Spirulina’s light absorption profile
Spirulina absorbs light through two pigment systems with distinct spectral peaks:
- Chlorophyll a: Peaks at 430–470 nm (blue) and 660–680 nm (red). Drives the Calvin cycle and ATP production.
- Phycocyanin:Absorbs maximally at 615–620 nm (orange-red). Transfers energy to chlorophyll a via FRET. Phycocyanin is spirulina’s primary light-harvesting antenna — it extends light absorption into the orange region that chlorophyll misses.
- Beta-carotene and zeaxanthin:Carotenoid accessory pigments that absorb in the 400–550 nm range and transfer energy to chlorophyll, also providing photoprotection from excess light.
The combined absorption profile means spirulina uses light across most of the visible spectrum, with strongest response to red (660–680 nm) and secondary strong response to blue (440–470 nm). Orange-red (615–630 nm) is uniquely efficient for spirulina due to phycocyanin.
Light intensity: the critical variable
For home and small-scale indoor growers, light intensity is almost always the primary yield-limiting factor — more than temperature, nutrients, or pH management.
Intensity guidelines for spirulina cultivation:
- Minimum for growth: 2,000 lux at the culture surface. Below this, growth rate is significantly suppressed and the culture may not remain viable.
- Optimal for productivity:5,000–15,000 lux. Most indoor grow setups achieve this range with appropriate LED placement.
- Upper tolerance: Up to 30,000–40,000 lux in well-mixed, shallow cultures. Above this, photoinhibition occurs — the photosynthetic machinery is damaged by excess light and yield actually drops.
- Outdoor sunlight reference:Full tropical midday sun is 80,000–100,000 lux. This is why outdoor cultivation in warm latitudes dramatically outperforms indoor — the natural light intensity is far higher than any practical indoor lighting.
Practical measurement: a cheap lux meter app on a smartphone gives a reasonable estimate. Hold the phone face-up at the culture surface level and check illuminance.
LED grow light selection
Full-spectrum LED grow lights designed for food plants are the best practical choice:
- Full-spectrum white LEDs(4000–6500K colour temperature) provide reasonable coverage across all spirulina absorption peaks. Easy to source, good for secondary plant growing alongside.
- Red-blue LED panels (660 nm + 450 nm) are more efficient for spirulina specifically — delivering energy exactly at the chlorophyll a absorption peaks with less wasted green output. However, they produce purple light that some growers find aesthetically unappealing indoors.
- Quantum board LEDs (Samsung LM301 and similar) provide high-intensity full-spectrum output at good energy efficiency. These are the current best option for serious home growers.
- T5 fluorescent tubes:A legacy option — lower heat than HPS, adequate spectrum, but significantly less efficient per watt than modern LED. Still usable for small cultures.
Calculating LED coverage for your culture volume
A rough practical guide:
- A 20W quantum board LED at 20–30 cm above the culture surface provides approximately 8,000–12,000 lux over a 30 × 40 cm area — suitable for a 20-litre culture.
- For a 50-litre raceway, a 50–75W full-spectrum LED panel covers the surface adequately.
- Place lights 20–30 cm above the culture surface for intensity without heat damage. More than 40 cm significantly reduces intensity (inverse square law).
Photoperiod: 16/8 is the standard
Spirulina is a photoautotroph and does not require a dark period for growth — it would grow 24 hours/day if light were available without photoinhibition. However, continuous light is not optimal:
- Continuous illumination causes circadian disruption in the biochemical cycles that regulate phycocyanin synthesis and cell division. A 16-hour light / 8-hour dark cycle is the most productive for most home growers.
- 18/6 is a useful alternative for maximising yield in small cultures during winter.
- Use a basic plug-in timer to automate the photoperiod — manual switching is unreliable and creates inconsistent growth conditions.
Light colour temperature and phycocyanin content
Higher orange-red light (600–630 nm, from phycocyanin absorption) has been shown in research to increase phycocyanin content relative to chlorophyll in spirulina cultures. If phycocyanin content is a priority (for anti-inflammatory use), a light source with enhanced orange-red emission is beneficial.
Practical implication: warm-white LED panels (3000–4000K) have more orange-red output than cool-white panels (6500K). For phycocyanin-focused home cultivation, a 3000–4000K LED is preferable to cool white.
Common lighting mistakes
- Under-lighting: The most common issue. At 1,000–2,000 lux, cultures grow slowly and have poor phycocyanin content. Invest in adequate lighting first.
- Using standard household bulbs:Standard LED bulbs produce 500–800 lux at a 30 cm distance — far below the minimum for productive cultivation.
- Inconsistent photoperiod:Irregular on/off cycles reduce growth rate and phycocyanin content. Always use a timer.
- Overheating from lights:If the culture temperature rises above 38°C from light heat, relocate the light or add cooling. High temperatures denature phycocyanin even before harvest.