By Allison Lee
It’s a hard thing to admit when a culture has officially died. A culture can die for many reasons, the most common being due to neglect. If a struggling culture does not receive the proper nutrients, light, or temperature for a long enough period of time, it will eventually give up the ghost.
Now, algae cultures are pretty resilient and if given new media, can sometimes make a recovery, albeit slowly. If there is any fleck of green, your culture could begin to grow if conditions improved.
However, if there is no recovery, then your culture has reached the official state of dead. Dead means a completely nonviable state with no chance of recovering to a healthy population.
Images: (left) Alive culture of Spirulina (Arthrospira), (right) Dead culture of spirulina
At this point the culture is usually white or cloudy in color. It can smell ‘off’. When shaken, the settled ghosts rise up and clump together into a flocculant.
An analytical method to determine viability is to use an Active Fluorescence measuring device, such as a Pulse Amplitude Modulated Fluorimeter, or PAM. This device measures the quantum efficiency or yield (Fv/Fm) of photosynthetic efficiency. Algae with chlorophyll-a will usually measure a 0.7 when they are healthy. The yield will drop to < 0.2 to 0 when they are not processing light anymore. Walz instruments makes a great instrument for determining this type of measurement, called Active Fluorescence. They are sold here in the US by PP Systems, there is a fella named Tim Doyle, in charge of the division. He can tell you more about the equipment (tell him we sent you!)
If you want to be really sure your culture is dead you could test with SYTOX, a dead cell stain. SYTOX will only stain nucleic material if it can penetrate a compromised cell wall.
What should I do?
It can be difficult to resurrect a nearly dead culture, and in most cases will be more time efficient to start over by purchasing a healthy culture, or finding a new seed stock from a colleague.