via a heterotrophic pathway should be further studied and considered as a transitional strategy” for the following reasons:

• First, fast-growing photosynthetic algal cells tend to produce poor amounts of oil, whereas those accumulating high oil content show little growth ability. Nitrogen starvation has been widely reported to trigger lipid accumulation but leads to a decrease in protein synthesis, chlorophyll content, and cell division.

• Second, light deficiency is common in photosynthetic cultivation. Thus, enough ATP and reductant for CO₂ fixation are not supplied. Thee synthesis of fatty acid needs a large amount of ATP and reducing power, they wrote, and when light is restricted, “it is not likely for alga to prefer synthesizing fatty acids”.

In prior studies, the Tsinghua team reported pilot-scale fermentation of heterotrophic microalgae with high biomass productivity and oil yield of up to 50% of dry cell weight. This new study investigated further improvements in algal oil yield, while minimizing the cost of fermentation. Key factors associated with algal oil yield are nitrogen content, C/N (ratio of carbon source/nitrogen source), and carbon source.

In their prior work, the team found that of three inorganic nitrogen sources (urea, potassium nitrate, and ammonium nitrate) and two organic nitrogen sources (glycine and yeast extract), yeast extract (YE) was the most suitable nitrogen source to provide high biomass and oil yield. Accordingly, they used YE as the nitrogen supplement in all cultures.

They found that the optimal oil production with the highest output-cost coefficient was achieved when C/N was 26.9 and the YE concentration was 0.60 g L^-1.

This work, for the first time, illustrates the feasibility of sugar cane juice served as a fermentable carbon source for oil production in C. protothecoides...The production pathway from sugar cane to biodiesel by microalgae fermentation has some advantages in comparison to fuel-ethanol production by yeast fermentation. For example, the heating value of alga-based biodiesel is much higher than that of yeast-based fuel-ethanol. Because algal cells are easily separated from medium and oil is easily extracted by solvent, it takes a lot of heating power to prepare ethanol by distillation from yeast fermentation medium.

...we are clearly conscious that this is just a beginning for biodiesel production using microalgae to meet the commercial needs. There are bottlenecks in this pathway. In comparison to yeast fermentation for ethanol production, microalgae accumulate oils as intracellular products. The oil/carbon source conversion is dependent upon the biomass/sugar conversion. As a result, the oil cell content typically ranges from 30 to 60%, leading to a limitation of oil/carbon source conversion.

Larger quantities of ethanol could be produced with relatively small amounts of microbial cells for ethanol fermentation. However, in the future, reasonable improvements can be introduced to the pathway of alga-based biodiesel production. For example, microalga is genetically modified, allowing the oil vesicle to be transported to an extracellular medium. On the other hand, the algal species used in this study cannot assimilate sucrose directly. Because of the lack of corresponding enzyme, pretreatment of SC is inevitable and requires time and labor. Genetic manipulation offering algal species with the ability of digesting sucrose is welcome in future studies.

—Xiong et al. (2009)

They also suggest that further in-depth research on the mechanism of oil production in heterotrophic alga should be undertaken—for example, genetic manipulation of critical enzymes for lipid synthesis may result in further improvements in oil yield. They also note that a strategy of phototrophic/heterotrophic tandem cultivation may be beneficial in both environmental and economic aspects.

Resources

• Yun Cheng, Yue Lu, Chunfang Gao and Qingyu Wu (2009) Alga-Based Biodiesel Production and Optimization Using Sugar Cane as the Feedstock. Energy Fuels, Article ASAP

• Wei Xiong, Xiufeng Li, Jinyi Xiang and Qingyu Wu (2008) High-density fermentation of microalga Chlorella protothecoides in bioreactor for microbio-diesel production. Journal Applied Microbiology and Biotechnology 78 (1) doi: 10.1007/s00253-007-1285-1

• Xiufeng Li, Han Xu, Qingyu Wu (2007) Large-scale biodiesel production from microalga Chlorella protothecoides through heterotrophic cultivation in bioreactors. Biotechnol. Bioeng. 98 (4), 764–771 doi: DOI: 10.1002/bit.21489