To fulfill the growing “energy” needs of Nevada, it is becoming increasingly clear that the state and country must invest in the development of alternative energy resources, such as biofuels, wind, solar and geothermal. Dunaliella has been proposed as a biofuels feedstock because it can produce high quantities of oil. Because this organism grows optimally in high saline environments, and is naturally occurring in various regions of the Great Basin, it appears to be well suited as a new alternative biofuels crop for Nevada.
Background on Dunaliella
Dunaliella spp. are halophilic unicellular green alga distributed worldwide that are the primary photosynthetic organism in many hypersaline environments and the cause of orange to red algal blooms in these ecosystems (Oren 2005). For example, the strain of D. salina whose complete genome sequence is being completed was isolated from salt lagoons in Australia. A draft genome sequence of D. salina is expected in 2007, funded through the Department of Energy’s Joint Genome Institute Community Sequence Program. A member of this genome sequencing team is John Cushman, a Co-PI on this proposal. This rationale for sequencing the D. salina genome reflects a growing interest by the research and commercial sectors. Two of the most important traits of interest are Dunaliella’s ability to produce high levels of oil (up to 30% of the organism dry wt), and its remarkable ability to grow in extreme saline environments (up to 5.5 M NaCl). These traits are of considerable academic interest in terms of understanding their contributing pathways and mechanisms. In addition, they have considerable commercial value because they provide the potential of producing a reliable feedstock for biofuel production, especially for regions with low quality salt water.
Dunaliella salina belongs to the same phylogenetic family as the well-studied fresh water green algae Chlamydomonas reinhardtii. However, it has several striking structural and physiological differences. Remarkably, Dunaliella enhances its photosynthetic activity in response to high salinity, whereas in most higher plants salinity stress inhibits photosynthesis (Liska et al., 2004). Increasing salinity can also increase the fatty acid chain length and degree of overall desaturation (Azachi et al., 2002). D. salina also has a unique osmoprotection system. It does not have a normal cell wall but rather a less rigid mucin layer, allowing it to inhabit a broad range of salinities from 0.5 to 5.5 M NaCl with optimal growth occurring at 1.2 M NaCl. D. salina appears to protect itself from osmotic shock and potential lysis by producing high concentrations of glycerol as an osmotic stabilizer. D. salina also produces high concentrations of carotenoids under high light and salinity conditions as a protectant against photooxidative stress. Another remarkable adaptation of D. salina is the high structural stability of certain extrinsic enzymes, such as carbonic anhydrase, that retain structural stability and solubility in low to high salt concentrations (Premkumar et al., 2005). Although it should be noted that other key enzymes from this organism are inhibited by high salt concentrations presumably because intracellular ionic concentrations of D. salina are maintained at very low levels. Most importantly, D. salina and related species have lipid and fatty acid compositions similar to plants and thus can serve as suitable feedstocks for biofuel production (Peeler et al., 1989; Thompson 1996).
Like some fresh water algae, some strains of D. salina have been found to produce hydrogen gas, raising the potential that this organism could be engineered to be an efficient converter of solar energy into a clean hydrogen gas (Ghirardi et al., 2000). The ability of D. salina and related species to produce hydrogen is currently being investigated by collaborators at the National Renewable Energy laboratory (Golden, CO). Thus, this unique alga has proven or potential relevance in regard to its role in the global carbon cycle, the production biofuels, and the synthesis of various economically valuable macromolecules and chemicals.
Despite the potential global economic importance of Dunaliella, relatively little research has been done to understand its unique physiology and exploit it tremendous potential. While the federal government is increasing their investment into biofuel research, most of the attention is being giving to fresh water dependent crops that will need to be grown using existing agricultural land and water resources. However, for Nevada, this is not a viable long-term strategy, as it will be politically and pragmatically un-reasonable to re-allocate significant amounts of its precious water and agricultural resources to feed the appetite of a new biofuels industry. Rather, it makes sense for Nevada to take the lead on developing an alternative biofuel feedstock that is more compatible with Nevada’s need to manage the unique resources available in the Great Basin.
Non-seasonal and local feedstocks: Traditional biofuel feedstocks are produced seasonally mainly in the Midwestern U.S. A major advantage of algal production systems is that they can be grown locally on a year-round basis with the aid of geothermal heating systems.
Halophytic production systems: Traditional agricultural productions systems require a high quality of fresh water. The alternative halophytic algal production system proposed here will utilize low quality waste water with preferably high salt content unsuitable for any other use.
Leveraging geothermal resources: In addition to abundance sunlight, Nevada has significant geothermal resources that could be leveraged to improve the economic viability of the proposed halophytic alga production systems. One of the major drawbacks of algal production systems investigated by the DOE in the mid 1990’s was the nighttime cooling of production ponds that limited algal growth rates. Effective and strategic exploitation of geothermal resources for heating of algal production ponds would overcome this drawback.
Biomaterials production: Aside from biofuels production, microalgae, in general, and Dunaliella in particular, are an attractive model to explore for it ability to produce a variety of “value-added” and antioxidant compounds with use in the pharmaceutical, neutraceutical, cosmetics, and biochemical industries (Borowitzka 1985; Hajazi et al., 2004; Hajazi and Wijffels, 2004). This alga serves as an important source of food dyes, vitamin A, tocopherols, lutein, and beta-carotenoids (Ben Amotz et al., 1991; Jinn and Melis, 2003). Following the extraction of oils, residual cellular components can be fractionated and selected “value-added” compounds can be purified including glycerol, pigments, and antioxidants (Herrero et al., 2006). Following this, residual materials could be fermented for the production of ethanol, an essential ingredient in the production of biodiesel. Local production of ethanol would avoid the transportation costs of shipping ethanol from outside locations. Alternatively, residual cell material could be utilized as an animal feed supplement rich in vitamins and essential nutrients and sold to local and regional cattlemen and dairymen.