By Victoria Gill
Science reporter, BBC News
Porphyra leucosticta is the nori seaweed often used to prepare sushi
A traditional Japanese diet could transfer the genes of "sushi-specific" digestive enzymes into the human gut.
This is according to researchers who discovered a substance in marine bacteria that breaks seaweed down into digestible pieces.
They say that by eating sushi wrapped in the seaweed, people probably ingested these bacteria along with the genes coding for that digestive enzyme.
The scientists published their findings in the journal Nature.
The team, led by Jan-Hendrik Hehemann from Universite Pierre et Marie Curie (UPMC) in Paris, France, was studying red algae known as Porphyra.
Among the various types of Porphyra is the nori seaweed traditionally used raw to wrap sushi rolls.
The team isolated the new enzyme, which they called porphyranase, from the bacteria living on the seaweed. These enzymes digest or break down the tough walls that encapsulate each algal cell - allowing the bacteria to feed.
Dr Mirjam Czjzek , also from UPMC, was a senior member of the research team. She and her colleagues searched through hundreds of databases of genes - looking for where else this unusual new enzyme might occur.
Bacteria on raw seaweed may have transferred genes to human gut flora
They finally "stumbled upon" their enzyme in the gut bacteria of a group of 13 Japanese people.
She explained that the gene for the porphyranase enzyme turned up in a database of all of the genes - the entire genome - of all of the bacteria living in each of these individuals' guts.
"Five out of the 13 people had this same gene [in their gut bacteria]. And the rest had similar genes that coded for enzymes with a similar function - to break down the algal cell wall," she said.
"When we looked at a [genomic] study of the gut bacteria of a group of American people, none of them had the gene."
Dr Czjzek explained that ingestion was "the most probable route" from the marine bacteria to the human gut - through traditionally prepared Japanese sushi.
"Sushi is the food in which Prophyria is used," she said. And when traditionally prepared, the seaweed is consumed raw.
Dr Czjzek said these seaweed-digesting genes could be beneficial - allowing their human hosts to extract nutrition from plant material they otherwise would not be able to digest.
These findings show, the scientists say, that food and the way we prepare it has the potential to influence the microbial environment (or the flora) in our guts.
Professor Justin Sonnenburg, a microbiologist from Stanford University in California, US, wrote an article to accompany the team's Nature paper.
He said that the study showed how important it was for the bacteria in the human gut to adapt to our changing environment and diet.
"Global travel and trade are providing unmatched access to new types of food and perhaps new microbes harbouring novel genes," he added.
"So the next time you take a bite of an unfamiliar food, think about the microbial inhabitants you may also be ingesting, and the possibility that you will be providing one of your ten trillion closest friends with a new set of [digestive] utensils."