https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4711437/

"Algae could be an alternative source of vitamin B12 and in this study, vitamin B12 from a halotolerant green alga, Dunaliella salina V-101 was purified and characterized. The extract of Dunaliella was purified by passing through Amberlite XAD-2 and EASI-extract vitamin B12 immunoaffinity column. The total vitamin B12content in purified sample fractions was 42 2 μg/100 g dry weight as determined by the chemiluminescence method which was almost close to 49 2 μg/100 g dry weight as estimated by microbiological method."
As always: what matters is whether or not this potential B12 source contains active, bio-available B12.

PMCID: PMC4711437
PMID 26788012