Why spirulina requires a specific medium
Spirulina (Arthrospira platensis) evolved in highly alkaline, saline, mineral-rich lakes — principally the soda lakes of East Africa and Chad. It cannot be grown in plain water; it requires a medium that replicates the ionic environment of its native habitat:
- High alkalinity: pH 9.5–11.5
- Bicarbonate/carbonate as the carbon source
- High sodium content as the primary cation
- Available nitrogen, phosphorus, and trace minerals
The Zarrouk medium: full composition
The Zarrouk formula (1966) is the standard laboratory and commercial cultivation medium. Per litre of water:
- NaHCO₃ (sodium bicarbonate): 16.8 g — primary carbon source and pH buffer
- Na₂CO₃ (sodium carbonate): 0.20 g — additional alkalinity
- NaNO₃ (sodium nitrate): 2.5 g — nitrogen source
- K₂HPO₄ (dipotassium hydrogen phosphate): 0.50 g — phosphorus source
- K₂SO₄ (potassium sulphate): 1.00 g — potassium and sulphur
- NaCl (sodium chloride): 1.00 g — ionic strength
- MgSO₄·7H₂O (magnesium sulphate heptahydrate): 0.20 g — magnesium
- CaCl₂·2H₂O (calcium chloride): 0.04 g — calcium
- FeSO₄·7H₂O (ferrous sulphate): 0.005 g — iron trace
- Na₂EDTA (EDTA disodium): 0.08 g — iron chelator (maintains iron solubility)
- A5 trace mineral solution: 1 mL — contains boron, manganese, zinc, copper, molybdenum, cobalt
The resulting pH after preparation is approximately 10–11.
Role of the key components
Sodium bicarbonate (NaHCO₃)
The most important ingredient. Provides inorganic carbon (CO₂ from bicarbonate) that spirulina fixes in photosynthesis, and maintains alkaline pH. In cultivation, NaHCO₃ is also the ingredient that needs regular replenishment as spirulina consumes it. pH dropping below 8.5 is the first sign of bicarbonate depletion.
Nitrogen source (NaNO₃)
Spirulina uses nitrate as its nitrogen source for protein synthesis. Nitrogen depletion is a common cause of culture yellowing and reduced growth. Urea (CO(NH₂)₂) is used as a lower-cost nitrogen alternative by some home growers.
Phosphorus (K₂HPO₄)
Required for ATP, DNA, and phospholipid synthesis. Phosphorus depletion causes growth reduction before visible colour change.
EDTA and iron
At high pH, ferrous iron (Fe²⁺) rapidly oxidises and precipitates as ferric hydroxide — becoming unavailable to spirulina. EDTA chelates iron, keeping it soluble and available. This is why EDTA removal from the formula typically causes iron deficiency symptoms (yellowing, reduced growth).
Simplified home grower alternatives
The full Zarrouk formula requires sourcing 10+ chemicals, some of which are difficult to obtain in small quantities. Two practical home alternatives:
Simplified Zarrouk (Jourdan’s formula)
Jean-Paul Jourdan’s simplified medium (from “Grow Your Own Spirulina”) reduces the formulation to the essential components accessible to home growers:
- NaHCO₃: 16 g/L
- NaNO₃: 2.5 g/L (or urea 0.9 g/L as equivalent nitrogen)
- K₂HPO₄: 0.5 g/L
- MgSO₄: 0.2 g/L (Epsom salt works as a source)
- Sea salt or NaCl: 1 g/L
- Iron supplement (ferrous sulphate + EDTA or chelated iron): trace
This omits the micronutrient trace elements, which are usually adequate in the water supply and any contamination from handling. Culture performance is somewhat lower than full Zarrouk but acceptable for home production.
Commercial spirulina nutrient packs
Several suppliers now offer pre-mixed spirulina nutrient packs that recreate the Zarrouk formula (or a close approximation) without the need to source individual chemicals. These are convenient and reliable for beginners and are the recommended starting point for new home growers.
pH management
pH is the most critical parameter to monitor in spirulina cultivation — and it is primarily managed through bicarbonate:
- Target range: pH 9.5–10.5 for optimal growth. Above 11 slows growth; below 8.5 stresses the culture and allows contaminant organisms to gain advantage.
- pH drop: Indicates bicarbonate depletion — spirulina has consumed the CO₂. Add NaHCO₃ in small amounts (1–2 g/L) and wait for equilibration before retesting.
- pH rise: Less common; may indicate reduced CO₂ fixation from low culture density, insufficient light, or temperature stress.
- Testing frequency: Daily during active growth; every other day in slower or cooler conditions.
Water quality
Tap water is generally acceptable for spirulina cultivation if it is not extremely hard (high Ca²⁺ and Mg²⁺ content) or heavily chlorinated. Let chlorinated tap water stand uncovered for 24 hours before use, or use a dechlorinator. Rainwater can work but may need pH adjustment before use.
Avoid using water softened with sodium-exchange units — the very high sodium content interferes with the intended ionic composition of the medium.
Medium replenishment after harvest
When spirulina is harvested, some medium is removed with the paste. To replenish:
- Add fresh water to restore volume
- Add nutrients proportional to the volume replaced (roughly 10–30% of the nutrient concentration of a fresh medium batch)
- Check and adjust pH
Over time (weeks to months), mineral salts accumulate in the culture medium as nutrients are added and water evaporates. Periodic partial replacement of the medium (30–50% dilution with fresh medium) prevents salt accumulation from impeding growth.
