pH calibration for spirulina culture.

Why pH matters in spirulina culture

• Optimal range pH 9.0–10.5: Within this range spirulina maintains maximum photosynthetic efficiency, protein synthesis, and phycocyanin production. CO₂ is available as HCO₃⁻ (bicarbonate) at these pH values, which spirulina fixes via the carbon-concentrating mechanism. Below pH 8.5, CO₂ becomes the dominant carbon species and culture can acidify further; above pH 10.5, bicarbonate availability drops and photoinhibition increases.
• Diurnal pH swing: In outdoor or high-light indoor culture, active photosynthesis consumes CO₂/HCO₃⁻, raising pH during the day by 0.3–0.8 pH units. At night, respiration adds CO₂, lowering pH. This is normal. Measure pH at the same time each morning (before illumination peak) for a consistent baseline. pH above 10.8 at morning measurement signals either high culture density, CO₂ deficiency, or overfeeding with bicarbonate.
• pH as culture health indicator: Sudden pH drop (below 8.5) suggests culture crash, bacterial contamination, or CO₂ flooding. Failure of pH to rise during daylight suggests photosynthesis is compromised (shading, contamination, nitrogen starvation). pH is a faster indicator than optical density for acute culture problems.

Two-point calibration: the correct method

• Why two points are required: pH meters measure voltage from a glass electrode and convert via a linear equation: pH = (E − E₀) / slope. A single-point calibration at pH 7 (the isopotential point) corrects the offset but does not correct the slope (electrode sensitivity, mV/pH). In new or ageing electrodes, the slope deviates from the theoretical 59.16 mV/pH. A pH 7-only calibration can introduce errors of 0.2–0.5 pH units at pH 9–10, which may cause you to misidentify pH 9.8 as pH 10.3 — a meaningful culture management error.
• Buffer solutions required: pH 7.00 (phosphate buffer; colour-coded yellow or clear by most brands) and pH 10.01 (carbonate-bicarbonate buffer; colour-coded blue). Do not use pH 4.00 buffer as the second point for spirulina work — pH 10.01 is the appropriate second calibration point for alkaline culture monitoring. Buy certified buffer sachets (±0.02 pH accuracy); avoid generic unlabelled bottles.
• Calibration procedure: Rinse electrode with distilled or deionised water; blot dry with lint-free tissue (never rub). Insert into pH 7 buffer, wait for stable reading (typically 30–60 seconds), confirm or calibrate. Rinse electrode. Insert into pH 10.01 buffer, wait for stable reading, confirm or calibrate. The meter now has a corrected slope for alkaline measurements. Check slope percentage: 95–105% is good; below 90% or above 110% signals electrode degradation — replace electrode.
• Calibration frequency: Calibrate before first use and thereafter weekly for routine monitoring. Also recalibrate after electrode storage, after any damage or drying, and whenever readings seem inconsistent with culture appearance. Budget pH meters (under £30) typically require more frequent calibration; laboratory-grade meters hold calibration longer but the two-point alkaline calibration principle is identical.

Daily pH monitoring protocol

• Timing: Measure at the same time each morning, before peak illumination (or first thing before lights-on in indoor culture). Record in a log. A consistent daily pH record is more useful than sporadic accurate measurements: you are looking for trends (gradual rise over days = CO₂deficiency; gradual fall = nitrogen depletion or bacterial acidification).
• Target range: Morning pH 9.0–10.2. Action at pH below 9.0: check for signs of crash (colour, smell, optical density); add bicarbonate slowly if culture appears healthy. Action at pH above 10.5 morning: slow CO₂ injection or dilute with fresh medium; check culture density (may need to harvest).
• Electrode storage: Never store the pH electrode dry. Store in electrode storage solution (KCl solution, usually supplied or sold separately) or, as a fallback, pH 7 buffer. A dry-stored electrode takes hours to re-hydrate and reads incorrectly until rehydrated. Keep electrode tip submerged at all times when not in use.

Managing pH excursions

• High pH (above 10.5): CO₂injection: CO₂ gas bubbled through the culture reacts with water to form carbonic acid, lowering pH rapidly. CO₂injection is the most precise pH control method in commercial-scale raceway production. For home culture: food-grade CO₂(SodaStream cylinder or aquarium CO₂kit) via a diffuser stone works. Add briefly in bursts and recheck pH — CO₂lowers pH quickly and can overshoot. Target pH 9.5–10.0 after correction.
• High pH: dilution with fresh medium: Replacing 20–30% of culture volume with fresh Zarrouk medium (pH typically 9.0–9.5 when freshly prepared) lowers pH and adds nutrients. Useful when culture density is high and harvest is due anyway.
• Low pH (below 9.0): sodium bicarbonate or sodium sesquicarbonate: Dissolve sodium bicarbonate (NaHCO₃) in a small volume of culture water before adding to the tank — adding dry powder directly causes local pH spikes. Add slowly, stir, recheck after 15 minutes. Sodium sesquicarbonate (Na₂CO₃·NaHCO₃·2H₂O, trona) is the preferred carbon and alkalinity source for Zarrouk medium and gives more stable pH than pure sodium carbonate.
• Avoid citric acid or vinegar to lower pH: Organic acids introduce carbon that supports bacterial growth and may selectively favour contaminating heterotrophs over spirulina. Use CO₂ or dilution with fresh medium only for pH reduction.

Alkaline shock for rotifer treatment

• Rotifer contamination: Rotifers (Brachionus spp.) are the most common spirulina predator in outdoor and semi-open indoor cultures. A heavy rotifer infestation can collapse a culture within 48–72 hours by grazing through the biomass faster than spirulina reproduces. Signs: culture rapidly clearing, sticky foamy residue, rotifers visible under 40x microscopy.
• Alkaline shock protocol: Raise culture pH to 11.5 by adding concentrated sodium carbonate solution. Maintain pH 11.5 for 2 hours with adequate mixing. Then restore pH to 9.5–10.0 with CO₂injection or dilution. At pH 11.5, rotifers die within 30–60 minutes; spirulina is damaged but can recover over 5–7 days if the treatment is brief. Recovery is faster if you dilute with fresh medium post-treatment to restore nutrients and remove rotifer debris.
• Prevention: Covered (but ventilated) cultures prevent rotifer introduction. Avoid adding untreated outdoor water or wild-collected spirulina inocula. Routine pH maintenance above 10.0 provides some natural rotifer suppression — rotifers reproduce poorly above pH 10.0.

Conductivity monitoring

• What conductivity measures: Electrical conductivity (EC, measured in mS/cm) reflects total dissolved solids — primarily mineral salt concentration in the culture medium. Fresh Zarrouk medium has EC approximately 3.5–4.5 mS/cm. As spirulina grows, it consumes nutrients but the culture also concentrates as evaporation occurs, raising EC. In harvest-and-refill management, EC rising above 6–7 mS/cm signals salt accumulation and the need for partial culture replacement.
• EC as contamination indicator: A sudden EC drop (without dilution) can indicate bacterial organic acid production (heterotrophs consuming organic matter). Combined pH drop + EC drop is a reliable crash warning sign. Measure EC weekly alongside pH for a complete culture health picture.
• Equipment: A combined pH/EC meter (around £40–60 for hobbyist grade) is a worthwhile investment for serious home growers. Calibrate EC probe with a 1413 µS/cm or 12.88 mS/cm standard solution depending on culture concentration.

Common mistakes

• Single-point pH 7 calibration only: Causes systematic error of 0.2–0.5 units in the alkaline range; may cause you to delay corrective action when pH has already reached dangerous levels
• Dry electrode storage: Causes slow response and inaccurate readings until electrode re-hydrates; always store in KCl solution
• Rubbing the electrode tip: Creates static charge on the glass membrane, causing unstable readings; blot only with soft tissue
• Adding acid to lower pH: Organic acids support bacterial contamination; inorganic acid (HCl) corrodes fittings and is hazardous; use CO₂ only
• Measuring only sporadically: pH trends over days are more informative than any single measurement; a culture log is the most valuable tool in home spirulina production