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A person diving into a plate of salmon sushi.

Japanese Guts Are Adapted to Sushi

Ocean microbe genes found in gut bugs of seaweed eaters.

Japanese intestines are adapted to sushi, says a new study that found genes from an ocean microbe in the gut bugs of long-term seaweed eaters.

In the wild, the marine bacterium Zobellia galactanivorans feeds on Porphyra seaweed, which includes the seaweed species used to make sushi wrappers, or nori.

Researchers were studying the microbe to find out which enzymes it uses to break down a carbohydrate abundant in Porphyra seaweed. (Watch explorers navigate the seaweed "forests" off Antarctica.)

When the researchers sequenced the genes for these enzymes, known as glycoside hydrolases, they made an unexpected discovery: The ocean microbe's DNA had already been detected in bacteria living symbiotically in Japanese people.

The scientists then compared the genes of intestinal microbes from 13 Japanese volunteers with those from 18 North Americans and found the marine genes only in Japanese guts. (Related: "Earliest Known American Settlers Harvested Seaweed.")

Seaweed has long been vital in Japanese culture, with eighth-century tax records listing the plant as a form of payment to the Japanese government. Today the average person in Japan eats 0.5 ounce (14.2 grams) of seaweed a day. (Take a Japan quiz.)

The researchers speculate that regular, long-term contact with ocean microbes via eating sushi and other seaweed-based foods provided a way for some strains of the trillions of gut bugs to acquire the seaweed-digesting genes.

Endowed with the ocean-based enzyme, Japanese gut bacteria can probably extract a small amount of energy from seaweed that gut bugs found in North Americans, for example, can't. But the extra energy is likely more of a boon to the bacteria than to their human hosts, the scientists say.

Gut Bugs and Human Evolution

The find marks the first known instance of genes from external microbes finding their way into human gut bacteria.

In addition, the discovery illustrates "how much more we humans are actually connected with the biosphere surrounding us then we think, and how much we may depend on its biodiversity, even at the microbial scale," said study co-author Jan-Hendrik Hehemann, a marine glycobiologist at the University of Victoria in Canada.

Future research could test the gut microbes of modern hunter-gatherers or even fossilized human feces to learn more about how regional food preferences might have changed gut microbes over time and influenced human evolution, Hehemann added.

The gut bug DNA study is detailed in this week's issue of the journal Nature.