Pandas, Cellulose, and Designer Bugs

September 1, 2011

About a year ago, I posted a note here about some research on the possibility of using genetically-modified “designer yeast”  to convert cellulose into ethanol (ethyl alcohol).   Cellulose, which is a polysaccharide (sugar polymer) containing from a few hundred to ~10,000 sugar units, is probably the most abundant organic compound on Earth.  It is estimated that something like 320 million tons of cellulose-containing agricultural waste, in the form of wood ships, corn stalks, and similar material, is thrown away every year just in the United States.  Unfortunately, the yeasts that can happily turn simple sugars from crops like corn, grapes, or sugar cane into ethanol cannot get at the glucose within the cellulose structure.  (Animals that consume this kind of plant matter are able to digest it thanks to symbiotic bacteria in their digestive tracts that produce enzymes capable of breaking down cellulose.)  The use of food crops for fuel production raises a number of issues, ranging from basic energy efficiency to the potential impact on food prices.  And it is clearly possible to break down cellulose — that plant matter does, after all, rot eventually.

Happily, a couple of recent developments may indicate there is another way to attack the problem.  The first is described in a press release from Tulane University in Louisiana, where a group of scientists have discovered a new strain of bacteria that are able to convert cellulose into butanol (butyl alcohol).

Tulane University scientists have discovered a novel bacterial strain, dubbed “TU-103,” that can use paper to produce butanol, a biofuel that can serve as a substitute for gasoline. They are currently experimenting with old editions of the Times Picayune newspaper with great success.

Butanol [C4H9OH] is a higher molecular weight alcohol than ethanol [C2H5OH]; it has a backbone made of four carbon atoms rather than two.  It is in many ways more attractive than ethanol as a fuel substitute; it is less corrosive, has a higher energy density, and can be used in existing engines with minimal modifications.  It is also relatively safe: more toxic than ethanol, but much less toxic than methanol.  The press release does not give the species name of the new strain TU-103, but it does describe it as “clostridial”, meaning presumably it is of the genus Clostridium, and notes that it can grow in the presence of oxygen.  Oxygen is toxic to many of the other Clostridium species, such as the organisms that produce tetanus (Clostridium tetani) and botulism (Clostridium botulinum).  This makes the idea of producing butanol on an industrial scale using TU-103 more plausible.

The other research news, noted in an article at Science Daily, was reported  in Denver at the 242nd National Meeting & Exposition of the American Chemical Society.  A group of researchers studied a pair of giant pandas at the Memphis Zoo, and analyzed the animals’ excrement for over a year.   The diet of adult giant pandas is almost entirely (99%) bamboo, of which they may consume 20-40 pounds per day.  Like ruminants, and some insects, pandas are host to bacterial symbionts that produce enzymes that can break down cellulose.

They identified several types of digestive bacteria in the panda feces, including some that are similar to those found in termites, which are renowned for their ability to digest wood.

“Our studies suggest that bacteria species in the panda intestine may be more efficient at breaking down plant materials than termite bacteria and may do so in a way that is better for biofuel manufacturing purposes,” said Brown [Ashli Brown, Ph.D, co-author of the study], who is with Mississippi State University.

The researchers report [summary and abstract] that some of the bacteria are able to break down lignocellulose, one of the toughest forms of the material.  The researchers are continuing their analysis, hoping to catalog all of the micro-organisms present in panda feces. (Apropos of nothing, I would love to see the reaction when they answer the standard, “What do you do?” question at parties.)   Eventually, they hope that genes for producing the relevant enzymes can be engineered into yeast, which can then be used for fuel production.

All of this is still at a relatively early stage, and there is much work yet to do.  Still, it is rather heartening to think that we may be able to meet some of our energy needs, and reduce our dependence on landfills, by using processes adapted from nature,

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