Nori sheets can be eaten in their natural form, but they are usually toasted and can contain high levels of iodine.
From Wikipedia: “Nori is the Japanese name for edible seaweed species of the red algae genus Pyropia, including P. yezoensis and P. tenera. Nori is familiar in the United States and other countries as an ingredient of sushi, being referred to as "nori" (as the Japanese do) or simply as seaweed. Finished products are made by a shredding and rack-drying process that resembles papermaking. Pyropia is also called laver in Wales and other English-speaking countries.”
Raw nori sheets:
Raw nori sheets are also available, but they can cost up to five times more than standard nori sheets. The sheets weigh between 2.5 and 3 grams per sheet. When the sheets are toasted, they are usually greenish and cost about 20 cents per sheet. In contrast, raw nori sheets are dark brown and can cost up to one euro per sheet.
“Nori is commonly used as a wrap for sushi and onigiri. It is also a garnish or flavoring in noodle preparations and soups. It is most typically toasted prior to consumption (yaki-nori). A common secondary product is toasted and flavored nori (ajitsuke-nori), in which a flavoring mixture (variable, but typically soy sauce, sugar, sake, mirin, and seasonings) is applied in combination with the toasting process. It is also eaten by making it into a soy sauce-flavored paste, nori no tsukudani (海苔の佃煮).
Nori is sometimes used as a form of food decoration.
A related product, prepared from the unrelated green algae Monostroma and Enteromorpha, is called aonori (青海苔 literally blue/green nori) and is used like herbs on everyday meals, such as okonomiyaki and yakisoba.
Since nori sheets easily absorb water from the air and degrade, a desiccant is indispensable when storing it for any significant time.”
“While seaweed has by far the highest proportion of iodine by weight of any food, Pyropia yezoensis has less than any other type of seaweed; it is nonetheless an excellent source of iodine.
It has also been found to contain sufficient vitamin B12 to prevent vitamin B12 deficiency in rats. Though nori has long been considered to be an important source of vitamin B12 for vegans, its vitamin B12 may actually not be biologically available to humans. It may contain cobalamin analogues which block absorption of B12. A study on mice found that Nori (Pyropia yezoensis) contains a significant amount of bioactive vitamin B12, not the inactive analogues. However, a study showed that in humans both dried and raw nori reduced the vitamin B12 status.”
Translated from “test.de/Jod-in-Gemuesealgen-Schock-aus-dem-Meer-1051651-2051651/”: Dried seaweed is available starting at 62 euro cents per 100 grams (not nori sheets), but these contain up to 3800 mg/kg dry weight of iodine. This is at least 150 times more than the German Federal Institute for Risk Assessment considers marketable.
However, there are also seaweeds that contain less than 20 mg iodine per kilogram of dry weight. Anything above this can be harmful to a person’s health. Kombu seaweed, in particular, contains much too much iodine.
“Fans of maki sushi don’t have to worry about getting too much iodine. The iodine content of the algae wraps, the nori sheets we tested, are safe to eat in moderation. A normal serving size is about three sheets or 7.5 grams. Wakame brown algae is also safe to eat in similar serving sizes. For Asians, especially the Japanese, iodine guideline values are not needed.”
Nori varieties for sushi (very low weight) and wakame for soups therefore contain safe amounts of iodine.
“Production and processing of nori is an advanced form of agriculture. The biology of Pyropia, although complicated, is well understood, and this knowledge is used to control the production process. Farming takes place in the sea where the Pyropia plants grow attached to nets suspended at the sea surface and where the farmers operate from boats. The plants grow rapidly, requiring about 45 days from "seeding" until the first harvest. Multiple harvests can be taken from a single seeding, typically at about ten-day intervals. Harvesting is accomplished using mechanical harvesters of a variety of configurations. Processing of raw product is mostly accomplished by highly automated machines that accurately duplicate traditional manual processing steps, but with much improved efficiency and consistency. The final product is a paper-thin, black, dried sheet of approximately 18 cm × 20 cm (7 in × 8 in) and 3 grams (0.11 oz) in weight.
Several grades of nori are available in the United States. The most common, and least expensive, grades are imported from China, costing about six cents per sheet. At the high end, ranging up to 90 cents per sheet, are "delicate shin-nori" (nori from the first of the year's several harvests) cultivated in Ariake Sea, off the island of Kyushu in Japan.
In Japan, over 600 square kilometres (230 sq mi) of Japanese coastal waters are given to producing 350,000 tonnes (340,000 long tons) of nori, worth over a billion dollars. China produces about a third of this amount.”