Eating seaweed may have conferred special digestive powers

Gut microbes found among Japanese can break down compound in nori

Eating seaweed appears to give some Japanese people digestive superpowers.

POWER SOURCE Colonies of the marine bacterium Zobellia galactanivorans (shown here in lab dish) carry an enzyme for breaking down porphyran in seaweeds. In a microscope image (inset), these bacteria appear like eyelashes growing on the outside of the central oval of a brown alga. Tristan Barbeyron

CHALLENGE Porphyra seaweeds like this one (drying out at low tide on the sands of Brittany) carry porphyrans, compounds which don’t show up in land plants and which can be digested by humans only if gut microbes have a genetic retrofit. Mirjam Czjzek

A paper in the April 8 Nature reports the discovery that bacteria in the guts of some Japanese people can break down porphyran, a compound found in seaweed. People who carry the bacteria may derive some nutritional benefit from the microbes, says coauthor Gurvan Michel, a biochemist at the Biological Station of Roscoff in France who is affiliated with the Pierre and Marie Curie University and the French national research agency CNRS.

The human gut bacteria probably acquired the gene centuries ago from marine microbes hitchhiking through the intestines on the seaweed abundant in the Japanese diet, says Michel.

“To our knowledge, it’s the first time there has been a demonstration of gene transfer from bacteria outside the gut to bacteria within the gut in connection with food,” Michel says.

“There has been speculation,” comments microbiologist Ruth Ley of Cornell University, who was not involved in the research. But she says she can’t think of any other work that has demonstrated an actual case and done it “so beautifully.”

All people rely on their gut microbes for doing tricky parts of digesting land plants, such as breaking down polysaccharides, a family of compounds that includes marine algae’s porphyran. “When you digest a salad, it’s not you that breaks down the vegetables; it’s the bacteria in your gut,” Michel says.

Land plants don’t make porphyran, but it shows up in seaweeds such as the dried, dark nori that wraps rice and fishy delicacies in sushi. Japanese people consume about 14 grams of seaweed per day on average and have been eating it for centuries. The Nature paper cites tax records from 8th century Japan that document levies paid in seaweed.

Michel’s team discovered the porphyran-digesting powers of the human gut as part of a long-term search for enzymes that breakdown polysaccharides. After finding genes for a pair of previously unknown porphyran-digesting enzymes in the marine bacterium Zobellia galactanivorans, researchers combed databases for similar DNA sequences in other species.

The team found a startling partial match from a human gut bacterium, Bacteroides plebeius. Mining the databases for further details, the researchers established that porphyran-digesting bacteria had been found in samples from Japanese subjects, but not yet in Westerners whose gut microbes had been sequenced.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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