When is my culture dead?
By Allison Lee
It is a hard thing to admit when a culture has officially died. A culture can die for many reasons, the most common being neglect. If a struggling culture does not get the right nutrients, light or temperature for long enough, it will eventually give up the ghost.
Before you give up on it. Algae cultures are resilient. Given fresh media they can often recover, slowly. If there is any fleck of green in there, it can come back once conditions improve. Work through the troubleshooting steps first, and give it a week.
If there is no recovery, then your culture has reached the official state of dead. Dead means completely nonviable, with no chance of returning to a healthy population.
Left: a living culture of spirulina (Arthrospira). Right: a dead culture of spirulina.
What dead looks like
At this point the culture is usually white or cloudy. It can smell off. When you shake it, the settled ghosts rise up and clump together into a flocculant.
If you want to be certain
Active fluorescence
An analytical way to test viability is a Pulse Amplitude Modulated fluorimeter, or PAM. It measures the quantum yield of photosynthetic efficiency, Fv/Fm. Algae with chlorophyll-a usually measure around 0.7 when healthy. The yield drops below 0.2, or to zero, when they have stopped processing light.
Walz makes a good instrument for this. They are sold in the US by PP Systems. Ask for Tim Doyle, who runs that division, and tell him we sent you.
A dead cell stain
SYTOX will only stain nucleic material if it can penetrate a compromised cell wall, so it stains the dead and leaves the living alone.
What should I do?
It can be difficult to resurrect a nearly dead culture, and in most cases it is more time efficient to start over from a healthy culture, or from a seed stock from a colleague.
This is why we tell everyone to keep a backup. Hold half a litre of healthy culture aside in a separate container under dim light and leave it alone. When the main one crashes, you restart in an afternoon instead of reordering.