Why Spirulina Is a Cyanobacterium, Not an Alga

The organism sold as “Spirulina” is a filamentous cyanobacterium in the genus Arthrospira (chiefly A. platensis and A. maxima). The market name is a historical mix-up: it was once classified in the genus Spirulina, and the label stuck. That taxonomy is not pedantic trivia — it changes the biology you are cultivating. Cyanobacteria are prokaryotes: they have no nucleus and no chloroplast. Their photosynthetic machinery sits on internal thylakoid membranes within the cell itself.

Three practical consequences flow from this. Pigments: cyanobacteria harvest light with phycobiliproteins — most famously the brilliant blue phycocyanin, which can reach roughly 10–20% of dry weight. Cell wall: Arthrospira has a soft peptidoglycan wall rather than the tough cellulose wall of green algae, which is why Spirulina is digestible without the cell-cracking step Chlorella needs. Niche: Arthrospira evolved in warm, highly alkaline soda lakes rich in bicarbonate, thriving near pH 9.5–10.5 and tolerating values up to about 11.

That alkaline preference is a gift to the grower. High pH is a built-in contamination defense: the same water chemistry Spirulina loves is hostile to most bacteria, fungi, and grazing protozoa. It also dictates the medium — Spirulina is fed bicarbonate rather than pumped CO2 alone, because dissolved bicarbonate is both its carbon source and its pH buffer. For an autonomous system like MAIN, knowing you are tending a cyanobacterium is what justifies the whole control philosophy: alkaline setpoints, bicarbonate dosing for carbon and pH together, and a light model tuned to a filament that self-shades.