4.1. INCREASE IN EFFICIENCY
As to the future role of microalgae as an alternative energy source, the key question is whether the production of biofuels by microalgal cultures will ever be able to compete on a large scale with petrofuels. The answer to this question is not easy. It depends on how its economical, ecological and political aspects are measured.
There are good reasons to assume that petrofuel will remain the most important energy source for transportation in the near future and that its price will keep going up. Therefore, it is a realistic goal to raise the efficiency of microalgal culture systems, mainly of indoor systems, in order to be able to compete with the price of petrofuels. Efficiency may be augmented by a lot of measures. The exploitation of light energy has to be made more efficient, e.g., by keeping algae in small layers as are tested in new types of bioreactors and by constructing fibre-optic devices that might increase the capacity of daylight. It is also important to avoid clumping of algae, adhesion to reactor walls, for example, by special coating techniques of inner tube surfaces and to establish an automatic control and harvesting system that guarantees a continuous operating. It will also be desirable to have nearby a cheap source of CO2, such as power and cement plants, although it should also be made clear that by algal cultures their output of CO2 never can be reduced to zero.
From the viewpoint of biology, it will be certainly promising to screen for further microalgal candidates since until now only a rather small part of natures fundus of about 30,000-40,000 algal species is used, mainly cyanobacteria (e.g. Spirulina) and green (chlorophycean) algae, such as coccoid (Chlorella, Scenedemus) and flagellate specimen (Dunaliella, Chlamydomonas). Other microalgal taxa are represented by a few xanthophycean ( e.g., Botryococcus), eustigmatophycean (e.g., Nannochloropsis), prasinophycean (e.g., Tetraselmis) and bacillariophycean (diatom) taxa such as Nitzschia and Phaeodactylum.
Efforts should be aimed at finding species that do not clump, show a lower photosynthetic compensation point to increase yield at less input of light energy and perform optimum growth at elevated temperatures to reduce costs of cooling. It will also be desirable to obtain a higher output of 'interesting substances' under standard conditions. If, for example, the content of processable oil in the algal biomass could be raised from 30% to 50% of the dryweight, the price of biodiesel would become competitive. However, this requires much better understanding of the physiological mechanisms in microalgae responsible for the synthesis of value products such as lipids and oil. Most algae produce substantial amounts of triacylglycerols such as the production of components of storage lipids only under stress conditions, e.g., nutrient limitation and photooxidative stress (Hu et al., 2008). Thus, the challenge will be to find culture conditions that combine both optimum growth of microalgae, i.e., high yield of biomass and high lipid content in cells.
As to the production of bioethanol from algal biomass, it will be recom-mendable to look by screening or by genetic engineering for algae producing carbohydrates that are more suitable to fermentation than the standard starch basis is. The same consideration holds also for other cell parts, such as the cell wall. Ideally, it consists of material that can also be easily fermented or alternatively used, for example by the BTE-technique, to produce energy.
It could also be promising to think about the problem whether the end product of fermentation has to be ethanol. Ethanol shows a high solubility in water and therefore is not easy to separate from it; it is poisonous to cells and has relatively relatively low energy content. Butanol could be a good candidate for ersatz.
A further yet speculative energy source produced by microalgae is H2. The photoproduction of H2 is mainly studied in Chlamydomonas sp. and represents a rather promising field of microalgal technology (Patel-Predd, 2007). One of the current problems to tackle is that H2 production is inhibited by oxygen. Various strategies to meet this obstacle are under study, for example, the insertion of leghemoglobin genes into algae. It still requires a lot of basic research and is far from being exploitable in practice.
In general, the leitmotiv should be to raise the efficiency of biomass processing. This means to increase the output of energy from algal biomass, either as biofuels (biodiesel, bioethanol, biogas) or directly in the form of heat energy that may be used in power plants. Each form of energy has to deal with a different and already successful competitor in the market. Therefore, it will be important to increase the current revenue from biomass by combining energy yield with the production of additional high-value products. Every component of the biomass should be exploited to add value ('biorefinery', Chisti, 2007).
An economic advantage of biomass production by microalgae in closed systems in comparison with other energy plants is doubtless the fact that production is constant, predictable and not dependent on such hazards as drought, diseases, and hurricanes.
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