The claims about how much B12 ulva contains don't really match each other...
www.seakura.net/Nutritional-Values.asp claims that ulca contains 8.6 micrograms B12 / 1g dried ulva, another source, which sells dry ulva for cattle feed suggests 7g B12/100g ulva, and then there are these sources:
http://www.encognitive.com/files/Nut...20Seaweeds.pdf :In Ulva sp vitamin A in 100g dry matter was 960 retinal U and B complex (B1, B2, B3, B12 and Folic acid) was (0.06, 0.03, 8.0, 6.3 and 11.8 mg, respectively)
Ulva lactuca can provide this vitamin in excess of the recommended dietary allowances for Ireland of 1.4 mcg/day with 5 mcg in 8g of dry foodstuff. This may provide an alternate source of vitamin B12 for vegetarians or vegans.