What spirulina needs: the macronutrients
Spirulina’s nutritional requirements as a photosynthetic cyanobacterium:
- Carbon:Sodium bicarbonate (NaHCO₃) is the primary carbon and alkalinity source. Spirulina uses dissolved CO₂ for photosynthesis, and in alkaline media this is in equilibrium with bicarbonate. Bicarbonate also buffers pH — a dual function. Standard concentration: 8–16.8 g/L depending on formula.
- Nitrogen:Sodium nitrate (NaNO₃) or potassium nitrate (KNO₃) is the nitrogen source. Spirulina is a nitrogen-fixing cyanobacterium in the wild — but in cultivation, providing readily available nitrate dramatically increases productivity. Standard concentration: 2.5 g/L NaNO₃.
- Phosphorus:Dipotassium hydrogen phosphate (K₂HPO₄) provides phosphorus for DNA, RNA, ATP, and membrane phospholipids. Also serves as a secondary pH buffer. Standard: 0.5 g/L.
- Sodium chloride:Provides ionic strength and is required for spirulina’s osmoregulation. Standard: 1 g/L.
- Sodium sulphate:Sulphur source for amino acid synthesis (methionine, cysteine). Standard: 0.2 g/L.
Zarrouk’s medium: the complete formula
Zarrouk’s medium (1966) remains the standard spirulina cultivation medium. Per litre of distilled/RO water:
- NaHCO₃ (sodium bicarbonate): 16.8 g
- K₂HPO₄ (dipotassium phosphate): 0.5 g
- NaNO₃ (sodium nitrate): 2.5 g
- K₂SO₄ (potassium sulphate): 1 g
- NaCl (sodium chloride): 1 g
- MgSO₄·7H₂O (magnesium sulphate): 0.2 g
- CaCl₂ (calcium chloride): 0.04 g
- FeSO₄·7H₂O (ferrous sulphate): 0.01 g
- EDTA-Na₂: 0.08 g (chelating agent to keep iron in solution)
- Trace mineral solution: 1 mL/L
The trace mineral solution contains: ZnSO₄, MnSO₄, H₃BO₃, CuSO₄, Na₂MoO₄, and CoCl₂ at very low concentrations. Pre-mixed trace mineral solutions for spirulina culture are available from aquaculture suppliers.
Simplified home formula
Many home growers achieve good results with a simplified formula. Per 10 litres of RO/low-mineral water:
- NaHCO₃: 100–168 g (10–16.8 g/L)
- KNO₃ (potassium nitrate): 25 g (2.5 g/L)
- K₂HPO₄: 5 g (0.5 g/L)
- MgSO₄·7H₂O (Epsom salt): 2 g
- Sea salt or NaCl: 10 g
- FeSO₄·7H₂O: 0.1 g (or iron-EDTA chelate)
- Spirulina trace mineral mix (commercial): 1 mL/L
Potassium nitrate (KNO₃) is available as food-grade (used in curing meats and for horticulture). Dipotassium phosphate is available as a food additive from brewing supply shops. Epsom salt provides magnesium sulphate conveniently.
Water quality
Tap water quality significantly affects media performance:
- Hard water (200+ mg/L calcium/magnesium):Excess calcium interferes with phosphate availability (calcium phosphate precipitates) and can cause culture turbidity. Use RO or distilled water for making up Zarrouk media.
- Chlorinated tap water:Chlorine and chloramine are toxic to spirulina. Let water stand 24 hours in open container (chlorine evaporates) or use sodium thiosulphate to neutralise chloramine.
- Soft water (below 50 mg/L):Low interference with media chemistry — but ensure all required minerals are in the formula (don’t rely on tap mineral content).
- RO water:Best starting point — removes almost all dissolved minerals. Add everything from formula. Most consistent results.
Replenishing nutrients after harvest
After each harvest, nutrients are removed with the biomass and the harvested volume. Replenishment strategy:
- Add fresh bicarbonate (proportional to harvest volume) to restore carbon and alkalinity
- Every 3–4 harvests, add a full nutrient top-up (all macro and trace minerals) to prevent cumulative depletion
- Signs of nitrogen depletion: culture lightens in colour (less chlorophyll, reduced growth rate) — add KNO₃ 1–2 g/L and monitor recovery
- Signs of iron deficiency: pale yellow-green tinge, very slow growth — add iron-EDTA or FeSO₄ at 0.005–0.01 g/L
Media pH after preparation
Freshly prepared Zarrouk media typically has pH 9.5–10.0 before inoculation. This is correct — do not lower the pH before adding the starter culture.
If preparing a large batch of media, it can be stored in a sealed container at room temperature for 1–2 weeks. pH should remain stable. Check before using — if pH has dropped below 8.5, add a small amount of NaHCO₃ to restore.
Cost comparison
For a 20-litre home culture changed monthly (approximate):
- Sodium bicarbonate: 340 g = approximately £0.40 (food-grade bicarbonate)
- Potassium nitrate: 50 g = approximately £0.20
- Dipotassium phosphate: 10 g = approximately £0.30
- Other minerals: £0.20–0.50
- Total media cost per month: approximately £1–2 for 20L culture
The media cost is negligible relative to the yield value — the primary cultivation cost is electricity for heating and lighting.
