Scientific research has found that a type of edible seaweed called Nori(laver) to contain substantial quantities of biologically active Vitamin B12.
This is unique because most plant-foods don’t contain any appreciable quantity of Vitamin B12 in the first place, let alone the active forms of B12 such as methyl-cobalamin and adenosylcobalamin.
If it indeed turns out that Nori is a robust source of active Vitamin B12 then it will be hugely beneficial for vegetarians and in particular vegans, who are extremely prone to developing Vitamin B12 deficiency according to a wealth of research.
The purple laver contained five types of biologically active vitamin B12 compounds (cyano-, hydroxo-,sulfito-, adenosyl- and methylcobalamin), in which the vitamin B12 coenzymes (adenosyl- and methylcobalamin) comprised about 60 % of the total vitamin B12. 
Research has also found that seasoned and toasted nori to contain lesser amounts of Vitamin B12.
However, according to the study, the decreased vitamin B12 contents in the seasoned and toasted laver products, were not due to the loss or destruction of vitamin B12 during the toasting process. 
Seaweeds are also a great dietary source of the mineral iodine, which is still a common deficiency in many parts of the world.
Although compared to other seaweeds, nori contains the lowest amount of iodine, so can be enjoyed more often without worry of consuming too much iodine.
Feeding dried purple laver (nori) to vitamin B12-deficient rats significantly improves vitamin B12 status.
A study from 2001 published in the British Journal of Nutrition investigated the bio-availability of Vitamin B12 in purple nori seaweed and the effects of feeding the nori to Vitamin B12 deficient rats.
The amount of total vitamin B12 in the dried purple laver was estimated to be 54.5 and 58.6 (se 5.3 and 7.5 respectively) microg/100 g dry weight by Lactobacillus bioassay and chemiluminescent assay with hog intrinsic factor respectively.
The purple laver contained five types of biologically active vitamin B12 compounds (cyano-, hydroxo-, sulfito-, adenosyl- and methylcobalamin), in which the vitamin B12 coenzymes (adenosyl- and methylcobalamin) comprised about 60 % of the total vitamin B12.
When 9-week-old vitamin B12-deficient rats, which excreted substantial amounts of methylmalonic acid (71.7(se 20.2) micromol/d) in urine, were fed the diet supplemented with dried purple laver (10 microg/kg diet) for 20 d, urinary methylmalonic acid excretion (as an index of vitamin B12 deficiency) became undetectable and hepatic vitamin B12 (especially adenosylcobalamin) levels were significantly increased.
These results indicate that vitamin B12 in dried purple laver is bioavailable to rats.
One thing to remember about the study above is that it was only done on rats and not humans. So at this point in time, we cannot say for sure if Nori seaweed will also improve Vitamin B12 status in humans.
However, if the same results can be replicated in the future with human clinical studies, Nori could potentially become an important dietary source of Vitamin B12 for those who are at risk of deficiency such as vegetarians and in particular vegans.
 Feeding dried purple laver (nori) to vitamin B12-deficient rats significantly improves vitamin B12 status.
 Dried green and purple lavers (Nori) contain substantial amounts of biologically active vitamin B(12) but less of dietary iodine relative to other edible seaweeds.
 Characterization of vitamin B12 compounds from Korean purple laver (Porphyra sp.) products.
The information in this article has not been evaluated by the FDA and should not be used to diagnose, cure or treat any disease, implied or otherwise.