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Meet the gribbles
Tiny bugs could supply the enzymes needed for modern bioenergy.
28 November 2012
It's said that great things happen from small beginnings. In this case, a tiny marine crustacean could revolutionise biofuel production and usher in a new generation of liquid propellants for buses, cars and aeroplanes that would effectively be powered by wood.
A huge amount of energy is stored in woody biomass. But getting at it is harder than it sounds because of the effort needed to prise the sugars in wood out of the tough structures in which it is encased. Enter the gribble: this 2mm creature, like a very small wood louse that lives in the sea, can digest wood all by itself.
BBSRC-funded researchers are looking to harness the power of the gribble's enzymes to release the sugars from wood that can then be fermented to make biofuels. The major advantage is that waste materials can be used to make fuels instead food crops, delivering a double bonus in not competing with land for food production as well as utilising unused materials from timber and agricultural industries.
The gribble, Limnoria quadripunctata, is one of the only animals that can digest wood.
Image: Simon Cragg/University of Portsmouth
The work is part of the BBSRC Sustainable Bioenergy Centre (BSBEC), a £24M investment that brings together six world-class research programmes to develop the UK's bioenergy research capacity.
The gribble is a marine isopod of the family Limnoriidae. They were the scourge of the naval world for centuries because of their abilities to eat entire ships or render them sluggish and useless for long voyages. In fact, their wood-boring abilities are legendary and still present a threat to piers and harbours today (ref 1).
The woody parts of plants are made of lignocellulose from cell walls. Energy-rich polysaccharide (sugar) polymers comprise up to 70% of plant cell walls, making them one of the most abundant reserves of biomass on the planet and ripe for industrial processes such as fermentation (ref 2). The snag is getting at those sugars requires energy to prise the polysaccharides from the tough lignin coating, which is currently achieved by heating at very high pressure.
The way the gribble attacks wood is much more efficient and almost unique. Other wood-digesting creatures such as termites possess gut microbes that do the digesting for them. Not so the gribble, which has a sterile gut – it must be processing the wood itself. This clue has led Professor Simon McQueen-Mason of the University of York and colleagues to take a very close look at the enzymes in the gribble gut.
"We've been working on this animal for two-to-three years," says Professor Simon McQueen-Mason. "At the beginning we did a profile of the genes in the digestive system to see the major enzymes are that are involved in digestion."
The species they are most interested is Limnoria quadripunctata, a convenient animal to culture in the lab, but a major pest species (ref 3) in temperate waters around the world where it is probably still dispersed by wooden ships. What they have found is that the gribble is exposing the wood it swallows to a kind of pre-treatment to loosen up the structure of the wood before the main enzymes get to work.
An electron microscope reveals round secretory structures in the gribble hepatopancreas that secrete unique wood-digesting enzymes.
Image: Simon Cragg/University of Portsmouth
Using electron microscopy as well as micro-computed tomography (ref 4), a miniature version of the 3D x-ray CT scans undertaken in hospitals, scientists have located a gland alongside the gribble gut called the hepatopancreas; not only does it absorb and stores toxins out of harm's way, it also secretes a chemical that breaks down the lignocellulose and allows other enzymes to break down the sugar components. A model of biological efficiency, McQueen-Mason thinks this chemical pre-treatment also helps to keep the gribble gut free of microbes.
While fishing around the gribble gut, researchers have also found cellulose-degrading enzymes from the glycosyl hydrolase family which had never been seen in animals before, only in higher wood-digesting fungi, and in protists that live symbiotically in the gut of termites. "Virtually all other examples of this enzyme family are non-marine and the only others produced by animals are from other crustaceans that are not capable of breaking down wood," says Dr Simon Cragg from the Institute of Marine Sciences Laboratories at the University of Portsmouth. "Wood probably represents the most challenging, though energetically rewarding of substrates."
Creatures great and small
The challenge now is to recreate the processes taking place in the gribble at an industrial scale. McQueen Mason thinks that technology from the gribble – in the way that it pre-treats wood before digestion proper – will allow industrial scale pre-treatment of woody biomass to happen at much lower temperatures. "We also hope that some of the enzymes we find in the gribble, which we know function better than the ones we have under particular conditions will also be applicable in this process," he says.
Another way of increasing efficiency is to decode the structure of the gribble enzymes. This is necessary so they can be cheaply produced in bacteria; it may also be possible to tinker with them to make them even more efficient at certain temperatures for example, or when breaking up harder substrates.
To delve into the enzyme in unprecedented detail, Cragg's colleague John McGeehan, also of the University of Portsmouth, has been using a particle accelerator, the Diamond Light Source synchrotron, based in Harwell, Oxfordshire; it's powerful x-ray beam lines can be used to elucidate complex molecular structures and their results are due to be published soon.
Researchers are using advanced techniques to solve enzyme structures.
Image: Richard Martin/University of Portsmouth
Sequencing the DNA of the gribble is also underway. "We now have the opportunity to build a whole genome for our creature, supported by our York-based colleagues, by The Genome Analysis Centre (TGAC) in Norwich, and US collaborators at three different institutions," says Cragg. Funded by a BBSRC Partnering Award (see 'International funding' in related links above), the partnering project started in May 2010 and the US partners will tackle DNA sequencing of the whole gribble genome; TGAC will then assist the University of Portsmouth team in assembling it. A workshop with all collaborating members will then tackle annotation of the genome (identifying important functional elements) around September 2013.
In the future, given the demands and complexities of providing sustainable biofuels, this type of fundamental work is unlikely to be restricted to just the tiny gribble. Enzymatic and genetic analysis can also be applied to the limited range of critters that can digest wood without microbes, which include some mussel-like molluscs.
"We have identified some other animals with remarkable abilities to deconstruct tough substrates which we know have unusual digestive systems and may have enzymes of biotechnological potential, " says Cragg. "Our insights will also drive innovation in benign wood protection methods for the marine environment."
- The Gribble Worm (external link)
- Molecular insight into lignocellulose digestion by a marine isopod in the absence of gut microbes
- Yarmouth pier saved by Heritage Lottery Grant
- Micro-CT (external link)
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