When Spirulina growers come to me for help, one of my first questions is what the strands in their culture look like. While photographs are helpful, it is often useful to be able to describe in a few words the shapes of strands, and how they may be changing. This is important because shape changes can indicate problems in your culture, and can even make harvest impractical, and so put your farm out of business, at least until you can grow a new culture with a better strand shape.
First, a note on terminology:
Because Spirulina has the “spiru-“ root in its name, it is tempting to refer to their shape as being spirals. Their actual shape, though, is not a spiral! Spirals either circle inward, or outward, with a constantly changing diameter, whereas the loops of a Spirulina strand have a constant diameter. The proper term for the three-dimensional shape of a Spirulina strand is “helix”.
Describing Spirulina Helixes — Parameters of Shape
Spirulina cells stack like little tuna cans; they grow in height until they are ready to divide, then they build a little round wall through their center to make two new tuna cans, stacked on top of each other and stuck together, so that the strand gets longer. A typical Spirulina strand has from a few dozens to a few hundred cells. They do not stack exactly on top of each other, and this slight twist gives the whole strand its helical shape.
Here is a typical Spirulina strand, along with its typical length variants through a growth cycle. A given Spirulina helix can be described by four parameters: cell diameter, helix diameter, wavelength, and length, as shown in the figure. Here are verbal descriptions of each shape parameter, as well as their typical values:
Cell diameter — the diameter of the cell in cross-section, of the strand itself. This generally does not change, so if you see strands of algae with a different cell diameter from your original culture, this is likely another species of algae. For all the edible Spirulina strains this is close to 8 microns.
Helix diameter — the outside diameter of the corkscrew shape. Typically 4-8x the cell diameter, so ~32-64 microns. It is not unusual for variants of a given strain with different helix diameters to show up in your cultures, and if they have an advantage, to multiply. This is not a major concern unless it results in an un-harvestable form (see Figures 62 and 63 for examples of variants with unusual helix diameters).
Wavelength — the distance between turns in the helix. It is not unusual for variants with a different wavelength to show up in your cultures (this is often accompanied by a change in helix diameter as well), and if they have an advantage, to multiply. Typically about 1.5 times the helix diameter, though I have seen it 2-3x the helix diameter in a few cases, and in the extreme opposite case, it can be exactly equal to the cell diameter in a supercoil (see below).
Length — number of turns in the helix. This constantly changes for each strand as the cells grow and the strands split; a given culture, subjected to a certain degree of hydrodynamic shear, will tend to have a typical length, so if this changes it is a sign of new genetics, or a change in shear. In most cultures, this is 4-8 turns per strand, though up to twice this (sometimes found in very low-shear settings) is generally not an issue. Below 4 turns, harvestability tends to suffer, and above 16, you run into the issues of “ultra-long” forms, which may catch on prefilters meant to protect pumps, etc. This is a very easy parameter to assess even in a low magnification image, and thus it is relatively easy to track.
Strand length says a lot about your culture; we'll talk more about this in future posts...
In the meantime, keep growing!
Cheers, Dr. Baum