The Gut(microbe)less Gribble – Biofuel Hero?
Behold the Gribble – a true gutless wonder. The Gribble (pictured here) is a marine wood-boring creature of around 2 millimeters in size. Scientists at the UK’s Biotechnology and Biological Sciences Research Council have been spending quality time with the Gribble because of its exceptional innards.
The tiny animal eats wood that finds its way into the sea. The wood can come from mangrove swamps or wash into estuaries from land. Gribbles, also called ship borers, have also been known to chow on wooden sailing vessels (including, rather famously, those of the Columbus voyages). “I’m sure they’ve taken down a few pirate ships, too” says Simon J McQueen-Mason a BBSRC researcher and materials biology professor at the University of York.
Most critters that eat wood or other lignocellulose plant material rely on symbiotic relationships with a diverse population of gut microbes - called the microbiome - to break down the tough-to-digest meal. When news reports suggest that pandas may hold the key to biofuels breakthroughs because they can live on tough bamboo, it’s really the microbes, and the enzymes made by the microbes, that are of interest.
(You can read a C&EN cover story about pandas, microbiomes and biofuels )
But the Gribble has no microbiome. And it doesn’t have the squishy, absorptive digestive system that most animals have. In fact, it digests its meals of wood in a sterile, hard-sided chamber in its hind gut. McQueen-Mason likens the environment to “a steel container you might use in an industrial lab.”
Instead of microbial helpers, the gribble has a separate organ where it produces the key enzyme itself. Termites do not do this (they have microbes). The gribble “must use quite aggressive chemistry; the enzyme is so harsh that it would kill any microbes” that might otherwise occupy the space, McQueen-Mason says.
The research team found the mystery organ and looked at the genes expressed there. Many of them encoded instructions for making what is called GH7 cellulase. This is a family of enzymes that are normally found in wood-degrading fungi. “These cellulases are abundant but were never reported in an animal before,” McQueen-Mason notes. “We were able to express the genes in a lab fungus and describe the properties.”
They also used X-ray crystallography to discover the structure of the enzyme and show how it binds cellulose chains and breaks them into small sugar molecules.
The Gribble lives in the sea and eats wood. Image: Laura Michie, University of Portsmouth
The Gribble’s enzyme appears to be very rugged and long-lasting, which is a good quality for an enzyme that might be used in an industrial setting to make biofuels from wood or straw, McQueen-Mason points out. It works very well in highly saline conditions and may also function well in ionic liquids. The use of salt water and ionic liquids for biofuels processing may cut down on the use of expensive, precious fresh water. And like a true catalyst, the enzyme may be reusable.
You can see a video of the Gribble – which I highly recommend – it’s kind of cute.
For more on the enzyme, check out the journal paper: 'Structural characterization of the first marine animal Family 7 cellobiohydrolase suggests a mechanism of cellulase salt tolerance' www.pnas.org/cgi/doi/10.1073/pnas.1301502110.
The GH7 cellulase, an enzyme made by the Gribble, breaks down cellulose into simpler sugars.