What it is
Protein-dense algae — spirulina was skimmed off Lake Texcoco by the Aztecs and Lake Chad by the Kanembu; chlorella is its modern cultivated cousin.
Why your biology objects
Genuinely nutrient-dense (~60–70% protein, iron, pigments like phycocyanin) and, for spirulina, an actual ancestral lake harvest. Two honest footnotes: most of spirulina's "B12" is inactive pseudo-B12 (don't count it as your source), and supplements can be contaminated with microcystin toxins unless third-party tested.