Spirulina, an algae available as a dietary supplement in tablet form, and nori, a seaweed, have both appeared to contain significant amounts of B12 after analysis. However, it is thought that this is due to the presence of compounds structurally similar to B12, known as B12 analogues. These cannot be utilised to satisfy dietary needs. Assay methods used to detect B12 are unable to differentiate between B12 and it's analogues, Analysis of possible B12 sources may give false positive results due to the presence of these analogues.
Researchers have suggested that supposed B12 supplements such as spirulina may in fact increase the risk of B12 deficiency disease, as the B12 analogues can compete with B12 and inhibit metabolism.
The current nutritional consensus is that no plant foods can be relied on as a safe source of vitamin B12.
http://www.vegsoc.org/info/b12.htmlSpirulina contains vitamin B1 (thiamine), B2 (riboflavin), B3 (nicotinamide), B6 (pyridoxine), B9 (folic acid), B12 (cyanocobalamin), vitamin C, vitamin D, and vitamin E.The bioavailability of vitamin B12 in Spirulina is in dispute. Several biological assays have been used to test for the presence of vitamin B12.The most popular is the US Pharmacopeia method using the Lactobacillus leichmannii assay. Studies using this method have shown Spirulina to be a minimal source of bioavailable vitamin B12.However, this assay does not differentiate between true B12 (cobalamin) and similar compounds (corrinoids) that cannot be used in human metabolism. Cyanotech, a grower of spirulina, claims to have done a more recent assay, which has shown Spirulina to be a significant source of cobalamin. However the assay is not published for scientific review and so the existence of this assay is in doubt.The American Dietetic Association and Dietitians of Canada in their position paper on vegetarian diets state that spirulina can not be counted on as a reliable source of active vitamin B12.
http://en.wikipedia.org/wiki/Spirulina_(dietary_supplement)#Vitamins
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