Occurrence of an uncommon Planktothrix (Cyanoprokaryota, Oscillatoriales) in a deep lake south of the Alps

While the widespread red filaments of Planktothrix rubescens are considered a typical cyanobacterium of the deep lakes of the subalpine district (Central Europe), we report a massive occurrence of unexpected Planktothrix specimens in Lake Idro (Northern Italy). Many strains isolated from water samples that were collected during two sampling campaigns developed filaments up to some milimetres long when placed in culture and developed clustering yellow aggregates, visible by eye in dense cultures. Among yellow strains, 40 were analysed genetically and showed unique patterns in the coding-genes rpoC1 and rbcLX. By combining morphological observations and genetic analyses, we provisionally ascribed these strains to P. cf. suspensa, a rare and poorly studied species, no culture of which is currently included in public algal collections.