Isolation and Characterization of Planktothrix Strains

All clonal Planktothrix strains were isolated by cutting out single filaments migrating on agar as described (Kurmayer et al., 2004 (link)), and cultivated at 15 °C under low light conditions (5–10 μmol m⁻² s⁻¹) in BG11 medium (Rippka, 1988 (link)). In total, 138 strains were analyzed, including 62 isolated previously (Christiansen et al., 2008a (link)); Table 1; Supplementary Table S1). Strains were analyzed for (i) the presence of the full mcy gene cluster and the mcyT gene, which occurred in toxic strains and as a remainder in nontoxic strains (Christiansen et al., 2008a (link)), (ii) the genetic variation within seven housekeeping gene loci and intergenic spacer regions (IGS): 16S rDNA, 16S rDNA-internal transcribed spacer region (ITS), phycocyanin (PC)-IGS, photosystem I (PSA)-IGS (in between photosystem I related psaA and psaB), RNaseP, rbcLX-IGS (in between the large subunit of the ribulose bisphosphate carboxylase/oxygenase and rbcX), and rpoC. In addition, the ability to express gas vesicle protein variants of 28, 20 and 16 kDa was revealed by the presence of the genes gvpC²⁸, gvpC²⁰ and gvpC¹⁶ as described (Beard et al., 2000 ; D'Alelio et al., 2011 ).