Download the Algal Biofuels Roadmap draft document - Sandia

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growth conditions dates back to the 1940s. Various reports during the 1950s and 1960s
indicated that starvation for key nutrients, such as nitrogen or silicon, could lead to this
phenomenon. The concept of utilizing these lipid stores as a source of energy only gained
serious attention during the oil embargo of the early 1970s, ultimately becoming the
major push of DOE‘s Aquatic Species Program.
The Aquatic Species Program represents the most comprehensive research effort to date
on fuels from algae. The program lasted from 1978 until 1996 and supported research
primarily at DOE‘s NREL (formerly the Solar Energy Research Institute). The Aquatic
Species Program also funded research at many academic institutions through
subcontracts. Approximately $25 million (Sheehan, 1998) was invested during the 18year
program. During the early years, the emphasis was on using algae to produce
hydrogen, but the focus changed to liquid fuels (biodiesel) in the early 1980s. Advances
were made through algal strain isolation and characterization, studies of algal physiology
and biochemistry, genetic engineering, process development, and demonstration-scale
algal mass culture. Techno-economic analyses and resource assessments were also
important aspects of the program. In 1998, a comprehensive overview of the project was
completed (Sheehan et al., 1998). Some of the highlights are described briefly below.
The Aquatic Species Program researchers collected more than 3,000 strains of microalgae
over a seven-year period from various sites in the Western, Northwestern, and
Southeastern U.S. representing a diversity of aquatic environments and water types.
Many of the strains were isolated from shallow, inland saline habitats that typically
undergo substantial swings in temperature and salinity. The isolates were screened for
their tolerance to variations in salinity, pH, and temperature, and also for their ability to
produce neutral lipids. The collection was narrowed to the 300 most promising strains,
primarily green algae (Chlorophyceae) and diatoms (Bacillariophyceae).
After promising microalgae were identified, further studies examined the ability of many
strains to induce lipid accumulation under conditions of nutrient stress. Although nutrient
deficiency actually reduces the overall rate of oil production in a culture (because of the
concomitant decrease in the cell growth rate), studying this response led to valuable
insights into the mechanisms of lipid biosynthesis. Under inducing conditions, some
species in the collection were shown to accumulate as much as 60% of their dry weight in
the form of lipid, primarily TAGs. Cyclotella cryptica, a diatom that is a attractive lipid
producer, was the focus of many of the biochemical studies. In this species, growth under
conditions of insufficient silicon (a component of the cell wall) is a trigger for increased
oil production. A key enzyme is acetyl-CoA carboxylase (ACCase), which catalyzes the
first step in the biosynthesis of fatty acids used for TAG synthesis. ACCase activity was
found to increase under the nutrient stress conditions (Roessler, 1988), suggesting that it
may play a role as a ―spigot‖ controlling lipid synthesis, and thus the enzyme was
extensively characterized (Roessler, 1990). Additional studies focused on storage
carbohydrate production, as biosynthesis of these compounds competes for fixed carbon
units that might otherwise be used for lipid formation. Enzymes involved in the
biosynthesis of the storage carbohydrate chrysolaminarin in C. cryptica were
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