Detection of Algal Lysing Biological Agents in Lakes by the Soft-agar Overlayer Technique

The detection of biological agents responsible for the deterioration of the blooms of blue-green algae in lakes was attempted by the soft-agar overlayer technique. One ml of algal suspension and a measured volume of sample were added into about 2ml of melted 0.8% agar cooled to 45°C-48°C, and the mixture was poured over the surface of a hardened layer of modified Detmer medium containing 1.5% agar. After the upper layer solidified the plate was incubated at 25°C to 30°C in the light of 3000 lux to 5000 lux. After 10 days incubation, plaques on agar plates were counted. Most of the agents detected by this technique were amoebae, bacteria and fungi. Especially, development of large plaques was accompanied by amoebae.