A Look Back at the U.S. Department of Energy’s Aquatic Species Program:
Biodiesel from Algae Biological Concepts
Photosynthetic organisms include plants, algae and some photosynthetic bacteria.
Photosynthesis is the key to making solar energy available in useable forms for all
organic life in our environment. These organisms use energy from the sun to
combine water with carbon dioxide (CO2) to create biomass. While other elements of
the Biofuels Program have focused on terrestrial plants as sources of fuels, ASP was
concerned with photosynthetic organisms that grew in aquatic environments. These
include macroalgae, microalgae and emergents. Macroalgae, more commonly known
as “seaweed,” are fast growing marine and freshwater plants that can grow to
considerable size (up to 60m in length). Emergents are plants that grow partially
submerged in bogs and marshes. Microalgae are, as the name suggests, microscopic
photosynthetic organisms. Like macroalgae, these organisms are found in both
marine and freshwater environments. In the early days of the program, research was
done on all three types of aquatic species. As emphasis switched to production of
natural oils for biodiesel, microalgae became the exclusive focus of the research.
This is because microalgae generally produce more of the right kinds of natural oils
needed for biodiesel (see the discussion of fuel concepts presented later in this
In many ways, the study of microalgae is a relatively limited field of study. Algae
are not nearly as well understood as other organisms that have found a role in today’s
biotechnology industry. This is part of what makes our program so valuable. Much
of the work done over the past two decades represents genuine additions to the
scientific literature. The limited size of the scientific community involved in this
work also makes it more difficult, and sometimes slower, compared to the progress
seen with more conventional organisms. The study of microalgae represents an area
of high risk and high gains.
These photosynthetic organisms are far from monolithic. Biologists have categorized
microalgae in a variety of classes, mainly distinguished by their pigmentation, life
cycle and basic cellular structure. The four most important (at least in terms of
· The diatoms (Bacillariophyceae). These algae dominate the
phytoplankton of the oceans, but are also found in fresh and
brackish water. Approximately 100,000 species are known to
exist. Diatoms contain polymerized silica (Si) in their cell walls.
All cells store carbon in a variety of forms. Diatoms store
carbon in the form of natural oils or as a polymer of
carbohydrates known as chyrsolaminarin.
· The green algae (Chlorophyceae). These are also quite
abundant, especially in freshwater. (Anyone who owns a
swimming pool is more than familiar with this class of algae).
They can occur as single cells or as colonies. Green algae are the
evolutionary progenitors of modern plants. The main storage
compound for green algae is starch, though oils can be produced
under certain conditions.
2 A Look Back at the Aquatic Species Program—Program Summary
· The blue-green algae (Cyanophyceae). Much closer to bacteria
in structure and organization, these algae play an important role
in fixing nitrogen from the atmosphere. There are approximately
2,000 known species found in a variety of habitats.
· The golden algae (Chrysophyceae). This group of algae is
similar to the diatoms. They have more complex pigment
systems, and can appear yellow, brown or orange in color.
Approximately 1,000 species are known to exist, primarily in
freshwater systems. They are similar to diatoms in pigmentation
and biochemical composition. The golden algae produce natural
oils and carbohydrates as storage compounds.