Gill bacteria enable a novel digestive strategy in a wood-feeding mollusk

Significance In animals, gut microbes are essential for digestion. Here, we show that bacteria outside the gut can also play a critical role in digestion. In shipworms, wood-eating marine bivalves, endosymbiotic bacteria are found within specialized cells in the gills. We show that these endosymbionts produce wood-degrading enzymes that are selectively transported to the shipworm’s bacteria-free gut, where wood digestion occurs. Because only selected wood-degrading enzymes are transported, the shipworm system naturally identifies those endosymbiont enzymes most relevant to lignocellulose deconstruction without interference from other microbial proteins. Thus, this work expands the known biological repertoire of bacterial endosymbionts to include digestion of food and identifies previously undescribed enzymes and enzyme combinations of potential value to biomass-based industries, such as cellulosic biofuel production.