ah....the B12 question yet again.
A common misconception in vegan circles is that fermented foods and spirulina contain B-12. This claim may, at times, be supported by lab tests for B-12 based on the USP (U.S. Pharmacopoeia) assay methods.
Unfortunately, as explained in Herbert et al.  and Herbert , the USP assay method for B-12 is unreliable. The assay measures total corrinoids--that is, true B-12 plus analogues (forms of B-12 that are not metabolically active in the body)--and the analogues have the potential to block the absorption of true B-12 by occupying B-12 receptor sites.
A preferred, reliable test that can differentiate between true B-12 and corrinoids is provided by differential radioassay. The assay problem must be considered in evaluating "old" studies on B-12.
Spirulina and tempeh contain mostly analogues of B-12. Herbert  which reports that tests on tempeh, a fermented soy product, and spirulina revealed that they contained almost no true B-12, i.e., the "B-12" they contained (per USP assay test) was predominantly analogues.
It is suspected that people taking spirulina as a source of vitamin B-12 may get vitamin B-12 deficiency quicker because the analogues in the product block human mammalian cell metabolism in culture [i.e., in the lab] and it is suspected that they will also do this in the living human.
The presence of analogues, rather than true B-12, in fermented foods therefore makes them unreliable sources for B-12.
However, in one of the more brilliant experiments in the field of vitamin B-12 metabolism in the 50s, Sheila Callendar in England found that colon bacteria make large amounts of vitamin B-12.
Although the bacterial vitamin B-12 is not absorbed through the colon, it is active for humans. Callendar studied vegan volunteers who had vitamin B-12 deficiency characterized by classic megaloblastic anemia. She collected 24-h stools, made water extracts of them, and fed the extract to the patients, thereby curing their vitamin B-12 deficiency.
This experiment demonstrated clearly that 1) colon bacteria of vegans make enough vitamin B-12 to cure vitamin B-12 deficiency, 2) the vitamin B-12 is not absorbed through the colon wall, and 3) if given by mouth, it is absorbed primarily in the small bowel.
Spirulina can cause B12 deficiency because of its "analogues" which "occupy" the receptor sites for the absorbable form available from other sources, including that made within the intestinal bacteria via intrinsic factor.