When most people eat seaweed it just passes through them without being digested. The reason for this is because there is a specific enzyme necessary for digesting the substance that specializes in breaking down algal cell walls and is only found in specific marine bacteria. The only place other than marine life that this enzyme is found is in the stomach of Japanese individuals!
Zobelia galactanivorans breaks down the carbohydrate found in cell walls of red algae. For Westerners, this algae is more commonly known as nori seaweed, and is most commonly seen in the wrapping of Japanese sushi. Because this microbe is necessary to break down seaweed in a digestive system, for most people it simply passes through their digestive tract. However, Japanese people are able to break it down.
The reason for this is believed to be because of evolution. For hundreds of thousands of years, Japanese individuals have consumed nori seaweed, and slowly gained the capacity to digest it by allowing the enzme to naturalize to their bodies!