Purification of freshwater picoplanktonic cyanobacteria by pour‐plating in ‘ultra‐low‐gelling‐temperature agarose’

SUMMARYSeven unialgal cultures of picoplanktonic cyanobacteria isolated from freshwater lakes in Japan were examined in the present study. They are assignable to the Cyanobium cluster of the Synechococcus group sensu Waterbury and Rippka (1989) based on morphology, GC content, fatty acid and quinon compositions. They include one blue‐green colored strain (NIES‐680) with only phycocyanin as well as the PCC strains of Cyanobium cluster (Waterbury and Rippka 1989) and six reddish colored strains (NIES‐715, ‐717, ‐718, ‐721, ‐722 and ‐723) with both phycocyanin and phycoerythrin. All the strains examined developed colonies in pour‐plates I of 1.4%‘ultra‐low‐gel‐ling‐temperature agarose’within 1 month. The colony numbers were 12–20 × 102 c.f.u. mL‐1. Most clones derived from the colonies were axenic by the bacteria‐free check. The length of this procedure, preculture excepted, was 4–6 weeks. Colony formation in pour‐plate II of 0.6% agarose was observed in the blue‐green strain (NIES‐680) and the three reddish strains (NIES‐717,‐721 and ‐723) after 1–2 months. The number of colonies formed on pour‐plate II were 1–2 × 102 c.f.u. mL‐1, much lower than those on pour‐plate I. Only a blue‐green strain formed colonies on spread plates of 0.6% agarose. The colony numbers developed were 10–15 × 102 c.f.u. mL‐1 and similar to those on pour‐plate I. The other reddish strains did not develop colonies on spread plates. The pour‐plating method developed provides a highly efficient and successful method for obtaining axenic clonal cultures of planktonic cyanobacteria.