“Probably most of . . . [the vitamin B12 content of blue-green algae] is due to the fact that the stuff is grown in . . . lakes . . . in Mexico that are contaminated with fecal matter; and the feces, of course, animals feces, have considerable B12 in them. . . . I suspect that's where most of it comes from, the stuff being not thoroughly washed before it's dried. . . . Spirulina is the biggie, of course. . . . Spirulina is a blue-green [alga], but not all blue-green algae are spirulina.”
Above passage was from this site
Swedish Medical Center info
Opinion from another doctor
Another page about questionable claims
It seems the jury is still out; not enough evidence exists so far to support either the efficacy or any inherent danger.
I'd act with caution here, and remember the age-old wisdom: If it sounds too good to be true...it probably is.
[edit on 16-8-2005 by Tinkleflower]