It is in your toothpaste and your ice cream. Japan has a very strong seaweed market. Most Japanese people eat seaweed daily. Seaweed farms are common. L. littorea feeds on sea lettuce.
Sea lettuce is a good indicator of pollution.
If there is a lot of sea lettuce then there must be a good source of nitrogen near by. Sea lettuce loves nitrogen but nitrogen is a pollutant in high doses. It is in the oxygen picture above. The white tips of the algae is calcium carbonate. That is the same stuff sea shells are made of.