Spirulina nutrient dosing and media formulation.

Macronutrient physiology and stoichiometry

• Nitrogen limitation and Redfield ratio: Spirulina biomass is ~50% protein (dry weight), and protein is ~16% nitrogen by mass. Optimal growth occurs when nitrogen is the limiting nutrient (prevents excess phosphate accumulation and toxicity). The Redfield ratio for marine phytoplankton (N:P:K = 16:1:8 by atomic mass) applies to spirulina. Deviating from this ratio causes stoichiometric imbalance: excess phosphate precipitates as struvite (MgNH₄PO₄·6H₂O, white crystals); excess potassium is expensive and raises ionic strength (osmotic stress).
• Nitrogen sources and bioavailability: Urea (NH₂)₂CO is the preferred nitrogen source (cost $1–2/kg, fully available). Dosing: 1–2 g/L per culture cycle (add on day 0, repeat day 5, 10). Sodium nitrate (NaNO₃) is an alternative (cost $3–5/kg, slower assimilation, adds sodium ion). Ammonium chloride (NH₄Cl) is avoided (lowers pH, causes acetification below pH 7). Each 1 g/L urea provides ~0.46 g/L nitrogen; for a 100L culture, 100–150g urea per cycle achieves optimal nitrogen flux.
• Phosphate balance and precipitation: Phosphate precipitates as struvite when [Mg²⁺][NH₄⁺][PO₄³⁻] exceeds solubility product (Ksp ≈ 2.5 × 10⁻¹³). To prevent precipitation: maintain K⁺:PO₄ molar ratio at 1.5:1 (potassium ions compete with magnesium, reducing struvite formation). Dosing: K₂HPO₄ 0.1–0.2 g/L per cycle (provides ~0.06–0.13 g/L phosphate). If white crystals form in tank, precipitation has occurred; acidify culture to pH 6.5–7 briefly (dissolves struvite), then return to pH 8.5–9 (may disrupt culture temporarily—avoid by careful dosing).
• Potassium supplementation: KCl is the source (cost $2–4/kg). Dosing: 0.3–0.5 g/L per cycle. Potassium is essential for osmotic regulation and enzyme cofactor (especially rubisco). Excess potassium (>2 g/L) raises ionic strength, increasing osmotic pressure and slowing growth.

Micronutrient dosing and toxicity windows

• Iron complexation and pH-dependent precipitation: Iron is supplied as ferrous sulfate (FeSO₄·7H₂O, cost $1–2/kg). Problem: ferrous iron (Fe²⁺) oxidises to ferric iron (Fe³⁺) at pH >8 in aerated cultures. Ferric iron forms insoluble hydroxide (Fe(OH)₃, brown precipitate). Solution: chelate iron with citric acid (1.2:1 molar ratio of citrate:iron) before adding to culture. Citrate maintains iron solubility at pH 9 by forming soluble citrate-iron complexes. Dosing: micronutrient stock solution containing 5 mg Fe per mL (citrate-chelated). Add 10 mL per 10L culture every 2–3 days (typical daily iron consumption is 0.01–0.02 mg Fe per mL culture). Excessive iron (>0.1 mg/mL) causes ROS production (Fe-catalysed Fenton reaction), damaging phycocyanin.
• Zinc, manganese, copper dosing: Supply as sulfate salts (ZnSO₄, MnSO₄, CuSO₄). Micronutrient stock (per litre): zinc 0.1 mg, manganese 0.05 mg, copper 0.01 mg (cost $10–20 per litre stock). Add 10 mL stock per 10L culture every 2–3 days. Zinc is essential for carbonic anhydrase (photosynthetic CO₂ fixation); deficiency causes pale culture and growth arrest. Manganese is the water-splitting oxygen-evolving complex cofactor; deficiency impairs photosystem II. Copper is cytochrome oxidase cofactor; deficiency is rare in cultures but causes anaerobic metabolism (undesirable). Toxicity windows: zinc >1 mg/mL inhibits photosynthesis; manganese >0.5 mg/mL is toxic; copper >0.1 mg/mL inhibits growth.
• Boron and molybdenum (trace elements): Boron (as boric acid, H₃BO₃): 0.02 mg per litre stock, add every 4–5 days (cell wall pectin synthesis). Molybdenum (as Na₂MoO₄): 0.001 mg per litre stock, add every 5–7 days (nitrogenase cofactor—relevant only if spirulina ever encounters nitrogen-fixing conditions, rare in closed culture). Boron deficiency causes cell wall weakening (cells lyse during harvest); molybdenum is rarely limiting.

Micronutrient stock solution preparation

• DIY stock recipe (per 1L distilled water): • FeSO₄·7H₂O 5 g (chelate with 6 g citric acid, stir 30 min) • ZnSO₄·7H₂O 0.1 g • MnSO₄·H₂O 0.05 g • CuSO₄·5H₂O 0.01 g • H₃BO₃ 0.02 g • Na₂MoO₄·2H₂O 0.001 g Dissolve each salt in distilled water (order: iron-citrate complex first, then others). Store in dark glass bottle at 4°C (light causes iron oxidation). Shelf life: 6–12 months. Cost: ~$15–20 per litre stock (single batch yields micronutrient supplement for 1000+ litres of culture).
• Commercial alternatives: Pre-formulated micronutrient mixtures (e.g., Hoagland’s solution trace elements): $30–50 per litre, ready-to-use, guaranteed chelation. Avoids DIY errors but costlier per application.

Nitrogen cycling and growth rate control

• Daily nitrogen consumption and depletion: Spirulina consumes nitrogen at ~0.1–0.15 mg N per mL culture per day (exponential growth phase, µ = 0.4 day⁻¹). A 100L culture (100,000 mL) consumes ~10–15 g nitrogen per day. Initial urea dosing (100–150g per 100L = 1–1.5 g/L) is depleted in 7–10 days. Replenishment schedule: day 0 (inoculation), day 5, day 10, then every 5 days. Monitor nitrogen status by Nessler test (colourimetric reagent, turns yellow in presence of ammonium; online kits available $20–40). When Nessler test shows faint colour, nitrogen is low; add urea.
• Nitrogen-limited plateau and growth rate: When nitrogen becomes limiting, growth rate drops from µ = 0.4 to µ = 0.05 day⁻¹ (10-fold slowdown). Culture reaches stationary phase: cells accumulate starch and lipids, phycocyanin is catabolised (colour fades from blue-green to yellow-green). This is the harvest signal: culture has maximal cell density (~1–1.5 g/L dry weight) and is ready for harvesting. Avoid remaining in stationary phase >3 days (cells die, contamination risk increases).

Phosphate management and crystal prevention

• Struvite crystal formation and consequences: Struvite (MgNH₄PO₄·6H₂O) forms when excess phosphate precipitates. Visible signs: white crystalline particles in tank, cloudiness, reduced light penetration. Consequences: reduced photosynthesis (shadowing effect), clogged airlift stone (air bubbles get stuck, circulation fails), equipment damage (sharp crystals scratch tank walls). Prevention: strict adherence to K⁺:PO₄ molar ratio 1.5:1. For typical dosing (K₂HPO₄ 0.1 g/L = 0.054 g/L PO₄³⁻ = 1.7 mmol/L PO₄³⁻), add KCl to maintain [K⁺] = 2.5 mmol/L (0.3–0.5 g/L KCl).
• Recovery if precipitation occurs: Add 5–10 mL concentrated hydrochloric acid (HCl, 37%) per 10L culture (acidifies to pH 6.5–7). Leave for 2 hours (struvite dissolves). Then carefully add sodium hydroxide (NaOH) solution to return pH to 8.5–9 (monitor with pH meter, slow addition to avoid overshoot). Resume normal dosing with improved K⁺:PO₄ ratio tracking. Acidification stresses cells temporarily but rarely causes permanent damage if pH is restored within 4 hours.

Sampling frequency and monitoring protocol

• Daily monitoring (pH and visual): Measure pH daily using calibrated meter (optimal range 8.5–9.2). Visual inspection: colour (blue-green = healthy; yellow-green = nutrient stress); clarity (cloudy = bacterial contamination or excess precipitate); bubble size (airlift function). Cost of pH meter: $30–80 (pocket-sized digital meter, sufficient accuracy ±0.1 pH units).
• Nitrogen assay (weekly, Nessler test): Purchase Nessler reagent test kit (~$25 per kit, yields 20–50 tests). Procedure: mix 5 mL culture sample + 0.5 mL Nessler reagent in test tube, wait 1 min, compare colour to standard chart (yellow = high NH₄⁺; colourless = depleted). Alternative: send samples to lab for ammonia-selective electrode assay (more accurate, $10–20 per sample, 2-day turnaround).
• Dissolved oxygen (2×/week, probe or chemical): Oxygen probe (digital meter, $200–500) provides real-time DO percentage. Chemical alternative: Winkler titration (lab service, $15–25 per sample, 3-day turnaround, gold standard accuracy). Target DO: 5–7 mg/L (too low <3 mg/L causes anaerobic stress; too high >10 mg/L oxidises phycocyanin and causes photooxidative stress).
• Phosphate assay (monthly, colorimetric): Phosphate test kit (Murphy-Riley method, $30–50 per kit, yields 25–50 tests). Mix sample + ascorbic acid + molybdate reagent, read absorbance at 880 nm using spectrophotometer ($200–600). Only necessary monthly unless struvite formation is suspected.

Cost-optimised formulations for small growers

• Budget option (DIY bulk salts, $2–5 per litre culture): Purchase urea, K₂HPO₄, KCl, and citric acid in bulk (25–50 kg bags). Make micronutrient stock from individual sulfate salts. This approach requires accurate scales (0.01 g precision, ~$40–80) and chemical knowledge but yields minimum cost. Annual cost for 1000L culture: ~$50–150.
• Mid-range option (commercial macronutrient blend + DIY micronutrient stock, $5–12 per litre): Purchase pre-formulated macronutrient packages (nitrogen + phosphate + potassium blended, $15–30 per litre stock, diluted to working concentration). Make micronutrient stock yourself. Reduces mixing error, slightly costlier than pure DIY. Annual cost for 1000L: ~$100–250.
• Premium option (Hoagland’s or equivalent complete nutrient solution, $15–25 per litre culture): Purchase pre-formulated complete solution (all macro- and micronutrients blended, chelated, pH-buffered). Zero dilution error. Costlier but suitable for research or commercial scales. Annual cost for 1000L: ~$300–500.

Nutrient depletion profiles and harvest triggers

• Sequential nutrient limitation (typical 14-day culture cycle): Days 0–5: all nutrients abundant, exponential growth (µ = 0.4 day⁻¹). Days 5–10: nitrogen depletes (Nessler test fades), phosphate and trace elements remain adequate, growth begins to slow (µ = 0.2 day⁻¹). Days 10–12: phosphate starts to deplete, nitrogen absolutely depleted, growth stalls (µ <0.05 day⁻¹), culture enters stationary phase (cell density plateaus at ~1–1.5 g/L). Day 12+: cells begin to die (natural senescence), contamination risk increases. Harvest day 10–12 is optimal (maximum cell density, minimal mortality).
• Extending culture lifespan by strategic nitrogen pulses: If harvest is delayed, small nitrogen additions (0.2–0.5 g/L urea) every 3 days extend culture viability to day 18–20. Beyond day 20, contamination (bacterial or fungal) is nearly inevitable; culture should be discarded and new batch inoculated.

Common nutrient deficiency symptoms

• Nitrogen deficiency: Culture colour fades from blue-green to yellow-green, growth rate drops sharply, cells accumulate starch (granular appearance under microscope), phycocyanin is catabolised (colour loss). Remedy: add urea immediately (0.5–1 g/L). Growth resumes within 24–48 hours.
• Phosphate deficiency: Growth slows but colour remains blue-green, cells are stunted (smaller than normal), no visible deficiency symptoms. Often masked by nitrogen depletion (occurs simultaneously). Remedy: add K₂HPO₄ (0.1–0.2 g/L). Rare in well-maintained cultures.
• Iron deficiency: Culture becomes pale yellow-green (bleached appearance), photosynthetic rate drops >50% (measured by oxygen evolution rate), cells die despite other nutrients present. Remedy: add iron stock solution (10 mL per 10L culture). Recovery: 3–5 days to restore blue-green colour.
• Zinc deficiency: Culture is slow to grow even after nitrogen addition, cells are small and pale, carbonic anhydrase activity is low (measured by ¹⁴C-labelled CO₂ fixation assay, research-level). Remedy: add zinc from stock solution (0.1 mg per 10L). Recovery: 1–2 weeks.