In my first post in this series, we talked about how much light Spirulina need to grow well. Light, though, is composed of many wavelengths, and which wavelengths you use can affect growth rates, as well as your electric bills if you’re using artificial light. What wavelengths Spirulina can use is also determined by its light-absorbing photopigments, and understanding these is useful for understanding how to supply light, and assessing the health of your cultures.
Light with a wavelength between 400nm and 700nm (blue to red light) can generally be used by plants and algae to grow (subject to the caveats below). Light in this range is called Photosynthetically Active Radiation (PAR), and we only need to consider these wavelengths—not infrared or ultraviolet—when growing algae. See the figure.
Plants and algae absorb light using photo-pigment molecules such as chlorophylls (green), xanthophylls (yellow), and carotenoids (orange). Blue-green algae like Spirulina also make bright blue pigments such as phycocyanins and allophycocyanins, which are also potent antioxidants, and sometimes sold in purified form as “Blue Spirulina.” Each pigment has a range of wavelengths of light that they absorb. This absorbance spectrum determines their color. The figure below shows the absorption spectra of the main pigments of Spirulina, compared with those of a typical green plant. As you can see, a typical green plant absorbs mostly in the red and blue parts of the spectrum. This is why plants look green as only green light escapes absorption by the plant, and it is why green plant-optimized LED grow lamps combine red and blue lights to make a violet color. In addition to the main pigments used by green plants, Spirulina’s deep blue pigments absorb red, orange, and yellow light well, in addition to some green. For this reason they have a blue-green color much darker than most green plants.
Due to the presence of these blue pigments, Spirulina will grow fine with white lights, and regular violet-colored grow lights, but it is a bit more efficient to grow them with lights that emit more red, orange, and yellow light — “warmer” light. If you use actual colored lights, with narrow ranges of frequencies emitted, it is essential to provide both blue and red light, as both activate different essential photosynthetic molecules.
When considering using artificial light to produce Spirulina commercially, it is important to take into account the very high levels of power required for fast growth. Direct sunshine is 1000+ watts per square meter. Though you don’t necessarily need this much, and you can increase the efficiency of light utilization by 20-30% by matching Spirulina’s absorption spectrum more exactly, matching this power level to reach maximum growth rates requires expensive lights and a high electrical bill. This usually makes the use of artificial light uneconomical unless your electricity is extremely cheap, though this cost can be offset by using the heat given off by the lights to heat your cultures as well. If slow, steady rates of growth are desired, 100 watts per square meter, or even less, can be used, though growth rates will drop below this level, and reach zero below around 20W/sq m.
Distributing the light more evenly through the culture, for example placing it in a clear container and illuminating it from all sides, or even using tubes or optical fibers to carry the light into the culture, can boost the efficiency of light usage, and maximize growth rates, by providing light levels closer to the optimum for all cells, but these systems are not practical for large-scale, economical production.
In our next post, we will be looking at inexpensive ways to measure light levels accurately.