The taxonomy confusion
The organism sold and cultivated as “spirulina” was reclassified from genus Spirulina to Arthrospira in the 1990s based on 16S rRNA gene sequencing. The two commercially relevant species:
- Arthrospira platensis:Originally from African soda lakes (Lake Chad, Lake Nakuru). The dominant species in commercial production worldwide. Grown at Earthrise Farms, Cyanotech, and most Asian production facilities.
- Arthrospira maxima:Originally from Central American alkaline lakes (Lake Texcoco, Mexico). Historically important — consumed by Aztecs as técuitlatl. Now a minor fraction of commercial production but still grown at some US facilities.
The name “Spirulina” persists in commerce, regulation (FDA, EFSA), and common use — so the old name remains functionally correct for labelling purposes.
Morphological differences
Both species are filamentous cyanobacteria with the characteristic helix (spiral) shape. Under a microscope at 400x:
- A. platensis:Tighter helix with more turns per filament length — typically 4–6 turns per 100 µm. Individual trichomes 6–8 µm wide.
- A. maxima:Looser helix with fewer turns — typically 2–3 turns per 100 µm. Trichomes slightly wider at 8–10 µm.
Home growers with access to a basic microscope can roughly distinguish the two species by counting helix turns, though strain variation within each species means this is not definitive for identification.
Temperature preferences
- A. platensis:Optimal 35–38°C. Grows well 30–38°C. Tolerates down to 20°C but growth slows significantly. Upper limit approximately 45°C before thermal stress.
- A. maxima:Optimal 32–35°C. Slightly lower optimum — marginally better for growers in cooler environments who struggle to reach 38°C. Similar lower and upper limits to platensis in practice.
For most home growers, the temperature difference between species is less important than the heater setup and ambient room temperature. Both species underperform below 25°C.
pH tolerance
- A. platensis:pH 9.0–10.5. The wider pH tolerance makes it more forgiving of CO₂ addition, bicarbonate depletion, and minor management lapses.
- A. maxima:pH 8.5–10.5. Tolerates slightly lower pH — some growers use maxima for lower-alkalinity water sources where reaching pH 9.5 requires excessive bicarbonate addition.
Phycocyanin and nutritional content
This is the most practically relevant difference for growers focused on nutritional yield:
- A. platensis:Phycocyanin typically 14–20% dry weight — among the highest of any strain. Protein content 60–70% dry weight.
- A. maxima:Phycocyanin typically 10–15% dry weight — slightly lower than platensis. Protein content similar at 60–68% dry weight.
For home growers prioritising phycocyanin (the blue pigment and primary bioactive compound), A. platensis strains have a measurable advantage. The colour of your paste should be intensely blue-green — this reflects phycocyanin content.
Growth rate comparison
Under optimal conditions:
- A. platensis: doubling time approximately 16–24 hours at 35–38°C with adequate light and CO₂
- A. maxima: doubling time approximately 18–28 hours — slightly slower under identical conditions at the same temperature
In practice at home scale with variable light, temperature, and CO₂ supply, the difference is rarely the limiting factor. Nutrient and light management matters more than species choice.
Strain variation within species
Within A. platensis, there is significant strain-level variation. Commonly available home culture strains include:
- UTEX 2340 (A. platensis):The most widely distributed home culture strain. Well-documented, forgiving, and the reference strain for most home growing guides.
- Paracas strain (Peru):Higher GLA content (some reports 2× standard platensis) — used by growers prioritising essential fatty acid content over phycocyanin yield.
- Commercial facility strains:Not generally available to home growers — proprietary strains optimised for raceway pond conditions differ from home tank cultivation requirements.
Practical recommendation for home growers
For most home growers starting out:
- Start with A. platensis (UTEX 2340 or equivalent) — the most documented, most available, and most forgiving strain for non-commercial conditions
- Source starter culture from an established home growing community or certified lab supplier — not from dried commercial powder, which is not viable
- Once culture is established and stable, maintain it — do not switch strains unless you have a specific reason. A well-adapted culture outperforms a fresh inoculant of a theoretically “better” strain
- Keep a frozen backup of your established culture — 10–20% DMSO cryoprotectant at −20°C can preserve a culture for years
