A recent article about a marine crustacean, the gribble, and making biofuel from wood came to my attention. I decided to follow up to see if it was a good topic for me to write about.

I quickly found articles from 2009 and 2013 that also seemed to be announcing the gribble’s usefulness. So what was the news in the most recent one?

My topic today, therefore, is not so much about the latest gribble research as the pace of scientific progress. On closer inspection, each of the three articles announced an important new breakthrough. Science is getting closer and closer to making biofuels from agricultural waste and non-food crops.

It hasn’t yet gotten there. To a chemist, the science may be progressing swiftly. To a non-scientist, it may appear glacially slow in bringing results.

President George W. Bush championed making ethanol from switchgrass in his 2006 State of the Union address. It grows on land farmers can’t do anything else with. And so it’s cheap. Wood chips and various kinds of agricultural waste can make ethanol, too.

Nearly 13 years after that speech, hardly any biofuels from these waste products are available commercially anywhere in the world. The project has encountered immense technological problems.

The basic problem of biofuel from biomass

Nearly all the gasoline available on the market today contains ethanol, a biofuel. Currently, ethanol comes from corn. It has at least two major drawbacks.

First, corn used to make ethanol can’t feed people or cattle. Production of ethanol directly competes with production of food.

Second, it requires energy, mostly from fossil fuels, to grow corn. Tractors and other farm equipment require some of that energy.

Most farmers use some kind of petrochemical fertilizer. We need to count the energy costs of manufacturing and transporting it as part of the energy required to grow corn. Making ethanol from the corn takes still more energy. The energy burning this ethanol provides is less than the energy required to make it.

No one except perhaps farm state politicians and lobbyists questions the need for ethanol from non-food sources.

But there’s a big chemical difference between corn and switchgrass or agricultural waste. Sugars humans can digest are disaccharides. That is, their molecules comprise two simple sugars. Our digestive system easily breaks them apart. I make chocolates every year and use an inexpensive enzyme to break the molecules in my creams and fudge.

The sugars in switchgrass, or for that matter, corn stalks and other inedible parts, are polysaccharides. Their molecules have at least ten simple sugars. It’s the simple sugars that ferment to make alcohols like ethanol. The enzymes that break down disaccharides don’t work on polysaccharides.

Agricultural wastes and non-food crops like switchgrass provide cheap cellulose in abundance. The chemistry of converting it to ethanol or other biofuel is complicated and expensive—and not the only technological challenge cellulosic ethanol presents.

Meet the gribble

In January 2009, the British government announced an investment £27 million to research and create biofuels that, unlike corn-based ethanol, would not compete with food production. In other words, these new biofuels would come from agricultural waste or from non-food crops grown on marginal land.

One of the research projects concerned a marine nuisance called the gribble. Gribbles eat wood that gets into the ocean, including piers and the bottoms of wooden boats. The scientists wanted to learn how they break down the cellulose in wood into simple sugars in order to get nutrition from it.

In principle, if we can make the chemicals a gribble does and replicate its processes, we can solve problems of both energy and waste management.

Scientists discover an enzyme in the gribble

A 2013 news story based on a paper from that same research project noted that the gribble’s digestive system does not resemble that of other cellulose-eating animals. Both pandas and termites rely on gut bacteria to digest the cellulose.

Gribbles have no gut bacteria. The authors of the paper noted that gribbles have a hard-sided chamber in their rear gut where digestion takes place. One of them compared it to steel containers used in industrial labs.

A separate organ secretes a very strong enzyme that accomplishes what a termite’s gut bacteria would do. It resembles a family of enzymes, called GH7 cellulases. GH7 is common in the kind of fungi that degrade wood. It was unknown in animals until scientists started to study the gribble.

On careful analysis, it turns out that the gribble’s GH7 is more acidic than that found in fungi. It would kill any bacteria that make it into the digestion chamber. Part of the research entailed using x-ray scanning to produce a 3D model of its molecule.

The scientists also looked at the gribble’s genetics to discover which genes encoded instructions for synthesizing the enzyme. Instead of obtaining it from gribbles, however, the team transferred these genetic instructions to microbes, which can make the cellulase in large quantities.

This enzyme has several advantages for industrial use. It works in a saline environment, for one thing. That means that using it to make biofuel would not require fresh water. Using salt water instead saves drinking water and costs much less. Also, the enzyme acts as a catalyst. That is, it is not consumed in the process of digesting cellulose and can be reused.

Scientists discover a protein in the gribble

The most recent article reports on further research, apparently by the same British team. The mechanism for the enzyme’s working involves a protein called hemocyanin.

In vertebrates, a blood protein called hemoglobin transports oxygen and, incidentally, contains iron that makes the blood red. Hemocyanins accomplish the same task in invertebrates. They contain copper instead of iron, which make invertebrates’ blood blue.

When gribbles chew the wood, they grind it into very small pieces. Oxygen carried by the hemocyanin begins to break down the cellulose. The GH7 completes the process and releases the simple sugars.

Current methods of obtaining simple sugars from cellulose call for a chemical pretreatment followed by different chemicals to release the sugars. The hemocyanin accomplishes the pretreatment for gribbles, and more efficiently than current industrial chemicals.

Can what happens in a gribble’s digestive system be scaled up for industrial production of glucose from cellulose?

So far, research is promising. But the authors of the latest paper can only say that their discovery may be useful in the long term.

Other research teams are exploring different aspects of the biochemistry of making biofuels. I suppose all of them are making similar progress in learning about other ways nature works. Maybe gribbles will become the model for digesting cellulose. Maybe something else will.

Scientific progress never comes as quickly as we would like. Maybe in five or six years, the British team will write another paper about something new they have learned about gribbles. Maybe by then, they will announce a discovery that industry can scale up and start using. Maybe.

Sources:

Sources:
Enzyme from wood-eating gribble could help turn waste into biofuel / Phys.Org. June 3, 2013
Gribble could hold the key to new generation of biofuels / Louise Gray, The Telegraph. January 27, 2009
The gut(microbe)less gribble – biofuel hero? / Melody Bomgardner, CleanTech Chemistry. June 11, 2013
Why a curious crustacean could hold secret to making renewable energy from wood / Physics.org. December 3, 2018

Photo credits:
Switchgrass harvest. Some rights reserved by eXtension Farm Energy.
Corn stover. Image courtesy of Idaho National Laboratory (INL) Bioenergy Program, via Flickr
Two yellow gribbles. Photo by Auguste le Roux from Wikimedia Commons
Single green gribble. Source unknown.