Gribbles can be thought of as ocean-dwelling termites. Their appetites for wooden boat hulls are well known to generations of sailors, but now it turns out that the gribble gut possesses wood-processing capabilities almost too good, and too weird, to be true.
These marine crustaceans are tiny (about a millimeter in length for the smallest, all the way up to 4 mm), come in more than 50 varieties, and belong to the category of “isopod,” which sounds prettier than “wood lice.” They will bore into and eat pretty much any kind of saltwater-soaked wood they can find, which makes them a pest to those who must care for vintage boat hulls, dock pilings and shoreline structures.
Others appreciate their ecological service in eating away the fallen trees that wash down estuaries and clog up the coastline.
And in recent weeks they have been in the spotlight because of new research suggesting that they may hold, in their hindgut, a simple solution to the enduring difficulty of extracting clean fuel from woody plants in a cheap, nonpolluting, sustainable way.
Things are not quite so simple as the headline in Popular Science would have it: “What Eats Wood and Poops Clean Energy?” But the findings do seem to carry a clear potential for breakthrough.
The problem of lignin
As a renewable and theoretically carbon-neutral fuel stock, the value of trees and other woody plants is vast. The research announcement from a team of scientists based at the UK’s University of York notes that:
Woody plant biomass is the most abundant renewable carbon resource on the planet, and, unlike using food crops to make biofuels [think corn-derived ethanol], its use doesn’t come into conflict with global food security.
However, the inputs of heat and strong industrial chemicals required to extract plant sugars have made large-scale production of biomass-derived fuel infeasible thus far; fossil competitors are still cheaper, and not always that much dirtier in accountings that consider the whole production cycle. So the main path for biomass energy thus far has been as another CO2-producing boiler fuel.
That cannot change until we find a way past the barrier of lignin — a group of tough polymers that create the structural strength of wood fiber, whether in a willow tree or a corn stalk, while locking the plants’ stores of photosynthesized energy away from hungry bugs.
It’s lignin removal at certain paper-making plants, using lye and other potent alkalis, that causes that outhouse stench but renders a strong and bright white pulp. Newsprint mills don’t bother, pulping the wood by mechanical grinding, but the result is a flimsy, dingy sheet with occasional slivers that are no problem embedded in the sports page but not necessarily something you would want to pump through your car’s fuel injectors.
For decades there have been high hopes for a low-heat, non-toxic way of breaking down lignins with enzymes. In 2001, at the National Renewable Energy Laboratory near Denver, I toured a lab-scale still that was using enzyme digestion to turn wood pulp and water into an ethanol precursor with a mildly nauseating resemblance to Guinness. It was said to be just a few years away from potential deployment at industrial scale, and I’m guessing it’s no closer today.
Lignins aren’t complex chemically, but structurally they are kind of random, and therein lies their resistance to enzymes. The best explanation I saw comes from Chuck Dinerstein over at the American Council of Science and Health:
Lignin is a particular type of polymer, still made up of identical components (monomers) but linked together without a regular repeat structure that enzymes can attack, making them more resistant to degradation.
But the gribble, or at least the Limnoria quadripunctata Holthuis variety studied at York, seems to have harbored the equivalent of such an enzyme along: a protein called hemocyanin that pre-digests wood pulp in the hindgut. The food is then sent for final processing, and without the bacteria and other microbes that you, me and every other animal on earth requires to get digestion done.
Unique digestive tract
Simon McQueen-Mason, a biologist at York and a leader of the research team, explained in the announcement:
Gribble are the only animal known to have a sterile digestive system. This makes their method for wood digestion easier to study than that of other wood-consuming creatures such as termites, which rely on thousands of gut microbes to do the digestion for them.
We have found that Gribble chew wood into very small pieces before using hemocyanins to disrupt the structure of lignin. GH7 enzymes, the same group of enzymes used by fungi to decompose wood, are then able to break through and release sugars.
His co-author and York colleague Neil Bruce added:
The cellulase-enhancing effect of the hemocyanin was equivalent to that of thermochemical pre-treatments used in industry to allow biomass hydrolysis, suggesting new options for bio-based fuel and chemicals production. In the long term this discovery may be useful in reducing the amount of energy required for pre-treating wood to convert it to biofuel.
Chemically speaking, hemocyanins are benign. They are oxygen-transport compounds, the invertebrate equivalent of human hemoglobin, with the chief difference being that copper, not iron, is the metal used to bind the oxygen. It’s the copper that makes gribble blood blue, or greenish blue, rather than the red of iron-rich human blood.
(Meandering ever so briefly from entomology to etymology: Chemistry has nothing to do with the term “blueblood” for aristocrats; it derived, in Europe, from the visibility of major blood vessels below the surface of lily-white skin).
While the York team devoted considerable attention to the “fecal pellets” produced by their research subjects, it was only to quantify the rate at which they digested wood, not for potential fuel value. Indeed, the sugars extracted in the digestive process went to power the crustaceans themselves.
Practicalities await
The paper is silent on the notion of exactly how its key discovery could be put to use in fuel production, but almost certainly the pathway would be about synthesizing hemocyanin on a large scale for treatment of ground pulp at industrial facilities — not pouring sawdust and cornstalks into watery gribble farms.
But just for a moment, let us join Chuck Dinerstein in laying aside these admittedly important practicalities and appreciating instead the way in which this discovery “makes you appreciate the wondrous quality of nature and evolution.”
Fungi, termites and our new friend the gribble all can digest these cellulose stores protected by lignins, but in different ways. Fungi go after the unprotected cellulose fractions first and then attack lignins with oxygen radicals, termites and beetles have symbiotic arrangements with their gut microbiome who do much of the heavy lifting. The gribble launches a full-on attack with hemocyanin.
There is an old saying that for the man with a hammer everything looks like a nail, but nature and evolution allow their tools to be a bit more like a Swiss Army knife.
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The full paper, “Hemocyanin facilitates lignocellulose digestion by wood-boring marine crustaceans,” can be read here without charge.
