Algae has plenty of B12. Also, this is a vitamin deficiency, compared to obligate carnivores like cat that can’t digest plants. It is a significant difference between the two.
> The usual dietary sources of vitamin B(12) are animal foods, meat, milk, egg, fish, and shellfish. As the intrinsic factor-mediated intestinal absorption system is estimated to be saturated at about 1.5-2.0 microg per meal under physiologic conditions, vitamin B(12) bioavailability significantly decreases with increasing intake of vitamin B(12) per meal. The bioavailability of vitamin B(12) in healthy humans from fish meat, sheep meat, and chicken meat averaged 42%, 56%-89%, and 61%-66%, respectively. Vitamin B(12) in eggs seems to be poorly absorbed (< 9%) relative to other animal food products. In the Dietary Reference Intakes in the United States and Japan, it is assumed that 50% of dietary vitamin B(12) is absorbed by healthy adults with normal gastro-intestinal function. Some plant foods, dried green and purple lavers (nori) contain substantial amounts of vitamin B(12), although other edible algae contained none or only traces of vitamin B(12). Most of the edible blue-green algae (cyanobacteria) used for human supplements predominantly contain pseudovitamin B(12), which is inactive in humans. The edible cyanobacteria are not suitable for use as vitamin B(12) sources, especially in vegans. Fortified breakfast cereals are a particularly valuable source of vitamin B(12) for vegans and elderly people. Production of some vitamin B(12)-enriched vegetables is also being devised.[1]
So it seems like that despite them containing substantial amounts of vitamin B12, they are not suitable for use as vitamin B12 sources because they predominantly contain pseudovitamin B12 which is inactive in humans. They recommend fortified breakfast cereals for vegans.
What you said about cats inability to digest plants is of no significance here. Vitamin deficiency - especially this particular one - is life-threatening. We need it. We can definitely have plants, and we may need plants as well, but we do need to eat something that contains substantial amounts of non-pseudovitamin B12, which is meat. Of course you can just be a vegan and supplement vitamin B12, but...
