Some Japanese people can digest the seaweed, used to wrap sushi, thanks to genes from marine bacteria.
The study has suggested that a marine microbe - perhaps ingested on a sliver of seaweed - probably transferred genes that encode algae-munching enzymes to bacteria that live in the human gut.
The enzymes break down algal carbohydrates including one found in red algae of the genus Porphyra, best known to sushi lovers as 'nori'.
Although gene transfer to gut microbes is suspected in other cases, this is the first clear-cut example in which a gut microbe has gained a new biological niche by snatching genes from an ingested bacterium, says Mirjam Czjzek, a chemist at the Pierre and Marie Curie University in Paris, one of the two researchers who led the study.
"Probably there are many more examples. It's only because of this exotic niche and the very rare specificity of this enzyme that we were able to pinpoint where it came from," Nature quoted her as saying.
Many of the microbes residing in the human gut are likely to be beneficial to their host. Some may give their host a calorific boost by breaking down ingested plant carbohydrates that human enzymes cannot touch.
In Japan, some of these indigestible carbohydrates come from the algae that wrap sushi rolls and form the basis for a variety of soups and salads.
Czjzek, Gurvan Michel, and their colleagues found a new class of algae-degrading enzymes called _-porphyranases while hunting for proteins that break down algal biomass.
In the genome of the marine bacterium Zobellia galactanivorans they found enzymes that were similar to those that degrade the algal compounds agarose and carrageenan.
But the enzymes lacked a crucial region needed to recognize these polysaccharides.
Instead, the enzymes broke down a Porphyra polysaccharide called porphyran.
The team searched databases for related enzymes and found that they are all also made by marine microbes, except one found in the genome of a human gut bacterium called Bacteroides plebeius.
The bacterium also contained an enzyme that breaks down agarose. Bacteroides bacteria containing these genes were only found in individuals with Japanese ancestry.
And on the basis of the similarity in DNA sequence and the finding that the two genes appear near others in the B. plebeius genome that appear to come from marine bacteria, the researchers concluded that the genes had been transferred from a marine bacterium to microbes living in the human gut.
The results suggest that ingested bacteria may have provided a valuable genetic resource for gut microbes throughout human history, said Justin Sonnenburg, a microbiologist at Stanford University in California.
The study has been published this week in Nature1.