Carbon supplementation for spirulina culture.

How spirulina fixes carbon

• Bicarbonate, not CO₂directly: At the alkaline pH of spirulina culture (9–10.5), the dominant inorganic carbon species is bicarbonate (HCO₃⁻), not CO₂gas. Spirulina has an efficient carbon-concentrating mechanism (CCM) that transports HCO₃⁻into the cell, dehydrates it to CO₂at the Rubisco site in the carboxysomes, and fixes it via the Calvin cycle. This gives spirulina a competitive advantage over many contaminants at high pH and high bicarbonate concentrations.
• Carbon consumption raises pH: As spirulina photosynthesises, it consumes HCO₃⁻and produces OH⁻as a byproduct of photosynthetic electron transport. This causes pH to rise during active photosynthesis (daytime). A rising pH is therefore a sign of active, healthy photosynthesis — but it also signals carbon consumption that will eventually limit growth if not replenished. Carbon management and pH management are inseparable.

Zarrouk medium carbon sources

• Sodium bicarbonate (NaHCO₃): Standard Zarrouk medium contains 16.8 g/L NaHCO₃, providing approximately 200 mmol/L bicarbonate and adjusting pH to 9.0–9.5. This is a large carbon reservoir that supports 3–5 growth cycles before depletion in typical semi-continuous production. When bicarbonate is consumed, pH stops rising during daylight, culture growth stalls, and colour may begin to pale (insufficient carbon for phycocyanin and protein synthesis).
• Sodium sesquicarbonate (Na₂CO₃·NaHCO₃·2H₂O, trona): Commercial spirulina production increasingly uses sodium sesquicarbonate as the combined carbon and alkalinity source in place of separate NaHCO₃and Na₂CO₃. Sesquicarbonate provides a more stable pH than pure carbonate and a more buffered alkalinity than bicarbonate alone. For hobbyist growers: sodium bicarbonate (food grade, widely available) is the practical standard.
• Sodium carbonate (Na₂CO₃, soda ash): Zarrouk also contains 0.5 g/L Na₂CO₃as an alkalinity buffer. Adding pure Na₂CO₃raises pH sharply (more alkaline per gram than bicarbonate); use carefully and in small dissolved amounts. At pH above 11, culture damage occurs rapidly.

CO₂ injection as carbon supplementation

• Dual function: CO₂injected into spirulina culture simultaneously provides inorganic carbon and lowers pH (CO₂+ H₂O → HCO₃⁻+ H⁺). This makes it the most efficient intervention for managing high-pH, carbon-depleted cultures. CO₂is the primary pH control and carbon management tool in commercial raceway production.
• Sources: For home growers: food-grade CO₂(SodaStream cylinders, aquarium CO₂kits with diffuser stones) work well. Inject in short bursts, recheck pH after each burst. Target pH 9.5–10.0 post-correction. At commercial scale: agricultural CO₂or waste CO₂from fermentation processes is piped to the raceway continuously via proportional controllers linked to pH sensors.
• CO₂and carbon enrichment: Beyond pH management, CO₂injection at elevated concentrations (1–5% CO₂in air) increases biomass productivity by 20–50% compared to air bubbling alone (which provides only 0.04% atmospheric CO₂). For indoor cultures using air pumps: adding aquarium CO₂injection alongside the air supply is a worthwhile productivity improvement.

Signs of carbon deficiency

• pH plateau: Morning pH no longer lower than evening measurement; pH fails to rise during daylight (photosynthesis not consuming carbon because carbon is exhausted, so no OH⁻production). This is a reliable sign of carbon exhaustion.
• Growth stall: Optical density and biomass stop increasing between harvest cycles despite adequate nitrogen, temperature, and light.
• Colour change: Pale green (not the yellow-green of nitrogen deficiency, but a washed-out pale green as carbon-limited cells reduce pigment synthesis). If colour shift is yellow-green: nitrogen deficiency is the primary problem; pale green more broadly: could be carbon, light, or temperature.

Practical carbon replenishment

• Semi-continuous production (20 L indoor tank): replenish carbon by replacing harvested volume with fresh Zarrouk medium (16.8 g/L NaHCO₃) immediately after each harvest. This simultaneously replenishes carbon, nitrogen, and micronutrients.
• If pH is consistently above 10.5 morning and growth is stalled: add dissolved NaHCO₃(5–10 g per 10 L, dissolved in warm water first) and recheck pH. The addition should lower pH slightly (bicarbonate acts as a pH buffer in this range) and provide carbon for photosynthesis.
• Never add dry NaHCO₃directly to the culture tank: local CO₂off-gassing and pH fluctuations can cause localised stress. Always dissolve first in a small volume of culture water.
• CO₂injection is preferable to bicarbonate addition if pH is simultaneously high: it corrects both problems at once.
• Keep a record of bicarbonate additions alongside pH and harvest data: carbon consumption rate helps predict replenishment schedule and optimise the feeding routine