Daily maintenance (5 minutes)
pH check
pH is the single most important daily measurement. Target: 9.5–10.5.
- Below 9.0:Add sodium bicarbonate (NaHCO₃) — dissolve 1–2 g/10L of culture volume in a small amount of warm water, then add. Below 9.0 is the contamination risk zone — act same day.
- 9.0–9.5:Normal range, mildly low. Add a small bicarbonate dose.
- 9.5–10.5:Optimal. No action needed.
- Above 10.8:Carbon limitation — add bicarbonate paradoxically (pH rise indicates CO₂ depletion; bicarbonate provides fresh carbon source and buffers the alkalinity).
pH test strips (6.0–11.0 range) are sufficient for daily monitoring. A calibrated pH meter is more accurate and worthwhile for serious growers.
Colour and visual check
Healthy culture: deep blue-green, opaque at 2–4 g/L density.
- Yellow-green or brownish tinge: nitrogen stress — begin nitrogen supplementation (see weekly tasks)
- Pale blue-green with visible clumping: check for contamination or temperature stress
- Sinking material at bottom: gas vacuole loss (see troubleshooting guide)
Temperature
Check water temperature (not just room temperature — the heater thermostat can drift):
- Target: 30–36°C
- If below 25°C: growth slows significantly
- If above 40°C: phycocyanin degrades rapidly
Weekly maintenance (20–30 minutes)
Nitrogen replenishment
Nitrogen is consumed by growing spirulina cells at a rate of approximately 10–15% of cell dry weight per generation. After regular harvesting, the medium depletes in 5–7 days.
- Add sodium nitrate (NaNO₃): 0.2–0.5 g per litre of culture after each weekly harvest
- Signs of deficiency before weekly replenishment: culture colour shifting toward yellow — check colour daily and add nitrogen immediately if yellowing starts
Bicarbonate replenishment
Even if pH is stable, bicarbonate is consumed as the carbon source for photosynthesis:
- Add sodium bicarbonate: 1–2 g/L of culture volume weekly as a maintenance dose (separate from any pH-correction additions)
Density assessment
Use the Secchi disc method (a small disc lowered into the culture — the depth at which it disappears indicates density):
- 2–5 cm visibility depth: 2–4 g/L density — in the harvest range
- >8 cm visibility: below 1.5 g/L — reduce or skip harvest this week
- <1 cm visibility: above 5 g/L — harvest 40–50% to prevent self-shading
Partial medium refresh
Every 3–4 weeks (not weekly), replace 30–40% of the culture liquid with fresh medium:
- Harvest 30–40% of volume normally (collect the spirulina)
- Replace with freshly prepared Zarrouk medium or simplified home formula — this replenishes all micronutrients (phosphate, trace minerals, sulphate) that are not covered by the weekly nitrogen/bicarbonate additions
Monthly maintenance
Microscope check
A 400x microscope inspection takes 10 minutes and catches contamination before it becomes a crisis:
- Look for characteristic Arthrospira spiral trichomes — count helix turns to confirm identity
- Check for rotifers (oval animals with cilia, clearly motile) — if found, raise pH to 10.2–10.5 for 24–48 hours to eliminate them
- Check for competing organisms: spherical green algae cells (contamination), straight or branched filaments (Oscillatoria or other cyanobacteria — potential microcystin risk)
Freeze a backup
Monthly: freeze a small portion of the culture as insurance:
- Mix 80% culture + 20% DMSO (dimethyl sulfoxide, cryoprotectant) in a sterile vial
- Freeze at −20°C or lower — viable for 1–3 years
- Label with date — use the oldest backup first if you need to restart
Seasonal maintenance in temperate climates
- Autumn:As days shorten and temperatures drop, reduce harvest frequency. Maintain culture at lower density to match reduced photosynthetic capacity.
- Winter (indoor only):Supplement with artificial light (LED, 12–16 hours/day). Reduce harvesting to monthly if growth is minimal. Maintenance mode is acceptable — the culture survives, just doesn’t produce much.
- Spring:Reinvigorate with fresh medium, increased light, and temperature. The culture returns to active growth within 2–3 weeks.
