Alkalinity as a Control Variable
For Spirulina, bicarbonate chemistry is carbon source, pH buffer, and biosecurity all at once — which is why MAIN controls alkalinity, not just acidity.
- Zarrouk's medium is dominated by ~16.8 g/L sodium bicarbonate — carbon source and buffer
- Optimal culture pH ~9.5–10.5, tolerant to ~11; carbon fixation drives pH up
- Sustained high pH suppresses most contaminants, a passive biosecurity layer
Most aquatic cultures fight to keep pH from drifting up. Spirulina cultivation does the opposite: it deliberately runs alkaline and manages the bicarbonate–carbonate system that holds it there. In Zarrouk’s medium the dominant ingredient is sodium bicarbonate at roughly 16.8 g/L — far more than any other salt. That bicarbonate is not just a buffer; it is Spirulina’s carbon source. As photosynthesis pulls carbon out of solution, the equilibrium shifts and pH climbs — so in a thriving culture, rising pH is the visible fingerprint of carbon fixation.
That gives alkalinity a dual role. The buffer resists sudden pH swings, keeping the culture in its productive band — optimally around pH 9.5–10.5, tolerant to ~11 — but it is consumed as carbon is fixed. Left unfed, a healthy culture drives its own pH up past 11 and stalls from carbon limitation. MAIN closes the loop by dosing bicarbonate: a single addition replenishes carbon and restores buffering at once.
The second payoff is biosecurity. High pH is a formidable passive defense — most bacteria, fungi, protozoan grazers, and competing algae cannot tolerate a sustained pH near 10, while Arthrospira is built for it. The control implication is subtle: pH is not an independent knob to force to a number, it is an emergent readout of the carbon budget. Treating alkalinity as the real state variable keeps carbon, buffering, and contamination defense aligned.