Spirulina growing: contamination control — predators, competitor algae, and bacterial infections

Why spirulina is naturally resistant but not immune

Spirulina’s high alkalinity (pH 9–10.5) and high sodium bicarbonate concentration exclude most freshwater organisms. The overwhelming majority of freshwater algae, bacteria, and invertebrates cannot survive at pH >9.5. This is spirulina’s primary defence.
The exceptions — organisms that have adapted to alkaline environments — are your contamination risk. Know them, recognise them early, and respond quickly.

What they are:Rotifers (Brachionus, Monostyla, Filinia) are microscopic filter-feeding animals (100–500 µm) that graze on bacteria and algae including spirulina. Alkaliphilic rotifers survive at pH 9–10 and can crash a spirulina culture within 2–4 weeks of establishment.
Recognition:Under 40× microscopy, rotifers are visible as transparent, wheel-shaped (corona) organisms with visible internal organs. They swim with a rolling motion. Spirulina filaments visible inside their digestive tracts confirm active grazing.
Signs in culture:Culture turbidity decreases (spirulina being consumed) without apparent cause (pH normal, nutrients normal). Culture appears cleaner and less green. Microscopy confirms rotifer presence.
Prevention:Rotifers enter cultures via contaminated water, open vessel exposure in outdoor environments, or contaminated inocula. Use RO or dechlorinated water; inspect inocula under microscope before adding to main culture; cover outdoor cultures with fine mesh or polycarbonate covers.
Treatment:Raise pH briefly to 11.5 (add sodium hydroxide in small increments) for 1–2 hours, then return to pH 9.5 with CO₂or citric acid. This kills rotifers (which cannot tolerate pH >11) while spirulina survives short alkaline shock. Alternatively: filter culture through fine mesh (25–50 µm) — spirulina filaments pass; rotifers are retained and removed.

Green algae (Chlorella, Scenedesmus):Unicellular green algae can survive at pH 9–9.5. They appear as tiny green dots under microscopy (5–10 µm spherical cells), distinct from spirulina’s helical filaments. At low levels, they are not harmful. At high levels, they compete for nutrients and light. Raise pH to 10–10.5 to selectively disadvantage most green algae while spirulina thrives.
Filamentous algae (Oscillatoria, Lyngbya):Other cyanobacteria that can establish in spirulina cultures. These are not harmful to consume but dilute spirulina biomass. They typically don’t compete well at spirulina’s preferred pH of 9.5–10.5. Maintain pH in the upper range during active growth to suppress them.
Diatoms:Brown single-celled algae visible under microscopy as rectangular cells with striated silica frustules. They can establish in low-sodium, lower-pH cultures. They are difficult to eliminate but do not usually dominate at spirulina culture pH.

Biofilm formation:Heterotrophic bacteria form biofilms on culture vessel walls and equipment. At low levels, bacteria are present in all cultures and do not impair spirulina growth. At high levels (particularly after a crash or nutrient depletion), bacterial overgrowth can acidify the medium and produce off-odours.
Recognition:Foul smell (egg/sulfur odour indicates hydrogen sulfide production by sulfate-reducing bacteria). pH drops unexpectedly. Culture appears turbid with white or grey floccules.
Prevention:Clean culture vessels with 10% bleach solution (chlorine neutralised with sodium thiosulphate before re-inoculation) between batches. Avoid organic contamination of the medium (food particles, organic residues). Maintain vigorous aeration (aerobic conditions disfavour sulfate-reducing bacteria).

Equipment needed:Compound microscope (100–400× magnification); basic glass slides and cover slips; a pipette for sample collection.
What to look for:At 40×: spirulina helical filaments (dominant); any rotifer movement; any unusual particles. At 100–400×: green algae cells; diatoms; bacteria (if present in unusual quantities); rotifer detail.
Healthy culture:>95% spirulina helical filaments; no rotifers; minimal competing algae.
Action thresholds:Any rotifers = immediate intervention; >10% competitor algae by visual estimation = pH adjustment; visible biofilm buildup on walls = vessel cleaning at next harvest.

If contamination is severe and cannot be remediated: restart from preserved stock culture. Maintain a small (<1 L) reference stock culture separate from main production culture, in a sealed container, refreshed every 2–3 weeks. This provides a clean inoculum if the main culture crashes.