Spirulina growing nutrients guide: nitrogen, phosphorus, potassium, iron, and micronutrients

The standard Zarrouk medium

The Zarrouk medium (1966) remains the reference for spirulina cultivation. Per litre of culture:

NaHCO₃: 16.8 g (carbon source, pH buffer)
KNO₃: 2.5 g (nitrogen source)
K₂HPO₄: 0.5 g (phosphorus, potassium)
NaCl: 1.0 g (osmotic balance)
MgSO₄·7H₂O: 0.2 g (magnesium, sulfur)
CaCl₂·2H₂O: 0.04 g (calcium)
FeSO₄·7H₂O: 0.01 g (iron)
EDTA (Na): 0.08 g (iron chelator — keeps Fe in solution at high pH)
Micronutrient solution: 1 ml/L (containing MnCl₂, ZnSO₄, CuSO₄, Na₂MoO₄, H₃BO₃, CoCl₂)

Source:KNO₃ (potassium nitrate) at 2.5 g/L provides nitrate as the primary nitrogen source. Spirulina can also use urea and ammonium, but nitrate is standard for food-grade production (ammonia can accumulate to toxic levels).
Deficiency signs:Nitrogen depletion causes chlorosis — cells turn yellow/orange as phycocyanin and chlorophyll synthesis both require nitrogen. This is the most common cause of yellow culture in home growing. Measure biomass colour change with a spectrophotometer or visually — vivid teal-blue-green = nitrogen replete; yellow-orange = deficient.
Top-up protocol:Add KNO₃at 1–2 g per litre of total culture volume weekly during active growth. Monitor colour recovery over 2–3 days.

Source:NaHCO₃ provides both the carbon for photosynthetic fixation (CO₂from bicarbonate dissociation) and the alkaline pH buffer (pH 9–10.5). Spirulina consumes bicarbonate rapidly in active growth.
Deficiency sign:pH rises above 10.8 as all CO₂is consumed and no more is buffered. Culture stops growing. This is the daily management challenge in outdoor systems without CO₂injection.
Correction:Inject CO₂gas to bring pH back to 9.5–10.2, or add 2–4 g/L NaHCO₃. pH-triggered CO₂solenoids automate this in photobioreactor systems.

Why iron matters:Phycocyanin and phycocyanobilin synthesis both require iron as a cofactor. Iron deficiency in the medium produces pale-blue spirulina with reduced phycocyanin content — the biomass is nutritionally impaired even if cell count is normal.
The solubility problem:At pH 9–10, free Fe³+ precipitates as Fe(OH)₃. Without EDTA (ethylene diamine tetraacetic acid) chelation, iron falls out of solution and becomes unavailable. Add FeSO₄always complexed with EDTA: the EDTA ratio should be 8:1 EDTA:Fe by weight to ensure chelation at high pH.
Deficiency sign:Pale blue-green instead of deep teal. OD&sub6;&sub5;&sub0;/OD&sub6;&sub2;&sub0; ratio shifts (chlorophyll A relatively preserved, phycocyanin reduced).
Excess iron:Excess unchelated iron causes Fenton chemistry (superoxide → hydroxyl radical → oxidative damage to spirulina cells). Use food-grade FeSO₄ at exactly 0.01 g/L with EDTA chelation.

K₂HPO₄provides both potassium and phosphorus. Phosphorus is essential for ATP, DNA, and membrane synthesis. Deficiency causes impaired cell division and reduced growth rate (not colour change). Standard 0.5 g/L is sufficient for most growth rates.
In very fast-growing outdoor cultures (doubling time <2 days), phosphorus can become limiting. Add 0.1–0.2 g/L K₂HPO₄weekly as maintenance in dense outdoor cultures.

MgSO₄provides magnesium (chlorophyll synthesis cofactor — the Mg at the centre of the chlorophyll porphyrin ring) and sulfur (cysteine, methionine synthesis). Standard 0.2 g/L is rarely limiting. Deficiency is uncommon but causes chlorosis similar to nitrogen deficiency.

Manganese (Mn):Photosystem II cofactor; essential for oxygen evolution in photosynthesis. Deficiency causes impaired photosynthetic efficiency.
Zinc (Zn):Carbonic anhydrase cofactor (CO₂ hydration to bicarbonate). Also RNA polymerase. Deficiency reduces growth rate at high pH where CO₂is rate-limiting.
Copper (Cu):Plastocyanin cofactor in photosynthetic electron transport. Very low requirement; excess copper is toxic to cyanobacteria (used as algicide in other contexts). Keep copper strictly at trace levels.
Molybdenum (Mo):Nitrate reductase cofactor — essential for converting nitrate (NO₃–) to ammonium for amino acid synthesis. Without Mo, nitrogen from nitrate cannot be assimilated.

Small-scale batch (1–10 L):Prepare fresh Zarrouk medium fully every 4–6 weeks (complete medium exchange during harvest). Top up NaHCO₃and KNO₃weekly between harvests.
Semi-continuous growing:After each 20–30% harvest, replace harvested volume with fresh Zarrouk medium. Nutrients are proportionally refreshed with each harvest.
Conductivity monitoring:Medium conductivity (measured with an aquarium EC meter) drops as nutrients are consumed. Target EC 6–8 mS/cm for Zarrouk medium. If EC falls below 4 mS/cm: refresh with full Zarrouk medium or add concentrated nutrient stock.