One of the good things about spirulina is the particular growth conditions that it prefers. What I mean is a pH of around 9.5 (most hydroponics systems are kept at a pH of only 6.5 or so!). However, this is a blessing in disguise because if the pH of the nutrient medium in which the spirulina grows is kept high then this will (I have read) discourage other forms of algae from taking root in the medium.
I have also read that Chlorella (another algae which people consume) does not have this high pH requirement and therefore it is more difficult to keep the growing medium monoalgal (all the same algae). Also Chlorella's cell wall is indigestible for us and so commercial growers must crack this cell wall using some type of process. And also I have read (all of this in Robert Henrickson's Earthfood Spirulina) that Chlorella is harder to harvest because the cells are small and so they cannot be "screened" through a filter like spirulina. They have to be centrifuged in some way and therefore spirulina seems to be much easier to get started with than Chlorella.
Offered by Glenn.