Reading Your Culture: Color, Density, Smell

You do not need a lab to read a Spirulina culture. Color is the headline indicator: a healthy culture is a deep, rich blue-green — chlorophyll green layered over phycocyanin blue — and deepens as biomass builds. A yellowing or olive cast often signals stress or nutrient limitation, frequently nitrogen. A blue tinge in the liquid itself (not just the biomass) suggests cells are lysing and leaking phycocyanin — a bad sign.

Density tells you about biomass and harvest timing. A simple field test is to note how far into the liquid a clean white spoon disappears; as the culture thickens, that depth shrinks. If you can measure optical density or turbidity, watch the trend rather than a single reading. Smell is an underrated diagnostic: a good culture smells fresh, faintly earthy or seaweedy. A sharp, rotten, sulfurous, or sour smell warns of a bacterial bloom, a crash, or anaerobic conditions — trust your nose, as a foul smell usually precedes a visible crash.

A few simple measurements round out the picture. pH is the most valuable: for Spirulina it should sit in the alkaline 9.5–10.5 range and tends to rise as the culture photosynthesizes. Temperature should hold in the 30–37°C band. Even inexpensive pH strips and an aquarium thermometer give you enough to act on. Build the habit of a quick daily look-smell-check — most problems announce themselves a day or more before a full crash.