affecting photosynthetic oxygen evolution, suggesting that net improvements in carbon fixation are reasonable. Chen and Blankenship (2011) made the challenging proposal that photosynthetic capacity might be expanded by engineering cells to use different chlorophylls to capture a broader range of the light spectrum than non-engineered cells.

CO2 abatement is a driver for developing algal biofuels. However, with current practices and species, CO2 often is limited in production ponds and photobioreactors, and addition of a CO2 source is a significant production expense. The effects of CO2 concentrations on algal growth are discussed in the cultivation section later in this chapter, and CO2 requirements and sourcing issues are discussed in Chapter 4. Improved carbon concentrating strategies would address this aspect of photosynthetic efficiency. The enzyme carbonic anhydrase is produced by several divisions of algae (Giordano et al., 2005). The enzyme converts bicarbonate to CO2 that is released intracellularly for fixation by Rubisco. Most algae possess C3 metabolism. That is, the enzyme Rubisco is solely responsible for CO2 fixation. The ability of some plants and microalgae (specifically diatoms and dinoflagellates) to use CO2 directly during C4-intermediate metabolism offers promise for reducing bicarbonate limitation (Zimba et al., 1990; Raven, 2010). A November 2011 press release from Iowa State University reports that Spalding et al. increased algal biomass by 50 to 80 percent in C. reinhartii by artificially increasing the expression of genes that encode components of the carbon-concentrating mechanism, which normally is induced only under low CO2 conditions. The cells presumably continue to actively scavenge CO2 even when it is at relatively abundant levels (Iowa State University, 2011). Algal Crop Protection

Events in which the crop dies (pond “crash” or culture collapse) take a toll on resources and could threaten the economic sustainability and the future potential of the algal biofuel industry (see section Cultivation in this chapter). One cause of such culture collapse is the activity of predators on high-density biomass cultures (see section Contamination and Stability of Culture in this chapter). Simple genetic modifications that affect cell size can improve resistance to grazers and could improve harvesting properties at the same time (Jurgens et al., 1999). Focused screens to find mutations that confer resistance to specific pathogens and grazers are likely to improve crop protection. Because their carbon- and nutrient-allocating traits are the results of domestication, crop algae might carry a heavier metabolic burden than invading weed species. For these reasons, trait modification to instill resistance to herbicides, production of antifungals, and anti-grazer properties could be important. Indeed, at least one company has developed a genetically engineered algal strain for use in open ponds that is resistant to herbicides (IP Monitor, 2009; Aravanis, 2011). Some algae are known to increase lipid content when they are exposed to low levels of herbicide (Ma et al., 2002, 2006). However, if residual biomass is to be used for food or feedstuff, possible negative consequences of these traits would have to be considered. Algal species production of allelopathic chemicals could be exploited to enhance or inhibit growth of other organisms in crop cultures (Gross, 2003). The activities, pathways, and genes related to the secondary metabolites of strains of interest need to be characterized to harness the potential of intrinsic growth modulators. Other Enhancements

The list of potential enhancements is open-ended and will expand as the specific algal species are chosen for cultivation and their attributes become apparent and as the

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