**9. Conclusion and future research directions**

Algal biodiesel continues to hold promise as a sustainable, carbon neutral source of transportation fuel. The technical feasibility of algal biodiesel has been demonstrated (Miao & Wu, 2006; Xiong et al., 2008), but the economics and energy demands of production require substantial improvement. The necessary changes appear attainable through the enhancement of productivity, the reduction of cost and energy demand for key processes and the application of the biorefinery concept (co-production of valuable products or processes). Current research is focussed on achieving this through a combination of biological and engineering approaches. The major challenges currently being addressed are:


194 Biodiesel – Feedstocks and Processing Technologies

2000), high light intensity (Rodolfi et al., 2009), high temperature (Converti et al., 2009); high

salt concentration (Takagi et al., 2000) and high iron concentration (Liu et al., 2008).

Fig. 4. Correlation of calculated lipid productivity with (a) biomass productivity and (b)

Nitrogen (N) deprivation is the most frequently reported method of enhancing lipid content, as it is cheap and easy to manipulate. N deficiency has a reliable and strong influence on lipid content in many species (Chelf, 1990; Rodolfi et al., 2009; Shifrin & Chisholm, 1981). Unfortunately, stress conditions that enhance lipid content, such as nitrogen deprivation, typically also decrease the growth rate, and thus the net effect on lipid productivity must be ascertained (Lardon et al., 2009). Maximum biomass productivity and lipid content in *Chlorella vulgaris* occur under different conditions of nitrogen availability, suggesting that a two-stage cultivation strategy may be advantageous. From studies we have conducted on *C. vulgaris*, it appears that an intermediate level of nitrogen limitation creates the optimum balance between biomass and lipid production. The optimum cultivation strategy tested was batch culture, using a low starting nitrate concentration (between 250 and 300 mg.L-1 nitrate), ensuring that nitrogen in the medium was depleted towards the end of exponential growth. Other cultivation strategies (e.g. two-stage batch, fed-batch or continuous) were found not to

Although high lipid productivity is a key factor in species selection, other characteristics such as ease of cultivation, tolerance of a range of environmental conditions (particularly temperature and salinity), flue-gas contaminants and high O2 concentrations, as well as

Algal biodiesel continues to hold promise as a sustainable, carbon neutral source of transportation fuel. The technical feasibility of algal biodiesel has been demonstrated (Miao & Wu, 2006; Xiong et al., 2008), but the economics and energy demands of production require substantial improvement. The necessary changes appear attainable through the enhancement of productivity, the reduction of cost and energy demand for key processes and the application of the biorefinery concept (co-production of valuable products or

lipid content under nutrient replete conditions

improve upon the productivity achieved in N limited batch culture.

**9. Conclusion and future research directions** 

resistance to contaminants and predators are likely to be equally as important.


These topics have captured the imagination of several researchers and some innovative solutions are being investigated. The overall goal of biofuel production is to optimise the conversion of sunlight energy to liquid fuel. In algal cultivation, techniques to improve light delivery include manipulating the reactor design, the use of optics to deliver light to the centre of the reactor, optimising fluid dynamics to expose all cells to frequent light flashes, increasing the efficiency of photosynthesis and carbon capture (e.g. enhancing the carbon concentrating mechanism), and using mixed-species cultures to utilise different intensities or wavelengths of light (Scott et al., 2010).

One of the major problems with light delivery is poor penetration of light into dense cultures due to mutual shading by the cells. Under high light conditions, microalgal cells absorb more light than they can use, shading those below them and dissipating the excess energy as fluorescence or heat. In nature, this confers individual cells an evolutionary advantage, however, in mass production systems it is undesirable as it decreases overall productivity. It would be advantageous to minimize the size of the chlorophyll antennae in cells at the surface, so as to permit greater light penetration to cells beneath (Melis, 2009). Reducing the size of the light harvesting complexes through genetic modification has been shown to improve productivity (Nakajima et al., 2001). The goal now is to engineer cells that change antennae size according to light intensity.

Although the TAG content of cells can be enhanced by manipulation of the nutrient supply, there is a tradeoff between growth and lipid production. For optimum productivity, cells that can maintain a simultaneously high growth rate and lipid content are required. Strategies to achieve this include screening for novel species, and genetic engineering of well characterised strains. The genes and proteins involved in regulation of lipid production pathways are currently being investigated through synthetic biology and the modelling of carbon flux through metabolism. Key enzymes and branch-points can then be manipulated to improve productivity. For example, carbohydrate and lipid production compete directly for carbon precursors. Shunting carbon away from starch synthesis by downregulation of the enzyme ADP-glucose pyrophosphorylase in *Chlamydomonas* has been shown to enhance TAG content 10-fold (Li et al., 2010).

The challenge of harvesting small algae cells from dilute suspensions has yet to be solved in a cheap, energy efficient manner. Ideally the addition of chemical agents that impede the recycling of the culture medium and nutrients should be avoided. A series of methods is likely to be used e.g. flocculation followed by sedimentation, or settling followed by

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The use of nutrients from waste sources (e.g. CO2 from flue-gas and nitrate and phosphate from wastewater) could help to reduce costs and energy input, as well as contributing to environmental remediation. Potential co-products include fine chemicals such as astaxanthin, B-carotene, omega-3 fatty acids, polyunsaturated fatty acids, neutraceuticals, therapeutic proteins, cosmetics, aquafeed and animal feed (Mata et al., 2010). Algae could also potentially be modified to synthesize other types of fuel e.g. ethanol, butanol, isopropanol and hydrocarbons (Radakovits et al., 2010) or downstream processing of algae could be modified to process the entire biomass to energy containing fuels through thermal processes.
