Troubleshooting Spirulina Contamination and Crashes

Spirulina is forgiving, but cultures still stumble, and most failures fall into a few recognizable patterns. pH swings are the most common and most informative. Spirulina’s productive band is roughly pH 9.5–10.5, tolerant to ~11. As it photosynthesizes, pH drifts upward — healthy — but if it climbs past 11 the culture becomes carbon-limited and stalls; the fix is to add bicarbonate, restoring both carbon and buffer. A pH that instead falls unexpectedly often means acid-producing contamination or a crashing culture; falling pH plus a foul smell is a strong contamination signal.

Contamination shows up as changes in color, clarity, and smell — foaming, stringy material, a film, a shift toward yellow or milky liquid, or a rotten odor. Because Spirulina thrives where competitors die, your best treatment is the same lever: push pH up. Raising alkalinity toward the top of Spirulina’s tolerance selectively suppresses most invaders while Spirulina keeps growing. Some contamination is unrecoverable, though; if badly fouled or strongly rotten, discard and restart from a clean starter.

Crashes — a sudden die-off where the culture pales and settles — usually trace to overheating (a small vessel in direct sun can pass ~40°C), too much light on a thin culture, oxygen supersaturation from poor mixing, nutrient exhaustion, or a large pH excursion. Diagnose each, correct the cause, and let the surviving fraction rebuild. A few habits prevent most trouble: keep the culture dense enough to self-shade, mix gently and steadily, feed before it starves, and keep a backup starter in a separate vessel — the single best insurance.