Spirulina growing troubleshooting: slow growth, colour changes, and culture crashes

Problem 1: culture turning brown or yellow

This is the most alarming change for new growers — the blue-green colour is fading.

Causes:

Nitrogen starvation:The most common cause. Spirulina degrades phycocyanin (a nitrogen-rich protein) to extract nitrogen when the medium is depleted. The culture turns yellow-green then golden-yellow. The cells are alive but stressed. Fix:Add nitrogen source immediately (sodium nitrate 0.5–1 g/L). Recovery colour within 3–5 days.
High temperature stress:Above 40°C, phycocyanin synthesis is suppressed and existing phycocyanin degrades. Fix: Move to cooler conditions or add shade cloth. Temperature monitoring is essential on hot days.
UV light stress:Direct unfiltered UV degrades phycocyanin on the cell surface. Glass and polycarbonate filter UV; open vessels are most vulnerable. Fix: Move to UV-protected environment or cover with translucent UV-filtering material.

Your density is stable or declining even after harvesting stopped.

Temperature too low:Below 20°C, growth essentially stops. Check actual water temperature — not just room temperature. Aquarium heaters set too low or failing. Fix: Verify heater function, insulate the container if needed, move to warmer location.
Carbon limitation:Spirulina uses bicarbonate as its carbon source. In a dense culture, bicarbonate can be depleted within 1–2 days without replenishment. Check pH — if above 10.8, bicarbonate is depleted and CO₂ production from the cells themselves is insufficient. Fix:Add sodium bicarbonate 0.5–1 g/L.
Light limitation:As density increases, self-shading limits photosynthesis for inner cells. The surface may look productive while deeper cells are light-starved. Fix: Increase agitation to cycle cells through the light zone; consider reducing density by harvesting more.
Nutrient depletion:After multiple harvests without full nutrient replenishment, phosphate or trace minerals become limiting. Fix: Do a partial nutrient refresh — remove 50% of culture liquid and replace with fresh medium.

Healthy spirulina floats — gas vacuoles provide buoyancy. Sinking filaments indicate:

Gas vacuole collapse from sudden light intensity increase:Moving from shade to full sun too abruptly causes rapid gas vacuole collapse. Fix:Gradually acclimate to higher light — increase light by 25% per day over 3–4 days.
Culture aging:Old cultures in stationary phase lose gas vacuole production efficiency. Fix: Harvest heavily (50%) to dilute old cells; add fresh medium to reinvigorate growth phase.
High dissolved oxygen:Supersaturation of oxygen in a dense culture in bright light can displace gas from vacuoles. Fix: Increase agitation/aeration to off-gas excess dissolved oxygen.

Healthy spirulina has a mild sea-vegetable smell. Strong sulphur, putrid, or ammonia smell indicates:

Bacterial contamination:Putrid smell with discolouration and cell clumping. May indicate a full culture crash. Fix: Test pH — if below 9.0, the alkaline protection is gone. Discard the culture if bacterial contamination is confirmed. Restart with sterile medium and clean inoculant.
Dead cell decomposition:If a large proportion of cells have died (post-crash or temperature shock), the smell is from decomposing protein. Fix: If some cells are still alive (green flakes or strands visible), transfer a portion of the culture to fresh medium. Discard the rest.

Rapid density loss, colour change, and smell together indicate a culture crash. Common causes:

pH crash to below 8.0 (often from CO₂ addition without bicarbonate buffering, or extreme rain dilution)
Temperature extreme — frost (cells die below 5°C) or heat spike above 45°C
Contamination with rotifers (microscopically small animals that eat spirulina cells rapidly — visible as flicker in culture under microscope)
Sudden freshwater dilution reducing salinity below spirulina’s tolerance

Recovery protocol:

Check pH — restore to 9.0–9.5 with sodium bicarbonate
Check for rotifers under microscope — if present, raise pH to 10.2–10.5 for 24–48 hours (rotifers cannot survive above pH 10)
Transfer the most productive-looking portion (greenest, most floating material) to fresh medium in a small volume (5–10L)
Use your frozen backup culture if all living cells appear lost

Daily pH check: 9.5–10.5
Weekly: add sodium nitrate (nitrogen) and verify phosphate and bicarbonate levels
Temperature log: stay within 25–38°C. Never below 20°C without insulation.
Monthly microscope check: verify spirulina trichome morphology and check for rotifer or other contaminants
Freeze a backup portion: 10–20% DMSO at −20°C — insurance against culture loss