According to http://wwwlib.murdoch.edu.au/adt/pub...45/02Whole.pdf, high concetrations of B12 is found in the Coccolithophorid algae, also called Pleurochrysis carterae: 0,0013% of dry cell weight. Hmmm... That would be 0.13g pr. 100g = 130 000 mcg, but this site (Characterization of a Vitamin B12 Compound from Unicellular Coccolithophorid Alga (Pleurochrysis carterae)) says that 100g Coccolithophorid Alga contains "only" circa 125 mcg B12 pr. 100 g dry cll weight, which is still a very high amount... equalling 500 times as much as Dr. Victor Herbert reckons that a human need to absorb (not consume) pr. day, and more than 50 times the amount of B12 that's considered the Recommended Daily Allowance in many countries (2.4 mcg/daily).
ETA - new link: http://researchrepository.murdoch.ed.../2/02Whole.pdf