Zarrouk Medium and Nutrients: Feeding a Spirulina Culture

Every thriving Spirulina culture is really a well-mixed chemistry problem. The classic recipe is Zarrouk’s medium, formulated for Arthrospira. The defining ingredient is sodium bicarbonate at roughly 16.8 g/L — by far the largest component. It is Spirulina’s carbon source and simultaneously the buffer that keeps the medium alkaline near pH 9.5–10.5, both where Spirulina grows best and where most contaminants die.

Next is nitrogen, the nutrient most tied to protein yield — sodium nitrate (around 2.5 g/L). Because Spirulina is 60–70% protein by dry weight, nitrogen limitation is one of the first things to cap growth and dull the culture’s color. Phosphorus comes from dipotassium phosphate (about 0.5 g/L). The recipe then supplies potassium, magnesium, calcium, and modest sodium chloride — Spirulina needs only mild salinity, not seawater. Iron (with a chelator like EDTA to stay available at high pH) is essential for photosynthesis and nitrogen handling, and a trace-metal mix supplies manganese, zinc, copper, molybdenum, boron, and cobalt.

Two practical points: at Spirulina’s high pH, iron and phosphate can precipitate and become unavailable, so chelation and reasonable dosing help; and you don’t always need full-strength Zarrouk — many hobbyists run reduced concentration — but bicarbonate and a nitrogen source are non-negotiable. A culture that stops deepening in color is often simply hungry: feed carbon and nitrogen first.