The growth rate of algae as a function of temperature. In a plot the rate of growth will look like a bell curve (see figure 1 below.) At lower temperatures the growth rate will be slow. Warm temperatures the growth rate will be higher. At hot temperatures the water will be too hot and the algae will not grow.
Here is a plot of phaeodactylum, a marine diatom as a function of temperature.
Optimum temperature for spirulina is between 80-95F (26-32C). As you can see from figure 2, heating above 100F can result in death of the culture as growth rates drop off quickly. We suggest using a couple different methods:
Glass heaters: Use a little caution here, you are likely going to be doing a lot of cleaning of the tank and moving the heater around. As a kid, I must have broken one a year in my aquariums. They are often a little bit less expensive than titaninium or plastic.
Plastic heaters: Plastic heaters are our favorites here at ARS. They work well to protect against an accidental dropping, you can soak them in acid to clean. and are generally indestructible.
Titanium heaters: Pretty much the same as plastic, but cost a little more. However they are pretty. After a while, they will get as ugly as a plastic one as you use it.
If you are using over 40 L of culture you may wish to give it a try, see our section on light.
Often you can set up your electricity driven lamps to be adjacent to a tank and transfer the heat into the water.