Source: <a href="http://www.algaeindustrymagazine.com/growing-spirulina-at-home/">Algae Industry Magazine</a>
Spirulina is in fact an extremophile, capable of growing in extremely alkaline water inhospitable to almost every other organism. Most other algae grow in essentially pH-neutral water, which supports the growth of a vast range of algae—including types that produce toxins—as well as doing nothing to inhibit the growth of other potentially harmful organisms such as bacteria. In my biofuel-algae work, we’re constantly fending off invasive species. It’s not just an academic concern. Since it is generally hard to control the growth of possibly harmful stuff (and although it’s fun, we think you should look at your culture under the microscope every day), this aspect of Spirulina cultivation is pretty key to growing pure and safe cultures on a DIY basis.