Flocculation of Chlorella vulgaris by Lactobacillus casei

Summary The flocculation of Chlorella vulgaris by Lactobacillus casei was studied to determine whether the latter could act as a suitable flocculant for the removal of Chlorella from algal ponds. The flocculating activity of the Lactobacilli was caused by the bacterial cells themselves, and not by diffusible products of bacterial metabolism. Diffusible products of algal metabolism inhibited flocculation. For algae resuspended in water, the best flocculation occurred at pH values less than 3.5 where the charges on the bacterial and algal cells were opposite. For flocculation at least one bacterium was required for every algal cell; in terms of cell concentrations,10 mg/l of bacteria were required to flocculate an algal suspension of 1,000 mg/l. The mechanism of flocculation implied by the results is that positively charged cells of L. casei adsorb to the surface of negatively charged cells of C. vulgaris neutralizing the charge and thus destabilizing the algal suspension. Because of the low pH required and because diffusible products of algal metabolism inhibit the flocculation, it is unlikely that L. casei could be usefully employed as a flocculant of Chlorella from algal ponds.