is the red algal species Kappaphycus alvarezii, a species cultivated for its high carrageenan2 content (Russell, 1983; Rodgers and Cox, 1999; Woo et al., 2000). Species of Spirulina have properties suitable for aquaculture, and they are grown at relatively large scales for sale as a nutritional supplement (Earthrise Nutritional, 2009a). Still, the spectrum of cyanobacteria that could be suitable for fuel production is largely unexplored. Prokaryotic algal species provide additional diversity in light harvesting, tolerance of growth habitat and pH, and facility of genetic modification.3 Moreover, some cyanobacterial species are diazotrophs; that is, they are able to fix atmospheric nitrogen (N). Although no current commercial operations rely on a nitrogen-fixing strain, several filamentous strains that have good lightharvesting properties and for which genetic methods are well developed are diazotrophic (Heidorn et al., 2011; Ruffing, 2011). The use of these strains as a biofuel feedstock or as a nitrogen provider for non-fixing strains (to reduce nutrient input) has received little attention.
Clear differences exist in carbon storage forms (important as fuel feedstock), dominant pigments (important for solar energy capture), and accessory pigments such as carotenoids (which can be valuable commercial products) among different algal divisions (Table 2-1). Furthermore, their pigmentation and composition are affected by growth conditions and environmental stress.
Emphasizing individual strains that are intended for monoculture discounts potential advantages that could be associated with mixed cultures. A recent study showed increased lipid production in algal cultures as a function of species diversity in mixed cultures under nutrient-limiting growth conditions (Stockenreiter et al., 2012). However, this effect has been demonstrated only at the laboratory scale or in low-density natural algal populations, and requires confirmation for extended periods of time and at relevant volumes. Moreover, lipid production of mixed algal culture could be different under the nutrient-replete conditions of ponds designed for maximal growth. Mixed cultures might facilitate crossprotection, diversity of products through product conversion, flocculation and harvesting improvements, and efficient use of light in the water column (Stomp et al., 2007). However, mixed cultures increase the heterogeneity of the potential product, which could affect the quality of yield and the ability to optimize the diverse characteristics of the mixture for a single product. The potential to enhance the supply chain of algal biofuel through growth of mixed cultures merits additional research to determine the effects on desirable product yield and biomass accumulation (see section Cultivation in this chapter). Because data are not available for large-scale, mixed-species systems, this report introduces the concept of mixed culture systems but focuses primarily on monoculture systems.
Among the biggest challenges for strain selection is the difficulty of translating desirable strain properties from the laboratory to the field. A desirable strain would have robust growth in open ponds under natural weather and cultivation conditions, and would retain attributes that are selected and measured in the controlled conditions of the laboratory. However, the ability to grow well and compete when exposed to environmental conditions is difficult to predict. Few strains are already proven to be robust in outdoor mass cultivation, and years of investment in time and process went into their commercial development.
2 A gelatinous substance extracted from red algae and widely used as a stabilizing or thickening agent in industrial, pharmaceutical, and food products.
3 Within the text of this report, the committee will distinguish whether it is discussing “genetic modification” or “genetic engineering” specifically. The committee considers genetic modification to be a general term and includes in its definition any organism whose genetic material has been altered through an array of approaches, including traditional cross breeding, mutagenesis, and genetic engineering. Genetic engineering is a modern technique that enables the introduction of a foreign gene or genes into the genome of an organism through recombinant DNA methods in an attempt to introduce a new trait into that organism.