Eating Sushi

Along the coasts of Japan, a reddish-brown seaweed clings to tide-swept rocks. Porphyra, better known as nori, has been consumed by the Japanese for hundreds of years. It is the edible paper-like casing which gives sushi its distinct structure and appearance. Despite sushi’s popularity all across the world, Japan’s long legacy of nori consumption has left its people especially well-equipped to digest the sea vegetable.

Nori contains unique carbohydrates (sulphated polysaccharides) that are not found in land plants. This means humans are poorly equipped to break down this energy source. It’s like asking our enzymes to sit a chemistry exam that they have not prepared for. However in the sea, there is a bacterium called Zobellia galactanivorans which is better-equipped for the job and loves to feast on nori.

Zobellia can enter the gut riding on morsels of sushi but it cannot survive long. Despite this, it is possible for Zobellia to share its genes with human gut bacteria that are able to build seaweed digesting enzymes called porphyranases. The process of sharing genes is known as horizontal gene transfer and it’s a cool feature that separates us from bacteria. I am not able to trade genes with you, even if I think your gene for eye colour is better than mine but bacteria can exchange DNA as we might exchange money or ideas. The genome of a typical bacterium is sprinkled with genes that arrived from its peers and bacteria can even scavenge discarded bits of DNA in their environment, left by dead or decaying neighbours.

The Japanese have been eating nori for so long that their gut microbes are peppered with digestive enzymes from oceanic species and they pass these to their children. Bacteria help us to take up exciting new evolutionary opportunities!

Inspiration: I Contain Multitudes: The Microbes Within Us and a Grander View of Life by Ed Yong

For the deeply curious:

Hehemann, J., Correc, G., Barbeyron, T. et al. Transfer of carbohydrate-active enzymes from marine bacteria to Japanese gut microbiota. Nature 464, 908–912 (2010).

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