Spirulina are photosynthetic, so light is their food. This may sound obvious, but to understand them one must keep in mind that they have an intimate relationship with light that is not obvious to humans. In my 20+ years growing algae, the issue of light—how to deliver it, how much, and what wavelengths—has come up again and again. It is pretty easy to mix the right proportions of nutrients into the water (especially with our recipes!), and to get the right temperature, but this is only two out of three of the key factors in Spirulina cultivation. I’ve seen many new growers lose their cultures, or get inadequate growth, due to mistakes with delivering light to their algae.
In this series, I’ll be covering every aspect of feeding your algae light: how much light to give, how this interacts with mixing, what colors/wavelengths to use, and how to measure light levels. When we’re done, you’ll have a big leg up on most Spirulina growers.
These posts are based on sections in my book Commercial Spirulina Production: From Flask To Farm, which contains everything you need to know to go from your first algae bottle to a profitable commercial farm.
Feeding Your Cultures Photons
Algae respond to light much more emphatically than people do. The human eye has a tremendous ability to adapt to low light levels, and sees highly divergent levels of light as almost the same. For example, though we can see fine by the full moon, it is almost a million times less bright than the sun. Even a room that seems brightly lit to us can be a hundred or more times less bright than full daylight. I often receive emails from would-be algae farmers who can’t understand why their algae is growing slowly; when I inquire more deeply, I often find that they put it in what they thought was a “bright” spot, but that doesn’t get any direct sunshine. Algae need lots of light to grow fast! Providing such light levels artificially is generally prohibitively expensive for large-scale production. There are some situations where artificial lighting is necessary, though, and sometimes low light levels are best, if fast growth is not desired. For these reasons we will discuss artificial lighting in a future post.
Algae growth depends on both the area of sun intercepted, and the hours of light the algae are exposed to. Both horizontal farms (such as ponds) and vertical (such as columns) can catch plenty of sunlight. Walls, buildings, trees etc. which block sunlight at certain angles from directly striking the algae culture will limit growth however; the important thing is the number of hours that sunlight strikes the culture each day.
Optimal light levels and mixing
The optimal level of light for a healthy individual Spirulina cell is approximately one-tenth of direct sunshine. (See the figure). This is roughly the maximum amount of light they can continuously absorb and use for cellular energy. If they are exposed to more light than this for very long, the extra energy “burns” the chlorophyl and other photosynthetic molecules, causing yellowing, stress, and a lightening of the culture, and if it continues too long, cellular death. If the cells are frequently exposed to high light levels, they will produce larger amounts of beta-carotene and other pigments that “ground out” the excess energy, giving them a more yellow-y color. With such adaptation, the cells can generally survive a few more minutes of direct sunshine, but not much more.
It is perhaps surprising, then, that the optimal illumination for a dense, healthy, well-mixed culture, is direct sunshine. This is because the cells in the top layers shade the ones below, so that while the cells at the top are getting more light than they can use, the cells at the bottom of the culture get almost no light. If properly mixed, cells in a culture will only spend several seconds at any given depth, so they will not be overwhelmed or starved for light for very long. So continual, thorough mixing is essential when a culture is in intense light.
Many systems for distributing light more evenly to algae have been proposed, that make sure a higher proportion of the algae get neither too much or too little light, using optical fibers, transparent tubes, etc. These schemes can increase the efficiency with which light is utilized by algae, but the added complication has never been economical at scale, especially taking construction and maintenance costs into account.
Protecting Cultures From Excessive Light
Excessive light is actually a fairly common problem for Spirulina cultures. In relatively thin cultures, where the light penetrates the entire culture, the cells cannot escape the excessive light provided by direct sunshine, so such cultures must be shaded to prevent stress from excessive light exposure. As a general rule, if the optical density in a culture (visibility depth, OD, or “Secchi disk” reading) is more than half the depth of the culture—meaning that you can see halfway through it—then it cannot handle full sunshine and should be shaded to some extent. For this purpose thin white fabric or white tarps can be used. For cultures so thin the bottom can be seen—i.e. the Secchi/OD reading is greater than the culture depth—or where the culture is stressed as well as thin, a 10x reduction from full sunshine is best. This is actually a good reason to start new cultures as thick as possible (density reading of less than half the culture depth)—so you don’t have to shade them, avoiding the extra labor and the growth slow-down caused by shading. For cultures being started from a test tube, even lower light levels may be needed (see the Growing Out From A Test Tube sections in either of my books).
Spirulina is more vulnerable to light stress when the temperature is below optimal growth temperatures, as this slows their ability to adapt. If the culture is below about 85F, take extra precautions to keep the cells from seeing excessive light levels.
Inadequate Light
By the same token, in a thick culture in minimal light, the cells will spend too much time in the dark zone. The cells will slow and then stop growing, instead investing in extra chlorophyll to catch what light they can. Eventually, light-starved cells will die, though they can hang on using stored energy for weeks.
If an culture is thin (OD of more than 4), and has been in low light levels or darkness for more than a day, it will be particularly sensitive to light overexposure until it adapts, which takes about a day.
That’s all for now! Stay tuned for the next installment, which will be about artificial lighting, and what wavelengths to use.