All kinds of animals, including people, rely on gut microbes for digestion. Now, researchers have found a novel digestive strategy in wood-eating marine bivalves (Bankia setacea) known as shipworms: Their helpful bacteria live in their gills. The findings, published in Proceedings of the National Academy of Sciences this week, show how bacteria outside of the gut -- and not in direct contact with the food item -- play a critical role in digestion.
Shipworms (which are actually worm-looking clams, not technically worms) are the termites of the sea, and like those house-eating insects, shipworms depend on their microbial helpers to break down tough lignocellulose for nutrition and nourishment. According to the U.S. Department of Energy, shipworms left feeding channels in 2,000-year-old Greek and Roman ships, ruined Christopher Columbus’s fourth trip to the Caribbean, instigated flooding of the Netherlands in the 18th and 19th centuries, and caused $15 million in damages to San Francisco wharves around 1920. It took the advent of copper sheathing to slow their destruction of naval vessels.
But strangely, there are no bacterial endosymbionts living in their guts. Rather, the bacteria live within specialized cells in their gills, and the wood-degrading enzymes they produce are selectively transported to a region of the shipworm’s gut where wood digestion occurs.
“No other animal in the world is known to rely on bacteria outside of its digestive system to produce its digestive enzymes,” Distel says in a DOE release, “and no other intracellular bacterium is known to produce enzymes that function in the outside world of the host.”
Distel and colleagues sequenced the genomes of the gill bacteria and identified nearly 1,000 different genes involved in breaking down plant matter. Then they searched the shipworm gut and found 45 of these same genes as well as the proteins that were encoded in the genomes of the gill bacteria.
The selective translocation of enzymes also suggests that the simple shipworm system is capable of identifying relevant enzymes and their combinations. "These enzymes turn out to be fairly complex proteins, made up of several active modules that are connected to each other," Distel tells Washington Post. These bits are configured into different combinations in order to improve function. "Other organisms do this, but these bacteria seem to take it to an extreme," he adds. But why this whole gill-to-gut long distance relationship? This strategy of remote enzyme production likely allows shipworms to capture liberated sugars from wood without competition from in-house gut microbes.
Once the team identified the enzymes involved in breaking down wood into sugars in the gut, they had a list of candidates for commercially-viable enzymes that would be incredibly useful for industries that convert plant biomass into renewable fuel, paper products, and even food.
Images: Wilson44691 via Wikimedia (top), Dan Distel, Ocean Genome Legacy Center of New England Biolabs via DOE (middle)