Microbes make cellulose from sunshine

By Jerry W. Kram | April 08, 2008
Web exclusive posted April 29, 2008 at 2:25 p.m. CST

Cellulose is the feedstock of choice for a new generation of biofuels. However, if the work by researchers at the University of Texas in Austin pans out, that cellulose may not come from wood or other plants. R. Malcolm Brown and David Nobles Jr. have engineered a microorganism that not only makes cellulose and sugars from sunshine and the carbon dioxide in the air but does it faster than just about any other organism alive.

Cyanobacteria, also called blue-green algae, are more closely related to bacteria than to algae and plants. Some cyanobacteria naturally produce cellulose, but the variety used by Brown and Nobles did not. The researchers inserted a gene from a species of Acetobacter which is a prolific producer of cellulose. "We have been studying cellulose biosynthesis for 40 years and the idea was to get the cellulose biosynthesis into the cyanobacteria that normally do not make cellulose," said Brown. The researchers have also created other modified strains of the organisms which secret sucrose and other simple sugars.

Brown and Nobles say the cellulose produced by the cyanobacteria will be easier to convert into biofuels than that of plants. The organisms produce a gel type of cellulose instead of the crystalline type found in the cell walls of plants. "There are a lot of other things in trees, such as lignins and hemicelluloses that need to be removed," Brown said. "You need either acids or cellulase enzymes to break them down. The more crystalline they are the harder they are to break down. We found our cellulose had zero crystallinity and very low molecular weight that made it easier to break down. It is still insoluble, but it is easier to break down with the cellulase enzymes."

The cyanobacteria, which requires a few micronutrients and fixed nitrogen to thrive, can grow in either fresh or briny water, Nobels said. In the lab, the cultures were able to produce prodigious amounts of cellulose from carbon dioxide in the atmosphere, but it may be possible to increase its productivity by enriching it with additional carbon dioxide.

The researchers plan to move their research outside of the laboratory setting. "We are working right now to get this out of the laboratory and into a demonstration scale facility," Brown said. There are many options, but the researchers say they may consider using photobioreactors to forestall objections to growing a genetically modified organism in open ponds. "Likely we will also require a closed system because we will be growing an organism that is secreting sugars," Nobels added. "That would create problems in an open pond."