Spirulina strain selection: Arthrospira platensis vs maxima, phycocyanin content, and sourcing

Species overview

Arthrospira platensis: The dominant commercial species. Found originally in African and Asian alkaline soda lakes (Lake Chad, Lake Nakuru). Morphology: tight, regular helical coil (helix pitch 40–60 µm). Higher phycocyanin content in most tested strains (15–25% dry weight). Wide pH tolerance (pH 9–11) making it more robust against competing green algae (which fail above pH 9.5). The species used in the majority of clinical trials and in the dominant commercial producers worldwide.
Arthrospira maxima: Found originally in Mexican soda lakes (Lake Texcoco). Morphology: loose, irregular helix (helix pitch 60–100 µm; appears almost straight in some strains). Slightly higher protein content reported in some analyses. Preferred by some growers for flavour (slightly milder marine taste). Narrower pH optimum (pH 9–10.5) and somewhat more susceptible to contamination than platensis in outdoor systems. Less studied in clinical trials than platensis.

Within A. platensis, C-phycocyanin (CPC) content varies from ~5% dry weight in low-performing strains to ~25% in high-performing strains. This fivefold variation is strain-specific and influenced by: growth conditions (temperature, light intensity, nitrogen source), culture age, and harvest frequency. When buying a starter culture, request information about the strain’s typical phycocyanin content under standard growing conditions.
Nitrogen limitation reduces phycocyanin synthesis. Nitrogen-sufficient Zarrouk medium conditions maximise phycocyanin. A culture showing yellowing (chlorosis) from nitrogen deficiency will have markedly reduced phycocyanin regardless of strain.
Temperature effects: phycocyanin content is higher in cultures grown at 25–30°C versus above 35°C. High temperature shifts the cell towards chlorophyll dominance over phycocyanin. Indoor LED growers can optimise for phycocyanin by maintaining 28–30°C rather than pushing to maximum growth temperature (35°C).

Certified culture collections: UTEX (University of Texas algae collection), CCAP (Culture Collection of Algae and Protozoa, Scotland), SAG (Germany), and commercial suppliers provide verified, identified strains. These are the most reliable sources for genetically confirmed A. platensis or A. maxima. Cost is higher but the strain identity is documented and cultures are typically clean and contaminant-free.
Community grower cultures: Many home growers obtain starter cultures from community sources (online spirulina growing groups, local growers). These are often viable and productive but carry risks: unknown strain identity, potential mixed cultures, and possible contamination from the source grower’s system. Before using community cultures for food production: examine microscopically for contaminants; observe for 2 weeks before first harvest.
Claimed “wild” cultures: Any culture described as “wild-collected” should be treated with extreme caution. Wild alkaline lake samples contain unknown cyanobacterial species mixtures, potential cyanotoxin producers (Microcystis, Anabaena), and environmental contaminants. Do not use wild-collected cultures for food production.

Visual assessment: Healthy culture should be a deep, uniform blue-green to bright green colour. Yellowing or orange coloration indicates nitrogen deficiency or dying culture. Brown patches indicate bacterial contamination. Pale blue indicates phycocyanin release (cell lysis).
Smell: A healthy spirulina culture smells mildly earthy and oceanic. Sulphurous or sewage odour indicates bacterial contamination (sulphate-reducing bacteria). Fishy smell can indicate green algae contamination.
Microscopy: Under 100× magnification, spirulina should appear as regular blue-green helical filaments. Random, non-helical green threads indicate green algae contamination. Small transparent wheel-shaped organisms (rotifers) indicate predator contamination.
Growth test: Transfer 100 ml of starter culture to 1 L fresh Zarrouk medium. If culture doubles in density within 3–5 days under standard light and temperature, the culture is viable and healthy.

Long-term continuous culture leads to gradual genetic drift and potential loss of desirable traits (phycocyanin content, growth rate). Refresh your culture from a preserved stock every 6–12 months. Store 500 ml of dense, healthy culture in a sealed, clean container in the refrigerator at 4°C (dormant but viable for weeks to months) as your reference stock.