Algae Pump Out Hydrocarbon Biofuels

When it comes to biofuels, ethanol from corn gets all the press, but redirecting the photosynthetic process to produce hydrogen or hydrocarbons is another potential source of biofuels.

Plants, cyanobacteria, and algae are all photosynthetic organisms that could be harnessed in this way. Of these, microalgae are the most efficient producers—not surprising considering that as much as 60–70% of each cell's volume is crammed with photosynthetic apparatus, and these organisms don't have roots, stems, or leaves to divert resources.

Anastasios Melis, professor of plant biology at the University of California, Berkeley, is engineering different microalgal species to maximize the generation of hydrogen and hydrocarbons. He described his group's work in a symposium sponsored by the Division of Biochemical Technology.

Although the energy content of hydrogen makes it an attractive fuel, Melis worries about the storage and transport issues associated with hydrogen. He is now investigating whether algae can produce hydrocarbon biofuels. In this context, the hydrocarbons can be viewed as hydrogen stored on carbon, Melis said.

To do this, he is engineering the alga Botryococcus braunii to produce more of the terpenoid C₃₀ botryococcene, a hydrocarbon that is similar to squalene in structure but difficult for cells to metabolize. Instead, the algae secrete droplets of the valuable oil. Melis' team has tinkered with the biosynthetic steps so that the algae produce more of the precursors required to make C₃₀ botryococcene.

Melis' group constructed a large doughnut-shaped polyethylene container to grow large amounts of the oil-secreting algae. They found that in the doughnut container, sunlight doesn't penetrate far into the high-density culture, and the algae beneath the surface are practically in darkness. To alleviate the problem, they engineered algae with smaller light-collecting antennae, which allow more light to penetrate the culture without sacrificing hydrocarbon production. Typically, Melis said, the algal photosystems contain antennae made up of approximately 600 chlorophyll molecules. But the number of chlorophyll molecules can drop to as few as 130, he said. They found that the gene tla1 is key to determining antenna size and is the first such eukaryotic gene to be identified.