Aerosakkonema funiforme gen. et sp nov (Oscillatoriales), a new gas-vacuolated oscillatorioid cyanobacterium isolated from a mesotrophic reservoir

A novel gas-vacuolated oscillatorioid cyanobacterium was isolated from the Nam Ngum Reservoir, Lao People's Democratic Republic. Five strains were tentatively assigned to this new cyanobacterium because they showed morphological characteristics different from previously described freshwater species of gas-vacuolated oscillatorioids. Their trichomes were solitary, blue-green or olive-green, 11.7-16.6 mu m wide, without sheaths, and were either not or slightly constricted at the cross walls. Notably, trichomes were sometimes twisted into a rope and contained gas vacuoles smaller than those of any known freshwater species of gas-vacuolated oscillatorioids. The occurrence of gas vacuoles was confirmed by transmission electron microscopy (TEM) and polymerase chain reaction detection of the gas vesicle gene (gyp). TEM observations also indicated an irregular arrangement of thylakoids. Phycobiliprotein analyses revealed that all strains contained two different biliproteins, phycocyanin and allophycocyanin. The cellular fatty acid compositions of these strains were 14:0, 16:0, 16:1 c, 18:0, 18:1 c, 18:1 c, 18:2, and 18:3 alpha, among which palmitic acid (16:0) was predominant. Phylogenetic analyses of 16S rRNA gene sequences revealed that all five strains had identical sequences and were only distantly related to other oscillatorioid cyanobacteria with gas vacuoles (e.g. Planktothrix and Planktothricoides). Instead, they showed the highest 16S rRNA gene sequence similarity to a non-gas-vacuolated oscillatorioid cyanobacterial strain, Phormidium sp. KS (93.8%). Based on their distinct morphological characteristics and the substantial sequence divergence of the 16S rRNA genes of these strains compared to other cyanobacteria, including oscillatorioids, we proposed a new genus, Aerosakkonema, which accommodated all five strains. The type species was Aerosakkonema funiforme and the type strain was NIES2861 (= Lao26).