Aposphaerospermum hubeiense gen . & sp. nov . (Aphanizomenonaceae, Nostocales), a novel geosmin-producing cyanobacterium from an eutrophic reservoir in China

A planktonic Dolichospermum-like strain FACHB-3676 was isolated from Xionghe Reservoir, an eutrophic drinking water reservoir located in Zaoyang city, Hubei Province, China. Morphologically, this strain closely resembled Dolichospermum smithii, differing in the dimensions of vegetative cells and akinetes, and also resembled members of the genus Sphaerospermopsis, from which it was primarily distinguished by the position of its spherical akinetes. Phylogenetic analyses based on 16S rRNA gene sequences revealed that strain FACHB-3676 formed a distinct clade within the family Aphanizomenonaceae, sharing less than 94.5% nucleotide similarity with all described genera. Phylogenetic reconstructions based on rbcLX and nifH gene sequences further supported its placement as a novel lineage within the Aphanizomenonaceae. Additionally, the strain exhibited unique ITS secondary structure features, including the D1–D1’ and Box-B helices, reinforcing its taxonomic distinctiveness. Based on this polyphasic approach, the strain is proposed as a new cyanobacterial taxon: Aposphaerospermum hubeiense gen. & sp. nov. Furthermore, PCR amplification and GC/MS analysis confirmed that strain FACHB-3676 produces geosmin, a key compound responsible for earthy-musty odours in freshwater systems.