In Part I we have seen why spirulina needed light and which light was the best for its growth. In Part II we have detailed the optimum heat condition for the best spirulina growth you can produce.

Today we will have a closer look to a very important component of your spirulina tank: the agitation of the water. It’s a critical component in your culture.

Spirulina is a photoautotrophic organism; from: photo- = light, auto- = self, –trophic = food => an organism that “makes” its food itself using light energy, an organism capable of photosynthesis. Basically, it means that Spirulina “makes” or “builds” its substances by photosynthesizing all the nutrients and minerals that are dissolved in its environment (i.e. water). To do that, Spirulina uses the energy from the sun gathered by different pigments. Spirulina has several pigments – chlorophyll a, β Carotene, γ Carotene, Phycoerythrins, Phycocyanin, allophycocyanins and about 11 different Carotenoids. These pigments will be activated at different wavelengths, this is why spirulina needs a specific type of light for optimum growth (see part I – The Light).

So, when the tank has efficient circulation, all the spirulina cells are mixed in the water column and get plenty of sunlight, each in turn. So no agitation in the tank means that spirulina cells won’t get as much sun as  they could and won’t produce as much as they could. A proper agitation is also very important to prevent spirulina forming clumps on the surface of your aquarium!

Illustration by Spirulina Division. Find out more about Spirulina Division –

But all photosynthetic cells have a limit to the amount of sun they can be exposed too. Indeed, too much light can damage the spirulina cells, it can lyse (or photolyse) the cells. It’s the same for us… Some sun (about 10min/day) is necessary to produce Vitamin D, too much can cause cancer!

So the trick is to find the right balance, give spirulina cells enough light to do the photosynthesis, grow and divide themselves but not too much to prevent photolyse.

One way to achieve this is by agitation (or stirring) of the culture. Stirring your tank will force spirulina filaments to go up and down in the water column, to go to the surface and then descend, limiting the time each filament will be exposed to the sunlight. Ideally, you want each spirulina filament to stay about half a minute at the surface (that is if you have very high light intensity (no clouds, midday summer sun)).

Practically, this can be done by using an air pump in your aquarium, or a paddlewheel if you have a larger tank. But it’s important to choose your air pump correctly! The size of the pump must be correlated with the size of your tank, up to 10 gallon – you can use the small pump available on the market (these cost $10 or less); above that you’ll have to get a larger one to make sure the agitation is correct. Don’t forget to check the noise level of the pump before buying one and make sure to pick a quiet one if your tank is placed in a living space or your office! It is also important to remember that spirulina is made of very tiny cells (they are less thanone-tenthh of the width of human hair across), which are fragile and not difficult to break. Some cheap pumps using centrifugal force to push water around are often dangerous for spirulina as they break the fragile filaments. A sign foam formation on top of your aquarium. More about how to remediate to this problem in our FREE email micro-course.

Interestingly, Spirulina has a built-in mechanism to protect itself against too much sunlight.
Most spirulina strains can exist under two forms – in spiral and straight. Each species of spirulina has it own specific spiral form. The same species can also have different forms in function of the environment (area in the world) where it is found – Fig 1.
When spirulina cells don’t have enough light they will straighten themselves so that they can absorb more light. When the sun is too intense, they will become more spirally, each turn of spiral providing some shade to the rest of the filament.


This is possible only to a certain extent such as when there is too much light, the cell will lyse and if the temperature becomes too high in the aquarium then your spirulina can also become straight. Even if straight spirulina has the same nutritional value as the spiral form, it is more difficult to harvest. So in general, this is something you want to avoid.

So keep a close eye on your culture and act quickly if you see some anomalies with your spirulina culture!


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