Complete technical guide: Zarrouk's Medium recipe, CO₂/O₂ exchange data, LED lighting protocol, preservation strategy, maintenance schedule, and a full troubleshooting guide.
A Liquid Tree is a photobioreactor filled with live spirulina culture. The spirulina (Arthrospira platensis) performs photosynthesis under LED light, absorbing CO₂ from pumped-in room air and releasing O₂ — exactly like a tree, but in liquid form and 10–50× more efficient per unit of space.
The concept was publicly demonstrated in Belgrade, Serbia, where 600-litre street-side Liquid Tree installations replaced benches and trees in high-pollution zones. The same biology scales down to a 1-litre desktop unit that can meaningfully contribute to air quality in a room.
The liquid medium (Zarrouk's Medium, 1966) provides all nutrients spirulina needs to grow, photosynthesize, and remain healthy long-term. The sections below give you the complete recipe, protocol, and operational guide.
The industry standard is Zarrouk's Medium (1966), used globally for Arthrospira platensis cultivation. It is prepared as two separate solutions to prevent mineral precipitation before autoclaving, then combined aseptically.
| # | Chemical Name | Formula | Amount per 1 L final | Role |
|---|---|---|---|---|
| 1 | Sodium Bicarbonate | NaHCO₃ | 16.80 g | Primary carbon source & pH buffer (alkaline ~9.5–10.5) |
| 2 | Dipotassium Hydrogen Phosphate | K₂HPO₄ | 0.50 g | Phosphorus source for cell growth & energy transfer |
| # | Chemical Name | Formula | Amount per 1 L final | Role |
|---|---|---|---|---|
| 3 | Sodium Nitrate | NaNO₃ | 2.50 g | Primary nitrogen source for protein & pigment synthesis |
| 4 | Potassium Sulfate | K₂SO₄ | 1.00 g | Potassium & sulfur supply |
| 5 | Sodium Chloride | NaCl | 1.00 g | Osmotic balance |
| 6 | Magnesium Sulfate Heptahydrate | MgSO₄·7H₂O | 0.20 g | Magnesium — essential for chlorophyll production |
| 7 | Calcium Chloride Dihydrate | CaCl₂·2H₂O | 0.04 g | Calcium for cell wall structure |
| 8 | Iron(II) Sulfate Heptahydrate | FeSO₄·7H₂O | 0.01 g | Iron for photosynthesis enzymes (ferredoxin) |
| 9 | Disodium EDTA Dihydrate | EDTA-Na₂·2H₂O | 0.08 g | Chelator — keeps iron & metals bioavailable to cells |
Prepare 1 litre of this stock separately. Store in 10 mL aliquots frozen. Add 10 mL to each litre of finished culture.
| Chemical | Formula | Amount in Stock (per 1 L stock solution) | Role |
|---|---|---|---|
| Boric Acid | H₃BO₃ | 2.86 g | Boron — cell wall integrity & pollen tube growth |
| Manganese Chloride Tetrahydrate | MnCl₂·4H₂O | 1.81 g | Manganese — oxygen evolution in photosynthesis |
| Zinc Sulfate Heptahydrate | ZnSO₄·7H₂O | 0.222 g | Zinc — enzyme cofactor (carbonic anhydrase) |
| Copper Sulfate Pentahydrate | CuSO₄·5H₂O | 0.079 g | Copper — electron transport (plastocyanin) |
| Molybdenum Trioxide | MoO₃ | 0.015 g | Molybdenum — nitrogen fixation & nitrate reductase |
The spirulina liquid medium functions as the living engine of your air purifier. Here is the complete operational procedure:
Run the air pump continuously or at minimum 18 hours/day (in sync with lights). The spirulina absorbs CO₂ from the air bubbles and releases O₂. Normal room air (0.04% CO₂) is sufficient for a small unit, though supplementing with 2–5% CO₂ greatly increases output.
Keep the live culture in medium at 25–30°C with reduced lighting (8 hours/day, dimmer) and gentle aeration. Replenish evaporated water with distilled water every 2–3 days. The culture stays active and healthy for several weeks with minimal maintenance.
Place a small sample (100 mL) in fresh medium, reduce light to 4 hours/day, and lower temperature to 15–20°C. The culture enters slow dormancy. Add a small amount of urea (0.01 g/L) and iron chelate once a month to maintain viability. Reactivate by gradually increasing light, temperature, and nutrients over 3–5 days.
For laboratory-grade preservation: cryopreservation in 10% DMSO at −80°C. For home use, maintaining a small 100 mL "mother culture" on a windowsill with occasional nutrient top-ups is the most practical backup strategy. Keep it in a sealed glass jar away from harsh direct sunlight.
Sterile Zarrouk medium (without spirulina inoculated) can be stored in a sealed glass container at 4°C (refrigerator) for up to 3 months without degradation. Label with preparation date.
These figures are sourced from peer-reviewed bioreactor studies of Arthrospira platensis under controlled conditions.
| Condition | CO₂ Consumed / day | O₂ Produced / day | Notes |
|---|---|---|---|
| Low — dim light, ambient CO₂ | ~0.025 g | ~0.018 g | Room air only, 12h light cycle |
| Medium — good LED, ambient CO₂ | ~0.1–0.5 g | ~0.07–0.4 g | 16h LED, aerated, ~500 mg/L density |
| High — 2.5% CO₂ injection | ~0.6 g | ~0.45 g | Active CO₂ enrichment, optimal temperature |
Spirulina stops photosynthesis in darkness and shifts to cellular respiration, which slightly reverses the process — consuming a small amount of O₂ and releasing CO₂. Keep the dark period to 8 hours or less. The net 24-hour balance is still strongly positive for O₂ production.
| LED Type | Color Temp | Performance | Recommendation |
|---|---|---|---|
| Daylight White LED | 6500 K | Excellent biomass & pigment production, most energy efficient | ✅ Best Choice |
| Warm White LED | 3000 K | Similar biomass growth, uses 4.53 kWh more per cycle | Good alternative |
| Red + Blue Combo | 660 nm + 470 nm | Excellent photosynthetic efficiency, higher cost | Good for advanced setups |
| Green LED only | 520 nm | Poorest growth rate — spirulina reflects green light | ❌ Avoid entirely |
Target 80–160 µmol photons m⁻² s⁻¹ (approximately 2,000–4,000 lux). For a small home bioreactor, place a 15–20W daylight LED strip 10–15 cm from the vessel surface. Too much light (>200 µmol m⁻² s⁻¹) causes photoinhibition — the spirulina shuts down its photosystems. Too little causes slow growth and thin culture.
Research confirms the best balance between biomass productivity and energy efficiency is the 16:8 photoperiod:
Use a simple plug-in outlet timer. For example: lights ON at 6:00 AM, lights OFF at 10:00 PM. The dark period is essential — it prevents culture stress and allows cellular repair processes.
| Photoperiod | Biomass Productivity | Energy Efficiency | Notes |
|---|---|---|---|
| 12L : 12D | Moderate | Good | Minimum for sustained healthy culture |
| 16L : 8D | High | Excellent ✅ | Best balance — recommended for all setups |
| 20L : 4D | Very High | Good | Best productivity, higher electricity cost |
| 24L : 0D | High (initially) | Poor | Culture stress long-term — not sustainable |
| Task | What to Do | Why |
|---|---|---|
| Check color | Deep emerald green = healthy. Yellow/olive = problem. Brown = emergency. Act immediately if off-color. | Primary early-warning indicator |
| Check aeration | Confirm air pump is running, bubbles are visible throughout the culture. | CO₂ supply & culture mixing |
| Top up water | Add a small amount of distilled water to maintain volume (evaporation loss). | Prevents salt concentration increase |
| Light timer | Confirm LED timer is working on 16:8 schedule. | Consistent photoperiod |
| Task | What to Do |
|---|---|
| pH check | Measure pH. Should be 9.5–10.5. Adjust with a few drops of 0.1M NaOH (raise) or 0.1M HCl (lower). |
| Temperature | Confirm 28–35°C. Below 20°C slows growth significantly. Above 38°C is damaging to spirulina cells. |
| Density check | Hold a sample jar to light — should be opaque green, not see-through. Aim for 400–600 mg/L dry weight. |
| Partial harvest | Remove 10–15% of the culture volume. Replace with fresh Zarrouk medium. Keeps nutrients balanced and culture young. |
| Nutrient top-up | Add a small dose of NaHCO₃ (1–2 g per 1 L) if culture looks pale — bicarbonate depletes as the primary CO₂ source. |
| Task | What to Do |
|---|---|
| Full medium refresh | Replace 30–50% of culture medium with freshly prepared Zarrouk medium. Prevents salt buildup and nutrient imbalance. |
| Clean vessel | Wipe inside walls of the bioreactor with a soft cloth to remove biofilm that blocks light penetration. |
| Check tubing & pump | Inspect air tubes for blockages, algae growth in lines, pump wear. Replace air stone if bubbling is uneven. |
| Backup culture | Save 50–100 mL of healthy culture in a small sealed jar in the fridge as emergency restart stock. |
| Micronutrients | Add 1–2 mL of micronutrient Stock C per litre to replenish trace elements consumed by spirulina growth. |
| Symptom | Likely Cause | Fix |
|---|---|---|
| Culture turns yellowish | Nitrogen depletion, pH drift, or photoinhibition | Add NaNO₃ (0.5 g/L), check pH, reduce light intensity to 80–100 µmol m⁻² s⁻¹ |
| Culture turns olive/brown | Temperature shock (<15°C or >38°C) or contamination | Restore temperature to 30–35°C, replace 50% with fresh medium + fresh inoculum |
| Foamy yellow liquid | Cell lysis — culture is dying or dead | Discard most of it, keep 10%, inoculate into 100% fresh Zarrouk medium with fresh starter |
| Culture too thin (pale green) | Low inoculum, nutrient depletion, or insufficient light | Add more spirulina starter, top up with Zarrouk medium, increase light to 120–150 µmol m⁻² s⁻¹ |
| Slow or no growth | Temperature too low, pH out of range, or light incorrect | Optimal: temp = 30–35°C, pH = 9.5–10.0, light = 80–160 µmol m⁻² s⁻¹. Adjust all three systematically. |
| White/gray residue on glass | Salt precipitation (mineral deposits) | Wipe clean with soft cloth. Ensure A and B solutions were mixed after autoclaving, not before. |
| Unpleasant smell | Bacterial contamination | Replace 80% with fresh medium. Ensure air inlet tube has an inline air filter (0.22 µm) to prevent bacterial entry. |